John Sequeira

 Oceanic Climate Change and its Organismal Effects

A State of the Art Review on Ocean Acidification

The Acidifying Depths: An Introduction

Considerable coverage has been given to the climate-altering effects of atmospheric CO2

for many years, and it has long been understood that the oceans are an effective CO2 sink. Yet a

deeper awareness of the extensive effects of carbon dioxide on our oceans has been, at times,

slow in coming to fruition. Ocean acidification, a term first coined in 2003 (Caldeira and

Wickett, 2003), might prove to be the most damaging of all of these. Modern laboratory

experimentation and in situ studies suggest that, despite variability among species response and

limitations to research, the effects of ocean acidification “due to historical fossil fuel emissions

will be felt for centuries” (Gattuso et al. 2012). A recent analysis of proxy records show that the

last time ocean acidification occurred at today’s rate was 252 MYA, at the Permo-Triassic

Boundary (Clarkson et al. 2015). It is estimated that 66% of terrestrial and 90% of ocean life

died in the greatest extinction event in the earth’s history.

The Chemistry of Acidification

 It is estimated that around 30% of atmospheric CO2 is absorbed by the world’s oceans

(Beaufort et al., 2011). This carbon dioxide can either be photosynthesized by aquatic plants and

algae or result in acidification. Ocean acidification is the process by which the dissolution of

atmospheric carbon dioxide creates a decrease in oceanic pH (Honisch et al., 2015). Upon

dissolution, CO2 reacts with H2O to create carbonic acid (H2CO3). Carbonic acid is then capable

of disassociating to form HCO3- (bicarbonate) which can further disassociate into CO3

2−¿ ¿

(carbonate ions). Another result of this disassociation is a release of hydrogen ions (H+). pH (-

log H+) is the logarithmic potential to create hydrogen ions with a pH of 7.0 being a baseline.

Any value higher implies that the substance is a base, while lower values mean the substance is

an acid. An increase in H+ thus decreases the water’s pH, further acidifying it from its already

currently decreasing pH of 8.1 (at mid-latitude; the pH has been recorded at below 8.0 around the

equator and Antarctica in certain parts) (Ying et al., 2012). It is important to note that pre-

industrial levels stood at around 8.2; this represents a 30% increase in acidity since the mid-18th

century (Feely et al., 2009).

Acidification and Calcium Carbonate

 A major effect of a decrease in pH is a decrease in the availability of calcium carbonate

(CaCO3). CaCO3 is formed from calcium atoms (Ca2+) and carbonate ions (CO3

2−¿ ¿) (Bednarsek et

al., 2014). As H+ are released, they recombine with available CO32−¿ ¿ to form bicarbonate, thus

limiting the availability of CaCO3. However, CaCO3 is a crucial component of numerous marine

ecosystems. Aragonite, calcite, and high magnesium calcite are three naturally created

polymorphs of CaCO3, and numerous organisms use them to create and maintain their shells and

skeletons (for research purposes, it is important to note that aragonite is more soluble than calcite

and aragonite saturation rates are). A low CaCO3 saturation state implies that these organisms

(which include crucial groups such as crustaceans, mollusks, and corals) will not only not have

the means to create a secure calcium carbonate covering to protect them from predators, but also

that their rate of calcification will fall below the rising rate of dissolution caused by the more

acidic environment.

 One such calcium carbonate precipitator is the shelled pteropod. As a planktonic

calcifier, pteropods construct their shells purely of aragonite and are one of the few pelagic

organisms to do so (Bednarsek et al., 2012). Because of this and the higher solubility of

aragonite, they are considered to be strong bioindicators of acidification. Laboratory experiments

in which pteropod samples were exposed to various CaCO3 saturation levels with all other

variables controlled showed that not only did the studied pteropods calcify at a lower rate, but

that their shells also exhibited a much higher dissolution rate when CO2 levels were elevated

(Bednarsek et al., 2014). Sample populations were incubated in tanks for up to 14 days and

exposed to three different levels of saturation (super-, transitional, and undersaturation), which

were created by supplying CO2 in 375, 500, 750, and 1200 ppm mixed ratios. Initial exposure to

undersaturated conditions resulted in immediate shell dissolution at the rate of 1.4% shell mass

per day. Extensive dissolution was apparent in samples held in aragonite-undersaturated tanks

for 14 days.

Figure 6. Dissolution is visible at sites 1, 4, 11, and 14 of the pteropod shell.

 Increased dissolution and decreased calcification in low CaCO3 saturated waters is not a

guarantee, however. Research shows that though calcifying phytoplankton such as

coccolithophores and foraminifera may face hardship from decreased CaCO3 saturation states

(Pinsonneault et al., 2012), some species may actually increase calcification under particular

circumstances. Planktonic algae known as coccolithophores, “considered to be the most

productive calcifying organisms on Earth,” serve a dual purpose in our oceans (Raven et al.,

2005) (Hutchins, 2011). Through photosynthesis, this phytoplankton assimilates CO2 and

produces organic carbon. However, it can also convert dissolved inorganic carbon (DIC) into

overlapping calcite plates known as coccoliths. Coccolith production is influenced by a number

of factors, including temperature, salinity, and seasonality (i.e. pH levels and CaCO3 saturation

levels are both lowest in the winter). Furthermore, coccolithophore blooms are known to increase

the ocean’s albedo due to the reflective property of coccoliths (Tyrrell, 1999), adding further

import to their study.

