EVSE 5309 Physical Environment New

8/9/2019 EVSE 5309 Physical Environment New

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EVSE 5309 Physical Environment, p. 1

EVSE 5309 Environmental Systems –Biological Aspects

Topic 2: The Physical and Chemical Environment

This section of the course focuses on the physicaland chemical environment that organisms experience.

Adaptation

People often speak of organisms “adapting” toconditions in their environment.

This use of “adapt” and “adaptation” can have twomeanings, one correct and one incorrect.

The correct meaning of “adaptation” is related toevolution by natural selection.

An adaptation is a heritable trait that increases

survival or reproduction. An adaptation increases fitness – the ability to

survive and reproduce. Adaptations are the result of evolution by natural

selection.They are the traits that the most successful

parents pass on to their offspring.


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Coping with environmental variation

There are many responses that individual organisms

can make to cope with changes in theirenvironment.

Successful change by an individual to cope withenvironmental variation is properly called“acclimation” or “acclimatization” (not“adaptation”).

The physical environment is highly variable andmay vary outside the limits for sustaining lifeor for efficient functioning.

Organisms must be able to maintain their internalenvironments within the limits for sustainingefficient function or even though theenvironment may fluctuate beyond those

limits.The study of the ability of organisms to sustain

function under variable abiotic environmentalconditions is known as PhysiologicalEcology.

Organisms can also respond to environmentalchanges by migration -- movement to morefavorable conditions.


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Example – regulation of blood osmolarity in amammal:

The hypothalamus (in the brain) acts as bothreceptor and control center.It detects changes in blood ion concentration,

and compares these to a set point.Thirst and the kidneys act as effectors.If blood is too concentrated (ions too high), thirst

directs the animal to drink water.If blood is too dilute, kidneys act to remove and

expel water.

from Mackenzie et al., 2001


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Conditions and Resources

For any species, physical and chemical factors in the

environment can often be classified as conditionsor resources.

Condition = an abiotic environmental variable towhich organisms respond.

These responses can consist of changes inphysiological rates, rates of body growth,

rates of reproduction or mortality, etc.Ultimately, conditions affect the total rate of

change of a species’ population. Populationpersistence thus depends these conditions.

Conditions can be regarded as causes of naturalselection, and organisms typically haveevolutionary adaptations to cope with a

certain range of conditions.Some of these adaptations involve phenotypic

plasticity, acclimatization, and internalregulation.

A condition is often something like temperature orsalinity. Organisms do not consume ordeplete such things.


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Resource = an environmental variable to which anorganism (or population) responds with increasedgrowth, and which is consumed by organisms in

the process.

Clearly survival depends on resources, but thepossibility of depletion leads to competition between individuals.

Such competition is also a cause of naturalselection, and organisms typically haveadaptations to cope with some degree of

resource depletion.Examples of resources include chemical

nutrients, food items, and space in which tolive.


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The Ecological Niche

The range of conditions and resources under which a

species can survive is related to the concept of aniche.The meaning of this concept is not fixed among

ecologists, but has evolved historically, and is notsettled yet.

Many modern concepts of the niche include an ideaproposed by Hutchinson: imagine a (high

dimensional) coordinate system in which eachcoordinate is one of the conditions or resourcesimportant to a species.

On each axis there will probably be a lower boundbelow which the species cannot survive, andperhaps an upper bound above which it cannotsurvive.

Within the high-dimensional space defined by thesecoordinates, there will be a high dimensionalhypervolume defining the conditions under whichsurvival is possible.


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A 2-dimensional illustration:

A more concrete example:


Condition 1

L1 U1

C o n d i t i o n 2

L2

U2


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Other ideas related to the concept of the nicheinclude:

The niche describes the habitat and function of a

species: physical location, interactions withinthe food web, means of obtaining resources,adaptations to its “lifestyle”.

Niches may be subdivided into the fundamentalniche, which is the idealized niche in theabsence of other species, and the realizedniche, which is actually achieved in thepresence of other species that are

competitors.

Resource 1

R e s o u r c e 2

Fundamental Niche

Realized

Niche


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The principle of competitive exclusion says that twospecies cannot occupy exactly the same niche.

One will be a superior competitor that drives theother extinct.To coexist, two species must therefore have some

differences in their niches. A high degree of nicheoverlap may indicate intense competition.

Many of these particular ideas of the niche aredebated among ecologists, and are not

universally accepted.Nevertheless, the general idea of a niche as a

summary of the conditions and resourcesrequired for survival is still used.

Condition or

Resource 1

C o n d i t i o n o r

R e s o u r c e 2

Species B

Species A


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But, contemporary research is moving from abstractrepresentations to detailed studies of conditions,resources, and organisms’ responses.

Two influential ideas summarize how organisms oftenrespond to the numerous conditions and thataffect survival and reproduction.

1. Liebig’s Law of the Minimum (1840)The total yield or biomass of any organism will be

determined by the nutrient present in the

lowest concentration in relation to therequirements of that organism.

This idea originated in agriculture: the yield of a cropis often governed by the soil nutrient in lowestsupply relative to the needs of that crop. Thisidea has been generalized to naturally growing

plants.It has also been generalized to state that the growth

rate of an organism is determined by the nutrientpresent in the lowest concentration in relation tothe requirements of that organism.

This idea applies best to plants and manymicroorganisms that require distinctly differentnutrient resources – e.g. phosphate, nitrate, …

This idea sometimes applies to animals, when onefood source, or one dietary component (e.g.protein, minerals) can be identified as limiting tobiomass or growth.


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Note that Liebig’s Law applies to what happens whenthe level of a nutrient or resource gets too low.

The next law applies to what happens when the level

of an environmental condition gets too high or toolow.

2. Shelford’s Law of Tolerance (1913)For an organism to succeed in an environment,

each of the conditions must remain within thetolerance range of that organism. If anycondition exceeds the maximum or minimum

tolerance of that organism, the organism willfail to grow and will be eliminated.

Environmental conditions which tend to follow this lawinclude examples such as:

TemperatureMoisture and relative humidity

pH (soil and water)SalinityCurrent flowSoil structure (e.g. grain size)Pollutants and toxins

Also, some substances that act as nutrients are toxinsat high concentration. For example, many metals(copper, chromium, selenium, etc.) are requiredas nutrients at trace concentrations. Elevatedconcentrations are toxic.


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Shelford’s Law is associated with a generalizedunimodal response of survival or productivity toan environmental condition:

from Jones, 1997

In the zone of tolerance, performance is high(consistent with survival). Outside of this zone,performance is low.

The lower incipient lethal level of the environmental

condition marks the lower limit of the zone oftolerance.

The upper incipient lethal level of the environmentalcondition marks the upper limit of the zone oftolerance.


