Photosynthesis and respiration

• Collectively, photosynthesis and respiration control bioelemental cycling.

Photosynthesis: 6CO2 + 12H2O C6H12O6 + O2 + 6H2O + heat

Sunlight

Respiration: C6H12O6 + 6O2 6CO2 + 6H2O + heat

Photosynthesis: 6CO2 + 12H2O C6H12O6 + O2 + 6H2O + heat

Sunlight

• Photosynthesis is initiated by the absorption of light energy by pigments or photochemicallyreactive proteins. The absorbed light energy is passed from the light harvesting pigments (antenna) to the photosynthetic reaction centers via the flow of electrons.

Phytoplankton taxonomy

• Phytoplankton include single cells or organized colonies; the vast majority are very small (<10 µm):– Picoplankton: 0.2-2.0 µm– Nanoplankton: 2.0-20 µm– Microplankton: 20-200 µm

The picoplanktonic photosynthetic cyanobacteria comprise a substantial component of plankton biomass and production in low nutrient systems.

Identifying phytoplankton

• Cell biomarkers (pigments)• Microscopy/Flow cytometry• Nucleic acids (rDNA)• Antibody probes

Major divisions and classes of photosynthetic plankton in the ocean

• Prokaryotes– Cyanophyta (includes

Prochlorophytes)

• Eukaryotes:– Chlorophyta (green algae); include

the following classes:• Chlorophyceae• Prasinophceae• Euglenophyceae

– Chromophyta (brown algae); include the following classes:

• Chrysophyceae• Pelagophyceae• Prymnesiophyceae• Bacillariophyceae (diatoms)• Dinophyceae (dinoflagellates)• Cryptophyceae (crytophytes)• Phaeophyceae (phaeophytes)

– Rhodophyta (red algae)-mostly macrophytes

1 µmMicromonas

Pelagomonas

Marine cyanobacteria

• Cyanobacteria: major groups of cyanobacteria in the oceans include: Prochlorococcus, Synechococcus, Trichodesmium, Crocosphaera, Richelia

– Characteristics: unicellular, filamentous, colonial

– 2 layer cell wall-inner murein and outer lipoprotein

– Some species fix N2

Synechococcus

Trichodesmium

Prochlorococcus

Richelia

Many images from: http://www.sb-roscoff.fr/Phyto/gallery/main.php?g2_itemId=19

Rhodophyta (Red Algae)• Rhodophytes

– In addition to Chl a, also contain phycoerythrin, phycocyanin, allophycocyanin

– Most marine representatives are macrophytes

Porphyridium

Rhodella

Chlorophyta (green algae)• Chlorophytes

– Contain Chl b– Uncommon in

open ocean; mostly freshwater.

– In marine environment, mostly tropical (Hawaiian coastal waters)

– Can be single cells or colonies, coccoid or flagellated

Nannochloris

Prasinophyceae

• Prasinophytes– Contain Chl b– Predominately

unicellular– Relatively

common, but not abundant in ocean

– Can be single cells or colonies, coccoid, biflagellated, or quadri-flagellated

Chromophyta (brown algae)• Pelaophytes

– Contain Chl c– Very common in

open ocean.– Coccoid or

monoflagellated

• Chrysophytes– Contain Chl c– Relatively rare in

open ocean– Mostly bi-

flagellated (flagella of unequal length)

• Cryptophytes– Contain Chl c– Contain

carotenoidalloxanthin

– Contain phycoerythrin or phycocyanin

– Flagellated unicells

Pelagomonas

Images from: http://planktonnet.sb-roscoff.fr/index.php#search

Dictyocha Rhodomonas salina

SkeletonemacostatumDate: 10/10/06 Owner: Gallery Administrator

3 votes4.333 N/A

Chromophyta (brown algae)-Cont.

• Prymnesiophytes– Mainly biflagellates– Very common in

open ocean– 2-5 µm– Some species form

CaCO3 plates (coccoliths)

• Bacillariophytes– Ubiquitous– All contain Chl c and

carotenoidfucoxanthin

– Rigid silica-impregnated cell wall

– Many form colonies– 2 prominent cell

morphologies: centric and pennate

Emiliana huxleyi

Coscinodiscus Rhizosolenia

Chromophyta (brown algae)-Cont.