 In situ observation of E. huxleyi coccolithophores in the Bay of Biscay, a northeastern

Atlantic Ocean gulf off the coast of France, was performed over the course of one year (Smith et

al., 2012). Monthly samples were taken of coccolithophores and seawater carbonate chemistry,

as well as other environmental variables such as DIC and alkalinity levels. Perhaps because

strong related research into coccolithophore response pointed to a positive correlation between

decreased pH and decreased calcification (Beaufort et al., 2011), this study’s researchers put

forth a hypothesis that lightly calcified coccolithophores would dominate winter samples.

However, analysis of the collected data proved the exact opposite to be true. Over 90% of

coccolithophores collected during winter months were heavily calcified; just as markedly, less

than 10% of summer samples, when pH and CaCO3 levels are higher, exhibited the same rate of

calcification. Subsequent laboratory testing could not duplicate the results utilizing the same

environmental conditions.

Figure 7. Map A shows the path and timeline of sample collection along the Bay of Biscay.

Box B shows the heavily calcified form of E. huxleyi on the left, with a lesser calcified

sample on the right. Box C shows a heavily calcified coccolith on the left, and a nominally

calcified coccolith on the right. Increased calcification is apparent in the lack of a central hole

and a thickening of spokes connecting the center mass to the outer edge.

A Research Review

 The previous study serves to highlight the variability of research results. As noted in the

introduction, ocean acidification has been a subject of study for just over a decade. In situ and

laboratory methodologies are both continuously being reworked so as to discover new

correlations and interactions in regard to decreasing global pH. Yet restrictions limit both in their

usefulness and feasibility at times.

 A 2013 “snapshot” of then-current research highlighted some worthwhile concerns. The

majority of research then and now has been focused on single species responses (Dupont et al.,

2013). Most laboratory research falls into what the authors term as “stamp collecting”- that is,

experimentation is performed on a single species under very simplistic conditions with changes

generally being introduced to only one or two variables. However, any oceanic ecosystem is in a

state of constant flux, being affected by inflow and outflow of currents, nutrients, organisms, and

myriad other factors. As the above coccolithophore study showed, results that could not have

been predicted through laboratory testing could not be replicated in the lab either. Furthermore,

many studies examined for this paper failed to observe sample populations for multiple

generations. This limited scope in turn limits the possibility of ascertaining whether adaptive

processes can be adopted by a species in time to prevent a catastrophic extinction event.

It is imperative to understand, however, that despite sometimes very wide-ranging

deviations in results, a general consensus does exist. Gattuso, Mach, and Morgan formulated a

detailed survey composed of 22 declarative statements (Gattuso et al., 2012). They submitted this

survey to 53 experts, previously participants in a 2011 IPCC workshop in Okinawa, Japan,

organized by Working Groups I and II.

 Results of the survey show that 90% of the experts agree with strong confidence that

ocean acidification caused by anthropogenic fossil fuel activities will continue to alter the

ocean’s chemistry for centuries. 14 of 19 experts knowledgeable on the subject confirmed with

high probability that “assuming business as usual…scenarios, anthropogenic ocean acidification

will continue at a rate faster than non-anthropogenic acidification has ever occurred in the past

55 Myr.” 26 of 28 surveyed experts expressed with strong confidence that “the magnitude of

future anthropogenic ocean acidification depends on CO2 pathways.”

Subsequent questions on specific biogeochemical issues achieved less solid consensus.

14 knowledgeable experts confirmed with strong to very strong confidence that acidification will

negatively affect calcification for most calcareous organisms. However, 21 instead placed some

to no confidence in this statement. “Anthropogenic ocean acidification will reduce biodiversity”

was said to be highly probably by four experts, while 14 responded with some or no confidence.

Despite this, 23 respondents placed high to very high probability on the statement that

anthropogenic “acidification will impact biogeochemical processes at the global scale.”

Refinements in research will have to be made to discover global trends related to pH and species

response. In this way, the science can be directed toward creating a more unified knowledge base

of and thus, approach to the issue of ocean acidification. As Dupont and Portner state:

“There is one overarching limitation. There is a lack of idea of and approach to

how the overarching principles of ocean acidification effects can be understood

across organism domains.”

 Perhaps when this is accomplished, more relevant research can be performed, and the

results grouped together to be succinctly presented to policy makers and the public. Ocean

acidification may be an old story for Earth, but it is a new one for us and it is our immediate next

few steps which will affect the next millennia.

References

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