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Many variations in the shapes of these curves can befound, depending on the species under study, thecondition under study, and the measure of

survival or productivity adopted.

For many resources, no upper incipient lethal leveloccurs at environmentally realisticconcentrations.

In such cases, a plot of the organism’s populationgrowth rate versus resource level is often anincreasing curve.

Resource Level

P o p u l a t i o n

G r o w t h R a t e , µ

µ = max. growth ratemax

K = resource level when

growth is 1/2 of max.

µ = µ R / (K + R)max

This equation applies well to the growth ofmicroorganisms and nutrient resources.

For example, simulation models of the growth ofalgae used to study water quality often use thisapproach.

It is sometimes also applied to growth of plants inrelation to soil nutrients, or of animals in relationto supply of particular foods.


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Solar Radiation and Climate

Many conditions of the physical environment are

related to climate.Climate itself is related to solar radiation, which heatsthe earth’s surface.

Winds and ocean currents distribute that heat.

Seasonal variation in solar radiation

The earth’s surface is heated most strongly when

solar radiation strikes directly (90° angle).The earth’s axis of rotation is tilted, so the angle of

solar radiation at any location varies annually.



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As a result of this geometry, total solar radiationreceived in a day varies with time of year andlatitude.

from Kirk, 1983


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Over a day, solar radiation varies with time of day andmeteorological conditions.

from Kirk, 1983


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Most sunlight passes through the atmosphere, so littleof its radiant energy is absorbed by air.

High energy wavelengths of sunlight are muchmore strongly absorbed by land and water.

Their radiant energy is converted to heat(molecular motions).

This heat is then transferred to the atmosphereby conduction and infrared radiation.

As a result, the atmosphere is heated primarily fromthe bottom.

Warmed air rises, and expands as pressuredecreases with altitude.

This expansion cools the air, a process calledadiabatic cooling.

Dry air cools 10 °C per 1000 m of elevation.Moist air cools about 6 °C per 1000 m of

elevation.


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As warmed air in one location rises, it is replaced bycooler air from another location, where the sunstrikes less directly.

If the earth did not rotate, winds would beoriented north-south.

The earth rotates, with surface moving at up to1500 km / h at the equator (slowing to 0 atpoles).

Therefore, the path of the wind with respect tothe ground surface is deflected (Coriolis

effect).

Because of differential solar heating at differentlatitudes, air at the earth’s surface has consistenthigh pressure at subtropical latitudes and at thepoles, and consistent low pressure at the equatorand in sub-polar regions.

Winds flow from high pressure regions to lowpressure regions.

The direction is not north-south due to theCoriolis effect, but is deflected.

Winds tend to blow west to east from about 30°to 60° latitude (“westerlies”).

Winds tend to blow east to west in tropicallatitudes (“trade winds”).

At the intertropical convergence zone (ITCZ) nearthe equator, the trade winds converge (the“doldrums”).


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In reality, wind patterns are not symmetric north-south

around the equator.

Land masses alter the wind direction.There is more land in the northern hemisphere.The ITCZ is north of the equator as a result.In the ITCZ air rises from the surface, collecting

moisture if it is over the ocean.

The Caribbean Sea is in the ITCZ, and theregular presence of moist, warm air makes itprone to hurricanes.


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Ocean currents also distribute heat around the earth.

If idealized wind patterns occurred (no land), then the

corresponding idealized surface ocean currentswould be:

from Press & Siever, 1978


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Drifts (consistent currents) would correspond to tradewinds and westerlies.

Gyres would arise in subtropical and subpolarregions.

Real patterns are much more complex, due to land:

from Press & Siever, 1978


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There are also vertical ocean currents.

At the equator, warm ocean water tends to rise. At the poles, cold ocean water tends to sink.This sets up a “conveyer belt”, where warm water

flows on the surface from the equator to thepoles, while cold water returns from thepoles to the equator along the ocean bottom.


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Earth’s is warmer at the equator, and cooler towardsthe poles.

But the distribution of climates is not symmetric north-

to-south, and is altered by the distribution of landmasses.

from Jones, 1997


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Further alternation of climate occurs due to elevationof the land.

As winds rise to pass over mountains, the aircools (adiabatic cooling).Cooler air is less able to hold moisture, so rain

becomes likely on the windward side. Air that descends on the lee side has lost

moisture, and as it heats up it will evaporatemore from the landscape.

Deserts tend to occur on the lee side of mountain

ranges, especially in areas of westerlywinds.



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Microclimate

Microclimate is the climate at the scale of an

organism (not the globe).Most organisms are small (


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A plant < 1 m tall can experience a large range oftemperature from its apex to its roots.



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Temperature prof iles in air

Temperature profiles develop in the air above the soil

surface or the top of the vegetation, over a scaleof several meters.

At night, the surface cools. Air at the surface conducts heat into the cooling

soil.This cools the air, and so a cold layer develops

just above the surface.

Thus the temperature increases with heightabove the surface.

During the day, the surface absorbs sunlight andheats up.

The surface conducts heat to the air immediatelyabove.

Warm air rises, but cools as it does.Thus the temperature decreases with height

above the surface.

These idealized patterns are altered between the topof the vegetation canopy and the soil surface.

They are also altered by wind.


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Properties of Water

Water is the only inorganic liquid that naturally occurson the surface of the earth.

Other inorganic liquids have lower boiling points.

Water is essential to life, and about 90% of livingmass is typically water.


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Water has many unique properties that result from itsmolecular shape and charge distribution.

The two hydrogen atoms are bonded to the oxygen at

an angle of 105°.Two sets of paired electrons lie “opposite” to thehydrogen atoms, generating a charge separation.

from Lehninger, 1982


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The polar nature of water means that it is a goodsolvent for ions and for other polar compounds.

The polar nature of water also permits “hydrogen

bonding”.

from Lehninger, 1982

Hydrogen bonding holds water molecules together,and raises freezing and boiling points comparedto other inorganic liquids.

Hydrogen bonding gives water a high surface tensionand high viscosity in comparison to molecular

weight.


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Hydrogen bonding also explains the unusualtemperature-density relationships of water

The solid form is less dense than the liquid (icefloats).For a certain temperature range, water gets

denser as it heats up.



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Ice floats because every water molecule forms 4hydrogen bonds in the crystalline structure of ice(maximum possible number of hydrogen bonds).

This holds the water molecules far apart, with lots ofempty space in between, making ice less dense.

As ice melts some of the hydrogen bonds break,and water molecules move closer.

So water just above freezing is denser than ice. As water warms from 0 to 4 °C, more hydrogen

bonds break, and water continues to get

dense as molecules can move closer.The maximum density of water is at 4 °C. Above 4 °C, further warming gives water

molecules enough kinetic energy to moveapart, and density decreases.


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When solar radiation passes through water, it isabsorbed.