• Dinophytes– Possess the

carotenoidperidinin

– Widely distributed (estuaries, open ocean)

– Mostly unicellular and autotrophic, but colorless heterotrophs can also be abundant

– 2 flagella– Many are

bioluminescent and some case toxic red tides blooms

Ceratium

Armored dinoflagellate-cellulose theca

Naked dinoflagellateGymnodinium

Active site of photosynthesis in eukaryotes is the chloroplast. Chloroplasts contain light harvesting pigments, electron carriers that capture energy for reducing power (NADPH2) and ATP, and enzymes that use NADPH2 and ATP to fix CO2and H2O to carbohydrate.

Components of the chloroplast:•Membranes•Thylakoids (lipids, chlorophyll, and protein)-contain the pigments and electron carriers•Stroma: gel-like matrix containing enzymes needed to fix CO2 (RUBISCO).

• Different algal groups distinguished by arrangement of thylakoids

– Red algae: single thylakoids– Cryptophyta: thick pairs of thylakoids– Other groups tend to have 3 thylakoids

stacked together– The green algae (and higher vascular plants)

have 5-20 small diameter thylakoids stacked together

Cyanobacteria have thylakoids free in the cytoplasm with phycobilisomes along the thylakoid surface.

Light harvesting photosynthetic pigments

• Chlorophylls• Carotenoids• Biliproteins

• Recently discovered photoreceptor proteins (Proteorhodopsin and Bacteriorhodopsin) serve as proton pumps, but do not appear to harvest energy for oxygenic photosynthesis.

Chlorophylls

• Cyclic tetrapyrolewith a magnesium atom chelated in the center of the ring

Phytol

Chlorophyll c lacks the phytolgroup

• All oxygen evolving photosynthetic plankton contain Chlorophyll a (peak absorption 450 and 660 nm)

• Chlorophyll b (peak absorption 500 and 640 nm) found mostly found in Chlorphytes, Euglenophyta, and cyanobacteria

• Chlorophyll c found in diatoms, dinoflagellates, chrysomonads, cryptophytes

• Chl a, b, and c all absorb strongly in the red (between 440-450 nm) and blue wavelengths (~645-660 nm). The presence of Chl b and cincreases the absorption between 450-650 nm.

Carotenoids• Isoprenoid compounds

(lipids)-not water soluble, embedded in hydrophobic membranes.

• Almost all chlorophyll and carotenoids occur as complexed proteins.

• β-carotene is found in all algal classes except the Cryptophytes

The double bonds absorb strongly in the short wavelength region of the visible spectrum

Carotenoids

• Photosynthetic carotenoids participate in photosynthetic electron transport .

• Non-photosynthetic carotenoids appear to protect photosynthetic reaction centers from oxidation via free radical scavenging

Phycobiliproteins

• Water soluble pigments that capture light energy and pass the energy to chlorophyll.

• Found in Rhodophytes (red algae), Cryptophytes, and Cyanobacteria– Red: phycoerythrin (red algae, often in

cyanobacteria); phycoerythrocyanin– Blue: phycocyanins, allophycocyanins

• Occur as aggregates, termed phycobilisomes, composed of 3 or more phycobiliproteins, that attach to the thylakoid.

Summary of photosynthetic pigments; pigments in bold are diagnostic

Monovinyl chlorophyll a and b, luteinChlorophytes

Monovinyl chlorophyll a and b, prasinoxanthinPrasinophytes

Monovinyl chlorophyll a, chlorophyll c2, peridininDinoflagellates

Monovinyl chlorophyll a, chlorophyll c2, alloxanthinCryptophytes

Monovinyl chlorophyll a, chlorophylls c1 and c2, fucoxanthin and violaxanthin

Chrysophytes

Monovinyl chlorophyll a, chlorophylls c2 and c3, 19-butanoyloxyfucoxanthin

Pelagophytes

Monovinyl chlorophyll a, chlorophylls c2 and c3, 19-hexanoyloxyfucoxanthin

Prymnesiophytes

Monvinyl chlorophyll a, chlorophylls c1 and c2, fucoxanthin

Diatoms

Eukaryotes

Monovinyl chlorophyll a, zeaxanthin, phycoerythrinSynechococcus

Dinvinyl Chlorophyll a and b, monovinylchlorophyll b, zeaxanthin

Prochlorococcus

Prokaryotes

Major pigmentsGroup

Carotenoids and biliproteins extend the spectral region able to support plankton growth; thus light forms an important environmental control on microorganism evolution.

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Through variations in pigment composition, phototrophic microorganisms differentiate their light harvesting capabilities, providing stable coexistence of multiple competing species

• Reaction center: the site in a photosystem where energy from absorbed light is transferred (via an electron) from one molecular (donor) to another (acceptor).

• Reaction center chlorophyll becomes excited with the receipt of light energy.