Absorption of visible light by pure water is relatively

small. Absorption of light is also due to dissolved substancesand suspended particles.

Irradiance at a given depth (Iz) tends to declineexponentially from the surface (Beer’s law):

Iz = I0e-ε z

where I0 = irradiance at the surface (W / m2)

ε = attenuation coefficient (m-1)

z = depth (m)


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The radiant energy absorbed by water heats it up.

If solar radiation were the only thing affecting laketemperature, we might expect to see:

But instead, temperature is typically constant for

some range of depth near the surface, anddecreases only in deeper waters:


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This creates stratification with a warm, less dense

water layer above the colder water layers below.Stratification in many climates varies seasonally,because solar radiation varies seasonally.

In a typical lake in a temperate climate, the followingpatterns occur.

In spring, the water warms up near the surface dueto heating by sunlight. This warm water forms

a hydrodynamically stable layer over colderwater below.

In summer, this stratified situation can become verystable, as the warmer epilimnion develops alarge temperature difference from the colderhypolimnion. Stable stratification isassociated with a thermocline – a region

where the temperature drops rapidly. Waterfrom the epilimnion does not mix with that fromthe hypolimnion.

In autumn, the epilimnion cools, and stratificationbecomes less stable as its temperatureapproaches that of the hypolimnion.Eventually, stratification breaks down entirelyand the lake mixes through its entire depth.

In winter, the lake mixes through its entire depth,unless it is cold enough for ice to form at thesurface.


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Jones (1997)

During winter there is little or no growth of algae (also

called phytoplankton) because of lack of light.Such organisms may die and release theirnutrient content to the water. Mixing of the entiredepth of the water transports oxygen from thesurface throughout the entire volume of water.


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In spring, light increases and the water heats up. Thecombination of high light, warm temperature, andhigh nutrient concentration typically stimulates a

spring bloom of algae.However, these algae consume nutrients so that bythe time summer stratification begins, nutrientsare depleted and algal growth slows down, andtheir abundance usually declines. This period iscalled summer stagnation.

During summer stagnation, stable stratificationprevents the epilimnion, which is oxygen rich,

from mixing with the hypolimnion. Decomposerorganisms consume oxygen in the hypolimnion,however, and oxygen can become depleted.These decomposers also recycle nutrients, sothat they hypolimnion may become nutrient-rich.

In autumn, when stratification breaks down, thenutrients from the hypolimnion are mixed into the

whole lake, and this can stimulate another bloomof algae.

The abundance of crustacean animals in the water(zooplankton) which eat the algae tends tofollow the seasonal peaks of algal abundance,with a time lag.


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There are many variations on these patterns, whichdepend on climate. For example, winterstagnation can occur if the climate is cold enough

to freeze the lake.In subtropical or tropical climates, lakes may neverstratify, or may do so only for short periods oftime.

Many parts of the ocean are permanently stratified,with a deep thermocline (100-1000 m). Coastalwaters and estuaries may have stratificationpatterns that vary seasonally, like those of lakes.

However, complications arise from tides andocean currents.

In some areas of the ocean, regular upwelling occurs, in which deep, nutrient-rich waters flowup towards the surface, stimulating the growth ofalgae. Such areas support some of the most

productive fisheries in the world.


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Plants and water

Most plants obtain water from the soil.

Soil water is held in pores by capillary action;otherwise it drains downward.Smaller pores hold water more tightly.Coarse, sandy soils have large pores, and water

tends to drain out.Fine, clay soils have small pores that might hold

water too tightly for plants to take up.

For a given soil, the field capacity is the amount ofwater the soil can hold against the force ofgravity.

The field capacity depends on the sizedistribution of soil pores.

The field capacity is an upper limit to the water

resource available to plants.

The lower limit of the available water resource is thepermanent wilting point.

This is the water content at which the bindingforce of capillary action just balances thesuction force that roots generate.

When water content drops to this point, plantscannot extract more and so they wilt.


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In arid environments or near the ocean coast, soilstend to be saline, which produces osmotic forcesthat act in addition to capillary forces.

Plants must generate enough suction force tobalance both capillary and osmotic forces.

This makes it more difficult for plants to extractwater in saline soils.


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As plant roots take up water, it becomes harder toextract.

When a root begins taking up water, the largerpores are emptied first because capillaryforces are weaker.

This creates a resource depletion zone aroundthe root.

Water is left in the narrower pores, which hold thewater more strongly.

Replenishment of water in the resource depletion

zone then slows down.

For aquatic plants, water per se is not scarce.However, regulation of ion content can be aproblem.

In freshwater or brackish habitats, plants are

hypertonic to their environment (higher saltcontent within).

Thus water tends to flow into the plant tissues,and must be expelled at a cost.

In marine habitats many plants are isotonic to theenvironment, and have no osmotic problems.

Some marine plants are hypotonic to theirenvironment (lower salt content within), andtend to lose water.

They must take it up from the environment liketerrestrial plants.


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Plants actually consume little of the water they take

up.

Most water passes through the plant only to keepits tissues moist.

In order to photosynthesize most plants mustopen pores in their leaves, called stomata, tolet in CO2.

While stomata are open, water can evaporate

out.Most of the water that plants take up is used to

balance this loss.This process is called evapotranspiration .

More available water means stomata can stay openand the plant can photosynthesize.

Therefore, production of organic matter by plantphotosynthesis tends to increase withprecipitation.


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The potential evapotranspiration rate measureshow much water plants “need” given the localclimate.

It is the rate water would be lost at a siteassuming soil water is not limiting (soil atfield capacity) and vegetation covered thesite.

Potential evapotranspiration rate depends ontemperature, solar radiation, air humidity,and wind.

The difference between potential evapotranspirationrate and precipitation is a “water deficit” that canprevent a site from developing full vegetationcover.

These climatic relationships govern the type ofvegetation that develops on a site, and are

summarized by Holdridge’s Life ZoneClassification.


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from Jones, 1997


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The productivity of organic matter by plants in thesedifferent vegetation types generally increaseswith temperature and precipitation.

Productivity is best measured as net primaryproduction, the rate of photosynthesis minusrespiration on the part of plants.

Vegetation Type Area(106 km2)

Net PrimaryProduction (g dry

mass m-2 yr -1)% of WorldProduction

Tropical Rainforest 17.0 2200 22.0%

Tropical Seasonal

Forest

7.5 1600 7.1%

Temp. GymnospermForest

5.0 1300 3.8%

Temp. AngiospermForest

7.0 1200 4.9%

Boreal Forest 12.0 800 5.6%Woodland & Shrub 8.5 700 3.5%

Savannah 15.0 900 7.9%Temp. Grassland 9.0 600 3.2%

Tundra 8.0 140 0.1%Desert / Semi-desert 18.0 90 0.9%Extreme Desert,Sand, Rock, Ice

24.0 3 0.04%

Cultivated Land 14.0 650 5.4%

ALL TERRESTRIAL 145.0 65.0%


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In many inland waters, salinity is less than 0.5 g / liter(fresh water). In some saline lakes, however,salinity may be 3-4X that of seawater.

Brackish waters are found in estuaries and othertransition zones between inland freshwaters andoceans. The salinity of brackish water oftenshows high variability.

Lagoons and tidal pools with high rates of evaporationof water may become very saline.

Aquatic organisms usually have specific adaptations

to live in waters of higher or lower salinity thanseawater, or in which salinity fluctuates widely.


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When an organism is hypotonic to its environment, ithas a lower salinity in its tissues than theenvironment. Water diffuses by osmosis from the

organism to the environment, concentrating saltswithin its tissues and disrupting physiologicalfunction.

Organisms that are hypotonic to their environmentmust have adaptations such as an ability toeliminate salts or consume water.

When an organism is hypertonic to its environment,

it has a higher salinity in its tissues than in theenvironment. Water diffuses by osmosis from theenvironment to the organism, increasing osmoticpressure within cells and risking rupture ofmembranes.

Organisms that are hypertonic to their environmentmust have adaptations such as an ability to pump

out excess water, or reinforcement of cellmembranes with cell walls.


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Stenohaline organisms are adapted to only a narrowrange of salinity. Examples include marineorganisms that live only in normal seawater, or

freshwater organisms that cannot tolerate salinity> 500 mg / liter.Euryhaline organisms are adapted to wider ranges of

salinity. Many inhabitants of brackish waters fallin this category, because they may regularlyexperience salinity < 500 mg / liter or > 34.5 g /liter.

Such adaptations involve two idealized patterns ofosmoregulatory response:

Perfect osmoregulators maintain a constant saltconcentration in internal tissues, regardless ofthe variation in environmental salinity.

Perfect osmoconformers allow their saltconcentration in internal tissues to vary and be

equal to the environmental salinity. They usephysiological adaptations to adjust theirbiochemistry to function at different salinities.

The following graphs show these idealized patterns,and some of the more realistic patterns thatorganisms actually achieve.


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from Jones, 1997


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Water balance in fish

A freshwater fish is hypertonic relative to its

environment – the solute concentration is higherin cells and tissues.Therefore a freshwater fish must excrete excess

water.The kidneys do this. They retain ions and remove

water to be expelled in urine.

Seawater fish are hypotonic relative to their

environment, with lower solute concentration incells and tissues, and thus tend to lose water byosmosis.

Seawater fish drink seawater to replace this loss.They have kidneys. These remove some of the

ions obtained in seawater, especially Ca+2,Mg+2 and SO4

-2.

Additionally, the gills remove excess Na+

, Cl-

andNH4

+.By drinking seawater, removing excess ions, and

excreting little urine seawater fish regulate bothwater content and solute concentration.


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Water balance in terrestrial animals

Terrestrial animals also tend to become dehydrated.

Exchange of O2 and CO2 takes place on moistsurfaces of large area (e.g. lungs in vertebrates).Inevitably, water is lost by evaporation from these

surfaces.This is replaced primarily by drinking, but also from

water released from digestion of food andmetabolism.

To regulate ion content while conserving water, urinewith high solute concentration must be produced.

Mammals such as humans use the nephrons in thekidneys to concentrate certain solutes in urine.

Tissue fluid enters the nephron from the blood atBowman’s capsule, and travels through the

loop of Henle.The loop passes through tissues that absorb

water and excrete certain solutes (e.g. K+)while retaining others (Na+).

At the end of this passage, the collecting tubulehas produced a urine with solutes about 4Xas concentrated as in blood.


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Mammals that live in deserts (e.g. kangaroo rats)have very long loops of Henle to produce veryconcentrated urine while excreting little water.

Mammals that spend much time in water (e.g.beavers) have short loops of Henle.


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Temperature and metabol ism

In common speech, we talk of “warm blooded” and

“cold blooded” organisms as those that can andcannot regulate their internal temperatures,respectively.

Biologists use more precise terminology tocharacterize the relationships between anorganisms internal temperature and theenvironmental temperature.

Homeotherms – organisms who maintain anapproximately constant internal temperaturewhile external temperature varies.

Poikilotherms – organisms whose internaltemperature varies with external temperature.

Problems with these terms:

For some animals, body temperatures vary butwith regulated levels: e.g. hibernatingmammals reduce body temperature duringhibernation.

Some animals that are poikilothermic exert somedegree of regulation of temperature.


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Endotherms – organisms who produce heatmetabolically within their own bodies to regulatetemperature.

Ectotherms – organisms that can’t do this.

By these definitions, only birds and mammals areendotherms.

Problems with these terms:Some insects and reptiles can obtain body heat

behaviorally (basking in the sun).

Some colonial insects produce heat within thecolony by vibrating (e.g. bees).


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Endotherms typically regulate their temperature at 35-40 °C.This is the maximum temperature of air or water

in many climates.Therefore, endotherms produce heat as neededwhen it is cooler.

However, fur and feathers act as insulators, so in hotweather endotherms must lose heat by panting orsweating.



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Regulation of temperature by endothermy is costly.Endotherms consume more metabolic energy

than ectotherms.

Within a favorable temperature range called thethermoneutral zone, endotherms consumeabout the same amount of energy at alltemperatures.

For temperatures outside the thermoneutralzone, metabolic energy consumption ismuch higher.

At low temperatures, heat production requires

metabolic energy. At high temperatures, cooling mechanisms also

require metabolic energy.

R

a t e o f E n e r g y C o n s

u m p t i o n

Temperature

Ectotherm

Endotherm

Thermoneutral Zone


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All organisms (endotherms and ectotherms) areaffected by various heat exchange processes.


Organisms can affect these processes by

fixed structural properties, e.g. surfacecharacteristics that affect reflection, radiationexchange, and evaporation.

physiological properties, e.g. methods of gasexchange, proximity of blood vessels to skin.

behavioral characteristics, e.g. choosing to baskin direct sunlight or go to the shade.


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Both endotherms and ectotherms achieve somedegree of temperature regulation by altering heatexchange.

But ectotherms generally regulate less effectively andso their temperature varies more in relation to theenvironment.

Ectothermshave fewer temperature regulation mechanisms

than endotherms.must rely to some extent on an external source of

heat.must consume energy to alter heat exchange, so

all regulation is a matter of cost and benefit.

Temperature thresholds set limits to the activity andeven survival of organisms.

High temperaturescan denature enzymes by disrupting protein

structure (this is uncommon in mostenvironments but can happen in hot springsand geothermal discharges).

can cause dehydration due to higher evaporation(this is much more common).


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Often, at a threshold temperature, water lossincreases dramatically.