• To reduce this energy, the excited chlorophyll can transfer an electron to an acceptor.

• The majority of energy captured by reaction center chlorophyll is transferred via antenna pigments.

• Summary of light reactions:2H2O + 2 NADP+ + 3 ADP +3 Pi O2 + 2 NADPH2 + 3 ATP

• Summary of dark reactions:CO2 fixation via the Calvin-Benson cycle

CO2 + 2 NADPH2 + 3 ATP H2O + (CH2O) + 3 ADP + 3Pi + 2 NADP+

8 hν

CO2 + 2 H2O (CH2O) + H2O + O28 hν

Key enzymes for photosynthesis

• RUBISCO (1,5-bisphosphate carboxylase/oxygenase)-key enzyme in the Calvin-Benson cycle, incorporates CO2 into 3-phosphoglycerate. Most abundant protein on Earth.

• Carbonic anhydrase: converts CO2 to bicarbonate (HCO3

-) and vice versa. HCO3- is

the most abundant form of inorganic carbon in seawater; most marine microorganisms take up HCO3

- and carbonic anhydrase dehydrates to CO2 near RUBISCO.

Net and Gross Photosynthesis• Net photosynthesis (PN): rate of photosynthesis

excluding material lost to respiration in the light (RL).

• Gross photosynthesis (PG): The total amount of light energy converted into biochemical energy. Measured by evaluating total CO2 fixed or O2 evolved, including that lost to respiration. PG = PN + RL

• Note that respiration also occurs in the dark (RD).

• Relatively straightforward to measure net or gross photosynthesis in pure cultures, but the oceans contain many organisms that contribute to respiration (e.g., bacteria, zooplankton).

Primary Production• Gross Primary Production: The rate of

organic carbon production via the reduction of CO2 due to photosynthesis.

• Net Primary Production: Gross primary production, less photoautotrophic respiration, integrated over time and depth.

• Net Community Production: Gross primary production, less all autotrophic and heterotrophic losses due to respiration.

• Units for all measures of production: mgC m-2 d-1

What methods are used to measure aquatic primary production?

• Typical rates of photosynthesis range between 0.2-2 µmol C L-1 d-1 or 0.3-3 µmol O2 L-1 d-1

• Analytical sensitivity of measurements:–CO2 by coulometry = 1 µmol C L-1

–O2 by Winkler titration = 0.01 µmol O2 L-1

Most Common methods used to measure photosynthesis:

• CO2 assimilation: radio- or stable isotopes of carbon (14C or 13C) – developed by Steeman-Nielsen 1951.

• Changes in O2 concentration - developed by Gaarder and Gran 1927)

• Oxygen isotope techniques (18O, 17O, 16O)

O2 light-dark bottle approachMeasures changes in oxygen concentrations in light and dark bottles following incubation period: •Light bottle = net community production (photosynthesis and respiration). •Dark bottle: community respiration. Light + Dark = Gross production

Several caveats: 1) assumes rate of respiration in dark = light.2) requires conversion from O2 to carbon units (photosynthetic quotient, PQ).3) Requires incubation and confinement of samples.

PQ values can vary from 1.1 to 1.4 depending on nutrient environment and products of photosynthesis.

Examine assimilation of 14C (as bicarbonate) by photosynthetic plankton in the light. •Add 14C labeled HCO3

- to bottles containing seawater; incubate and harvest plankton biomass by filtration.•Count (using scintillation counter) the amount of radioactive 14C assimilated into plankton biomass during incubation period.

Several caveats: 1) Always returns a positive result, even if net community production is negative.2) Does not discriminate light and dark respiration.3) Unclear whether net or gross photosynthesis is measured.4) Generally ignores organic carbon produced and excreted or lost during incubation.5) Requires incubation and confinement of samples

14C-bicarbonate assimilation

Oxygen isotopes• Addition of 18O labeled H2O: light bottle/dark

bottle incubation approach; traces splitting of H2O by photosynthesis yielding 18O2. By simultaneously examining changes in O2concentrations, can determine gross primary production.

• Triple oxygen (16O, 17O, 18O): aquatic photosynthesis has a characteristic fractionation of H2O in production of O2; can use the ratio of the different isotopes to derive photosynthetic O2production.

What factors limit primary production?

• Nutrients– Protein (~50% of cell biomass): H, C, O, N, S,

metal co-factors– Lipid (~10% of cell biomass): H, C, O, P– Nucleic acids: H, C, N, O, P– Carbohydrates (~20% cell biomass): H, C, O

• Light• Temperature