Specialized structures such as seeds can sometimeswithstand dehydration, but many tissues andstructures cannot.


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At extreme low temperatures, water freezes withincells.Ice crystals expand, which can rupture cells.

As ice forms, remaining cell water has high soluteconcentration, which may cause problems ofosmotic regulation.

Organisms adapt to low temperatures bybehavioral changes, such as moving to warmer,

insulated locations (e.g. under the snow atthe ground surface).

producing solutes that prevent formation of icecrystals (“antifreeze”).

producing specialized structures that aredehydrated so ice crystals cannot form (e.g.seeds).


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Responses to temperature

Rates of biological processes have a characteristic

response to temperature.

Rates of enzyme-catalyzed biochemical reactionsfollow the same kinetic laws as other chemicalreactions.

Arrhenius equation:

( ) a E

RT

ak T Ae

−

=

where k(Ta) = reaction rate at absolutetemperature Ta

A = constantE = activation energy

R = ideal gas constant


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Biologists rarely work with the Arrhenius equationdirectly.

Instead they work with the implication that the ratio ofthe rate at one temperature Ta2 to the rate atanother temperature Ta1 depends only on thetemperature difference.

( )2 12

1

( )

( )a a

T T a

a

k T

k T θ

−

=

where θ = constant derived from the Arrheniusequation.

This further implies that if you increase thetemperature by a specified difference, the rateincreases by a specified factor.


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Biologists measure this increase as Q10:Q10 = the factor by which rate increase when

temperature increases by 10 °C.

For many biochemical reactions, Q10 is about 2 (ratedoubles for every 10 °C temperature increase).

For aggregated processes like metabolism,respiration, development rate, or reproductiverate Q10 is again roughly 2 (range 1-3).



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For organisms whose development rate is a strongfunction of temperature (e.g. many ectotherms),the period of time required for maturity varies with

temperature.This leads to the concept of physiological time,measured in degree-days (i.e. a day at 20 °Ccounts twice as much as one at 10 °C).

Biological rates do not increase indefinitely withtemperature. At sufficiently high temperatures, enzymes are

denatured.If an organism becomes dehydrated,

physiological rates are often impaired.Metabolic and genetic regulation is impaired at

high temperature.Therefore, most biological rates show a unimodal

relationship to temperature.

The following graph shows the rate of populationgrowth and abundance achieved by populationsof the toxic alga Prymnesium parvum.


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from Baker et al., 2001


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In some cases, the distribution of a species is directlylimited by temperature, i.e. the maximum orminimum likely to occur at a given location is

lethal.In other cases, there are indirect temperature effectson a species’ performance that make it a poorcompetitor or otherwise unable to persist.

In other cases, there are indirect temperature effectsbecause variation in some other factor is relatedto temperature.

For example, at higher temperatures oxygen isless soluble in water.

Many fish species cannot inhabit warm waterbecause it will not have enough oxygen forthem.

Only fish species tolerant of low oxygen typicallyinhabit warm waters.

In many cases, the proximate limit of a species’distribution is uncertain, but there is a correlationbetween its range limit and an isotherm.

Isotherm – line on a map joining places that have thesame temperature, usually the same annualmean temperature.


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There are also generalized changes in organismalcharacteristics in relation to temperature.

Al len’s rule – endothermic animals (birds andmammals) from cold climates tend to haveshort extremities (limbs, ears, etc.).

Short extremities reduce surface to volume ratioand reduces heat loss relative to production.

Bergmann’s rule – endothermic animals fromcold climates tend to be larger.

Geometry dictates that larger objects have lowersurface to volume ratios.

Both rules have many exceptions.


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Solar radiation and plants

For plants, light is an energy resource.

Photons are absorbed by pigments such aschlorophyll a. Thus light is “consumed” byshading individuals or leaves below.

The energy of the photons is converted to the energyof reduced organic compounds such as glucoseby the process of photosynthesis.

hν + 6CO2 + 6H2O C6H12O6 + 6O2

Plants, algae, and some bacteria carry outphotosynthesis.

Light is not a “simple” resource. Instead it is aresource continuum, distributed continuouslyover different wavelengths of the electromagnetic

spectrum.Visible light is typically defined as that with

wavelengths between 400 and 630 nm.


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Photosynthetic organisms use the green pigmentchlorophyll to capture light energy by absorbingphotons.

Wavelengths between 400-500 nm (violet) andbetween 600-700 nm (red) are the mosteffectively used wavelengths for photosynthesis.

Most photosynthetic organisms have a number ofaccessory pigments in addition to chlorophyllwhich help to absorb more of the light between400 and 700 nm.

Therefore, most photosynthetic organisms use at

least some light with wavelengths 400-700 nm,and this waveband is called Photosynthetically

Active Radiation (PAR).The spectrum of solar radiation striking the earth has

abundant visible light and PAR.



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In aquatic habitats, water absorbs light, limitingphotosynthesis to relatively shallow waters.

Water absorbs different wavelengths unequally.

Violet and red wavelengths that are mosteffectively used in photosynthesis arestrongly absorbed.

Green and blue wavelengths transmit moreeffectively through water.

Algae living at different depths have differentaccessory pigments to try to absorb as much

radiant energy as possible, given what theyreceive.


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from Jones, 1997


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In terrestrial ecosystems there are importantinterrelationships between the biochemistry ofphotosynthesis, the typical irradiance and

temperature of a habitat, and water availability ina habitat.Leaves have a surface that is relatively

impermeable to gases.In order to carry out photosynthesis, plants need

CO2, and must open “pores” in their leavescalled stomata.

While CO2 diffuses in, water vapor diffuses out, so

that plants inevitably lose water whilephotosynthesizing. This water loss must bemade up for by absorption in the roots.

Habitats that have bright light tend often to berelatively warm and dry. Thus plants tend to losewater at the times and places when the most

energy for photosynthesis is available.Many of the plants that live in hot dry habitats have

biochemical adaptations to avoid drying out.


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“Normal” plants have “normal” photosynthesis inwhich CO2 is first incorporated into a 3-carbonmolecule called phosphoglycerate. Sugars and

carbohydrates are formed by subsequentreactions. This kind of photosynthesis is called C-3 photosynthesis.

Plants with C-3 photosynthesis do not perform well intropical habitats because the chemical reactionsinvolved slow down when the temperature isabove about 28 °C.

Also, the C-3 process does not operate well when

CO2 concentration is low, so such plants mustkeep their stomata fully open during the day, andrisk losing water.

For these reasons, C-3 plants are foundpredominantly in moist, non-tropical habitats.


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An alternative method of photosynthesis is the C-4 process, in which CO2 is first incorporated intophosphoenolpyruvate, and then transferred to

form oxaloacetate, a 4-carbon molecule. Sugarsand carbohydrates are formed by subsequentreactions.

The formation of phosphoenolpyruvate can take placeat very low CO2 concentrations. This allowsplants to shut their stomata for periods of time,reducing water loss.

C-4 plants are usually very productive under dry

sunny conditions, and tend to be found in suchhabitats.

Many crops that produce well in hot climates are C-4plants (e.g. maize, millet, sugar cane), so aremany weeds.



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Some plants use CAM photosynthesis, in which CO2 is taken up only at night, with the stomata open,and incorporated into organic acids and stored.

During the day, light energy is used to convert theorganic acids to sugars and carbohydrates.Stomata are kept closed.

With stomata only open at night when it is cooler,CAM plants reduce their water loss.

Such plants are common in arid regions, e.g. manyspecies of cactus do this.

Some CAM plants also use C-3 photosynthesis when

conditions permit this.

Plants have evolved suites of adaptations to sunny orshady conditions.

Sun plants:more likely to use C-4 or CAM photosynthesis.

have high maximum rates of photosynthesis.hold leaves at an acute angle to the sun,

spreading incident radiation over a largerarea.

arrange leaves in a multi-layer canopy.Shade plants:

more likely to have C-3 photosynthesis.have low maximum rates of photosynthesis.use low levels of irradiance more efficiently.


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Also, individual plants often produce both sun andshade leaves for different locations within the leafcanopy.

Sun leaves:smaller, thicker, denser, more cells, more

chloroplasts, more veins.Shade leaves:

larger, translucent, less dense, lower rate ofphotosynthesis.

The energetic efficiency of photosynthesis can becalculated theoretically.

To make one molecule of glucose, 24 electronsmust be energized twice, so 48 photonsmust be absorbed.

The energy of these photons varies inversely with

wavelength, but is a minimum of 172 kJ permole of photons.

Therefore, at least 8256 kJ of radiant energymust be absorbed per mole of glucoseformed.

Free energy of formation of glucose is 2872 kJper mole.

Efficiency = 2872 / 8256 = 35%


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In reality, this theoretical efficiency of photosynthesisis never achieved.

Of the radiant energy arriving at a 1 m

2

surfaceoccupied by vegetation, less than about 3%is converted to the chemical energy oforganic matter.

Much of the energy is not absorbed byphotosynthetic pigments.

Either it passes through the vegetation or it isabsorbed by other cellular components and

structural materials (e.g. stems).

Rate of photosynthesis by a plant depends on waterand CO2 availability, in addition to light energy.

Plants reduce the rate of photosynthesis iftheir energy needs are low.opening the stomata risks too much water loss.

CO2 is at low concentration (a problem foraquatic plants and algae in water, at times).


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Sources and cycles of nutrients

A long list of elements is known to be essential to

living matter.

Bulk elements, present in relatively highproportions: C, H, O, N, P, S.

Elements occurring as ions, present in varyingproportions: Na, K, Mg, Ca, Cl.

Trace elements, typically present in very lowproportions: Fe, Cu, Zn, Mo, Co, I, Mo, V, Ni,

Cr, F, Se, Si, Sn, B, As.

Not all types of organisms require all of the traceelements, and some of these elements are toxicin high concentrations.

At regional and global scales, nutrient elements flow

in biogeochemical cycles.Nutrient elements are absorbed by plants, algae, or

microorganisms. These organisms are consumedby other organisms.

Death, decomposition, and excretion of wastes returnnutrient elements to the environment.

Nutrient elements also cycle geologically, from rocksand soil, to water, maybe to air and back again.

Heavy metal contaminants often have cyclesresembling those of chemically similar nutrients.


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Most of the earth’s surface is covered with oceanwater, and collectively the oceans hold hugeamounts of biologically important elements.

Inputs of materials from land occur along the coasts,and exchanges with air take place over the entireocean surface.

Output occurs when materials sink to the sedimentsand are buried. Once buried, materials mayreturn to land only over geological time, whenmarine sediments are uplifted during mountain-building.

The large mass of elements stored in the oceansimplies that they respond only very slowly tochanges in inputs and outputs.

The residence time (average time that an atomspends in a compartment) for most elements inthe oceans is typically 100’s to 1000’s of years.

Fresh waters have much lower volume than theoceans, and much greater surface area inproportion to volume.

Because inputs and outputs occur along the surfaces,they respond much more rapidly to changes inthese processes.

Residence times of elements in freshwater aquaticecosystems are typically < 100 years, and maybe shorter (a few days or less).


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Some of the processes and transformations involvedin biogeochemical cycling are illustrated in thisfigure:



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Cycles of particular elements and substances

The water cycle is linked to the cycles of many othersubstances, due to the importance transport inthe dissolved phase, especially for ionic chemicalforms.

from Ricklefs, 1979


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The Carbon Cycle

In a sense, there are two carbon cycles.

1. “Slow” geological cycle, involving acid-basereactions and precipitation-dissolution reactions.

Atmospheric CO2 dissolves into water.CO2 in water hydrates to H2CO3, carbonic acid.Carbonic acid can lose two protons successively

to form bicarbonate and carbonate ions.

Carbonate ions can precipitate with calcium ionsto form calcium carbonate, CaCO3.

CaCO3 sinks to sediments and is compressedinto rock.

Uplift raises carbonate rocks, and thenweathering, erosion, and runoff transportions to water.

Water Surface

CO2

CO2 H2CO3 HCO3- CO3-2

CaCO3

Sedimentation

Rock Formation

Uplift

CaCO3

Rocks & Soils

Weathering & Runoff

Ca+2, HCO3-, CO3-2


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2. “Fast” biological cycle, involving oxidation-reductionreactions.

Photosynthesis by plants, algae, andphotosynthetic bacteria reduces CO2 tocarbohydrates and other organic matter.

hν + 6CO2 + 6H2O C6H12O6 + 6O2

This organic matter is oxidized back to CO2 whenorganisms use the energy of organic matter.

Many organisms use aerobic respiration:

C6H12O6 + 6O2 6CO2 + 6H2O

In the absence of oxygen, various series of otherreactions are used to oxidize organic matter.

Plants respire some of the organic matter theyproduce by photosynthesis.

Herbivores and carnivores also respire some of theorganic matter they consume in food.

Excreted wastes and dead bodies are decomposed,which also oxidizes organic matter.


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Plants & Algae

Herbivores

Carnivores

Decomposers

(bacteria &fungi)

CO2

Photosynthesis

Respiration

Decomposition

Over daily time scales, photosynthetic organisms

remove CO2 from air and water and reduce it toorganic matter.

About equally rapidly, much organic matter is oxidizedby respiration to meet the energetic needs oforganisms.

Over longer time scales (years to decades), C is

stored in the tissues of long-lived organisms(especially trees) and in detritus (dead material)in soils and aquatic sediments. Eventually mostof this C is oxidized to CO2 by decomposition.


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A small fraction of detritus does not completelydecompose and is eventually buried insedimentary rocks, and forms oil and coal.

Thus the biological carbon cycle intersects thegeological carbon cycle.Under pre-industrial circumstances, very small

amounts of petroleum were brought to thesurface (in oil seeps, etc.) and oxidized to releaseCO2.

Modern industry releases C from petroleum at

unprecedented rates. CO2 is increasing steadilyin the atmosphere, and is expected to increasethe average temperature of the atmosphere by

0.5-2°. Atmospheric CO2 traps outgoing infrared radiation,

affecting the net radiation balance of the planet(“greenhouse effect”).


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Another intersection between the biological andgeological carbon cycles involves calcifyingorganisms.

Precipitation of carbonate minerals is often initiated bycalcifying organisms (plants, animals andmicroorganisms) that form hardened walls orshells.

Deposition of such biogenic carbonate in productiveaquatic habitats leads to formation of limestoneand related rocks.

The C content of such rocks is a large reservoir of C,

in addition to that in the atmosphere, dissolved inwater, bound in organisms, and in petroleum.

from Ricklefs, 1979


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The Nitrogen Cycle

This cycle is notable for the large number of chemical

transformations that take place, the roles ofspecialized bacteria that perform thesetransformations, and the great importance of theatmospheric reservoir of N2.

N2 composes about 80% of the atmosphere, and theatmosphere holds most of the earth’s nitrogen.

Most of this N is unavailable to almost all types ofliving organisms. Most plants andmicroorganisms must obtain N in the form nitrate(NO3

-) or ammonium (NH4+) dissolved in water.

Many animals and microorganisms obtain Nprimarily in the form of amino acids (R-NH2) intheir food.

In living things, N occurs mainly as proteins(composed of amino acids) and as nucleic acids,such as DNA, which make up the genes.


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Blue-green algae (cyanobacteria) and some othertypes of bacteria can reduce atmospheric N2 toNH3. The NH3 is incorporated into amino acids.

This reaction is called nitrogen fixation.Cyanobacteria are the most important N-fixingorganisms in aquatic habitats. When consumerseat them, N is cycled into the biosphere.

On land, bacteria are the most important N-fixingorganisms. They often live symbiotically withplants, in root nodules or other structures. Theyreceive carbohydrates from plants and supply

fixed N in return. Plants with this relationship,such as legumes, are often found in soils poor inN, where this relationship is a competitiveadvantage.

When animals consume organic matter with excessamino acids, or when bacteria decompose such

matter, NH3 (or a waste product at the same levelof oxidation) is often released (ammonification). An example reaction is

C2H5NO2 + 1½ O2 → 2CO2 + H2O +NH3

Specialized bacteria oxidize ammonia, eventually tonitrate, in the nitrification reactions:

NH3 + 1½ O2 → HNO2 + H2O

KNO2 + ½ O2 → KNO3


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Nitrate (a dissolved form) is converted to gaseousforms in the denitrification reactions, whichcertain bacteria carry out under anaerobic

conditions. This is an important means ofdecomposition (oxidation) of organic matterwithout oxygen, as nitrate is used as an electronacceptor.

C6H12O6 + 6KNO3 → 6CO2 + 3H2O + 6KOH + 3N2O

5C6H12O6+24KNO3 → 30CO2 + 18H2O + 24KOH +12N2

Nitrogen moves among the various compartments ofthe environment, with these chemicaltransformations occurring at critical steps.


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Graphical summary of the nitrogen cycle

from Schlesinger, 1997


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The Phosphorus Cycle

Phosphorus in the environment occurs almost

exclusively as the non-volatile phosphate ion(PO4-3).

Oxidation/reduction reactions are not directlyimportant, nor is the atmosphere.

Living things incorporate PO4 groups in a wide varietyof biochemicals: nucleic acids, phospholipids(which compose cell membranes), bones, the

energy carrier molecule ATP, and many others.

Phosphate weathers out of rocks and soils. It is takenup by plant roots, and by aquatic algae andmicroorganisms.

When these organisms are consumed, phosphate isoften released in excreted wastes, and

phosphate is released by decomposition. It isthen available for uptake again.

Such cycles may take place rapidly and repeatedly.Small fractions of phosphate are buried insediments, form rock, and only weather out againafter millions of years.


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The phosphorus atoms do not generally participate inredox reactions in the environment. However,redox reactions of other constituents in soils and

sediments do strongly affect the availability ofphosphate to plant roots and microorganisms.For example, reducing conditions prevail below the

sediment-water interface in aquatic systems.Under these conditions, iron and manganese aresoluble, and dissolved phosphate is often presentat high concentrations.

When the sediment-water interface is oxidized, iron

and manganese form solid oxyhydroxides thatsorb phosphate, and keep its dissolvedconcentration low.

If the sediment-water interface is reduced due toanaerobic conditions, iron, manganese, andphosphate are released to the overlying water.

These substances then become available foruptake by aquatic microorganisms.


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Graphical summary of the phosphorus cycle:



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Eutrophication

The N and P cycles are greatly altered by human

activities.

Fertilizer production is now massive compared tonatural N fixation and weathering of P. Largeamounts of fertilizer spread on land eventuallywash into lakes, rivers, and coastal oceans.

Burning of fossil fuels releases nitrogen oxides, whichenter rainwater, form nitrate, and are deposited

on the earth’s surface.Intensive animal husbandry produces ammonia

vapors that also dissolve in rainwater and aredeposited.

Availability of N and/or P often limits the productivityof autotrophs in ecosystems.

Excessive fertilization resulting from human activityhas many adverse consequences.

In terrestrial ecosystems, it causes plant speciesreplacements and changes in vegetationcomposition.


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The effects in aquatic ecosystems are better known,however. Excessive nutrients stimulate theproliferation of undesirable algae.

In freshwaters, cyanobacteria are often stimulated.These algae can form unsightly scum, and arepoor food for aquatic herbivores, compared toother types of algae.

In coastal marine systems, blooms of toxic algae (“redtides” and “brown tides”) can occur. They poisonmarine animals, and can be deadly to humans

who consume tainted seafood.

The Sulfur Cycle

Sulfur is a relatively common element on the earth’ssurface. Sulfate (SO4

-2) is a major ionicconstituent of ocean water and of most inland

waters.

Sulfur in living things is mostly in a reduced chemicalstate. The organic sulfhydryl group (-SH) is animportant constituent of many proteins.

Plants and many microorganisms take up SO4-2 and

reduce it as it is incorporated into organic matter (reductive sulfate assimilation).

Animals typically consume sulfur in their foods.Many decomposers release excess sulfur as SO4

-2, ina dissimilatory oxidation process.


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When organic matter is decomposed anaerobically,some bacteria can use SO4

-2 as an electronacceptor. Such sulfate-reducing bacteria often

release hydrogen sulfide (H2S) as an end-product.Chemosynthetic bacteria can do the reverse. H2S

serves as an electron donor, to reduce CO2 in theproduction of organic matter, releasing SO4

-2.Some chemosynthetic bacteria can also oxidize

sulfide or elemental sulfur in minerals, releasingSO4

-2. This release may acidify streams draining

regions rich in sulfide minerals. Such aciddrainage may also occur where sulfide rich“tailings” (wastes) from mining have beendumped.


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Graphical summary of the sulfur cycle:



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Plants and consumers

Plants, algae, and some bacteria are autotrophic.

They produce organic matter from CO2 byreducing it. This requires an input of energy, anda source of electrons.

Photosynthesis is the most important autotrophicprocess. Sunlight is used a source of energy, andwater is used as a source of electrons. Oxygen isan end product. Photosynthesis often producescarbohydrates such as glucose:

hν + 6CO2 + 6H2O C6H12O6 + 6O2

In nearly all such organisms, light energy is trappedby the pigment chlorophyll a and other accessorypigments, used to remove electrons from water,energize them, and then reduce CO2.

Chemosynthesis is carried out by some specializedtypes of bacteria, by using a highly reducedchemical compound as a source of energy andelectrons. In environments where sulfide isavailable, a chemosynthetic reaction is

6CO2 + 6H2O + 6H2S + 6O2 C6H12O6 + 6H2SO4


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Autotrophs are also called primary producers.The total amount of energy produced by autotrophs is

called gross primary production (GPP).

Some of this energy is used by the autotrophsthemselves for their own maintenance, and thebalance is called net primary production (NPP).

All heterotrophs in an ecosystem ultimately rely onNPP. They consume it and oxidize some of it toobtain energy, while “repackaging” some of it asthe organic compounds of their own cells and

bodies.The amount of repackaged organic matter produced

by heterotrophs is called secondary production.

Some heterotrophs are herbivores, or primaryconsumers, obtaining organic matter by directlyingesting autotrophs.

Heterotrophs that consume herbivores are calledsecondary consumers, those that consumesecondary consumers are called tertiaryconsumers, and so on.

All organisms produce non-living matter calleddetritus, in the form of dead bodies, tissues thatare shed while alive, and feces and liquid wastes. Animals that consume detritus are calleddetritivores. Microorganisms that consumedetritus are sometimes called saprotrophs .Collectively, organisms that rely on detritus areoften called decomposers.


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All organisms also produce waste heat, which isessentially an energy loss from the ecosystem.

This picture summarizes these energy flows (eachbox is called a trophic level):

The flow of energy here is paralleled by the flow ofcarbon.

Primary producers take CO2 and convert it to energy-rich organic compounds.

These compounds pass to consumers and todecomposers.

Production of heat during metabolism is accompaniedby release of CO2, completing the biologicalcarbon cycle.

Energy input (E)

Primary Producers

Energy transfer

Primary Consumers

SecondaryConsumers

TertiaryConsumers

Heat

Heat

Heat

Heat

D e c om p o s er s

Heat

Detritus

Detritus

Detritus

Detritus


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Flows of other elements (e.g. N, P, S) also runparallel to those of energy and carbon.

In most ecosystems, much of the energy andmaterials reside in detritus and in decomposersconsuming the detritus.

The release of elements from detritus bydecomposers is essential to the biological cyclingof nutrients, and makes them available forautotrophs to use.

In aquatic habitats, some of the detritus is in theform of dissolved organic matter, and someof the decomposers are microorganismssuspended in the water.

But more detritus and more decomposers aretypically found in the sediments.

In terrestrial habitats detritus and decomposersaccumulate on the ground and contribute tosoil formation.


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Soil formation, properties, and classification

The physical properties of soil or sediment particles

are important conditions for many organisms thatattach or burrow.Chemical properties may also be important, and

influence such things as adhesion properties, andavailability of nutrients.

Some organisms are adapted to living on or amonglarge rocks, and cannot tolerate the abrasion

caused by the shifting grains of sediments.

Particle size is an important characteristic of soils andsediments. The basic classification of soilsbegins with particle size:


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from Jones, 1997


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Soil particle size covaries with soil organic mattercontent. Coarse soils and sediments tend to havelow organic content

Large particle size and low organic content leadsto soils and sediments that are unstable andmove around. The resulting abrasion mayprevent some organisms from surviving.

Soils with small particle size and high organiccontent bind together and are more stable.

Soils with large particle size and low organiccontent dry out quickly and often have lowwater content.

Soils with small particle size and high organiccontent retain water by capillary action andtend to be moister.

However, movement of water in soils of small particlesize is restricted, and aeration may be poor.

Many animals that live in mud or silt (with smallparticle sizes) must dig tunnels or burrows tocirculate water to obtain oxygen.


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Soils consist of weathered bedrock minerals, andorganic matter that derives largely fromvegetation. The weathered minerals in soils are

often various types of clay, but compositionvaries widely and depends on bedrock materials.Weathering also depends on rainfall, temperature,

and vegetation type (i.e. whether a lot of organicacids are produced by plants and microbes).

In many types of soils, more or less distinct layerscalled horizons occur.

A horizon – has much organic matter, includingundecomposed plant litter and partiallydecomposed humus; also contains mineralparticles; contains shallow plant roots; highlysoluble substances are somewhat depleted byleaching.

B horizon – contains more mineral matter, includingclay minerals formed reactions with bedrockminerals; substances from above often depositedhere; contains some humus and plant roots.

C horizon – contains primarily weathered bedrockminerals, formed by oxidation or by deposition ofevaporites in dry climates; has fewer plant roots.

R horizon – parent material, or bedrock.


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References:

Baker, J.W., J.P. Grover, B.W. Brooks, F. Ureña-

Boeck, D.L. Roelke, R. Errera, and R.L. Kiesling.2007. Growth and toxicity of Prymnesium parvum (Haptophyta) as a function of salinity, light andtemperature. Journal of Phycology 43: 219-227.

Jones, A.M. 1997. Environmental Biology. RoutledgePress, London & New York.

Kirk, J.T.O. 1983. Light and Photosynthesis in AquaticEcosystems. Cambridge University Press,

Cambridge.Lehninger, A.L. 1982. Principles of Biochemistry.

Worth Publishers Inc.MacKenzie, A., A.S. Ball, & S.R. Virdee. 2001. Instant

Notes in Ecology, 2nd ed. BIOS ScientificPublishers, Oxford.

Press, F., & R. Siever. 1978. Earth, 2nd ed. W.H.

Freeman & Co., San Francisco.Ricklefs, R.E. 1979. Ecology, 2nd ed. Chiron Press,

N.Y.

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