Marine and Freshwater Acidification: Effects on Marine Ecosystems
Several studies have investigated the response of corals to a decline in oceanic pH, and like the studies cited above, their results indicate the model-projected decline is highly exaggerated.
Suwa et al. (2010) employed controlled infusions of pure CO2 to create mean pH values of 8.0, 7.6, and 7.3 in filtered seawater that flowed continuously through three sets of multiple tanks into which they had introduced the gametes of two Acropora coral species (A. digitifera and A. tenuis) they had collected during a natural spawning event. Seven days later they determined the percent that survived; after ten additional days, they documented the size of the developing polyps; and after 14 days they documented the percentage of polyps that had acquired zooxanthellae that the researchers had collected from the giant clam T. crocea and released into the several treatment tanks.
Results indicated “A. digitifera larval survival rate did not differ significantly among pH treatments,” and the graphs of their data indicate survivorship in A. tenuis was about 18.5 percent greater in the lowest pH (highest CO2) treatment than in the ambient seawater treatment. At the end of the subsequent ten-day study, however, polyp size was reduced in the lowest pH treatment, but by only about 14 percent, not too bad for an atmospheric CO2 concentration of more than 2,000 ppm. And in the A. tenuis coral, this reduction in individual size was more than compensated by the even greater percentage increase in survivorship. In addition, after only four days of being exposed to the zooxanthellae derived from giant clams, all polyps in all treatments had acquired a full complement of the symbiotic zooxanthella.
In discussing their findings, the seven scientists state “the survival of coral larvae may not be strongly affected by pH change,” or “in other words,” they continue, “coral larvae may be able to tolerate ambient pH decreases of at least 0.7 pH units.” That, in fact, is something that will likely never occur, as it implies atmospheric CO2 concentrations of more than 2,000 ppm. And in the unlikely event that such high concentrations ever were to happen, they would be a long, long time in coming, giving corals more than sufficient time to acclimate—and even evolve (Idso and Idso, 2009)—to cope with the slowly developing situation.
In another study, zooxanthellate colonies of the scleractinian coral Astrangia poculata were grown by Holcomb et al. (2010) in controlled laboratory conditions under all four combinations of ambient and elevated (5 µM NO3-, 0.3 µM PO4-3, and 2 nM Fe+2) nutrients and ambient and elevated (~780 ppm) pCO2 for a period of six months. Coral calcification rates were measured via two different techniques—both one month after the start of the experiment and again five months later—and the carbonate chemistry and saturation state of the seawater of each treatment were calculated from measured values of alkalinity, salinity, phosphate, and pCO2.
The results of these operations indicated mean calcification rates of the studied corals were 2.1, 0.7, 1.4, and 1.3 g/m2/day for the ambient, CO2-treated, nutrient-treated, and CO2-plus-nutrient-treated corals, respectively, so that relative to ambient conditions, calcification rates were reduced by the CO2 treatment to approximately 33 percent of the ambient rate, but with the addition of nutrients bounced partway back to 62 percent of the ambient rate.
In light of their findings, Holcomb et al. conclude “nutritionally replete corals should be able to compensate for reduced saturation state under elevated pCO2 conditions.” As pCO2 increases and seawater saturation state declines, for example, they write, the “availability of DIC [dissolved inorganic carbon] to the zooxanthellae will increase, potentially allowing increased photosynthesis which provides both alkalinity and energy to help drive calcification.” Thus, if corals are experiencing carbon limitation, “elevated pCO2 could even positively impact calcification.”
It becomes clear, therefore, as the three researchers continue, that “saturation state alone is not an effective predictor of coral calcification.” They note “the interaction between nutritional status of the coral, DIC availability, and saturation state may help to explain the wide range of calcification responses seen in published acidification and nutrient enrichment studies.”
Two earlier studies reported similar findings. Atkinson et al. (1995) showed “nutritionally replete zooxanthellate corals in naturally low [aragonite] saturation-state seawaters are capable of accreting skeletons at rates comparable to those achieved by conspecifics in high-saturation-state seawaters.” And Cohen and Holcomb (2009) reported “today, several reefs, including Galapagos, areas of Pacific Panama, and Jarvis (southern Line Islands), experience levels of aragonite saturation equivalent to that predicted for the open ocean under two times and three times pre-industrial CO2 levels (Manzello et al., 2008; Kathryn Shamberger [PMEL/NOAA] and colleagues, pers. comm., August 2009),” and “available data on coral colony growth rates on these reefs, albeit limited, suggest that they are equivalent to and sometimes even rival those of conspecifics in areas where aragonite saturation states are naturally high, such as the western Pacific warm pool.”
Probably the most important deduction to flow from these observations is the fact, in the words of Cohen and Holcomb, that “naturally elevated levels of inorganic nutrients and, consequently, high levels of primary and secondary production, may already be facilitating high coral calcification rates in regions with naturally high dissolved CO2 levels.”
In another study, Kreif et al. (2010) collected two colonies of massive Porites corals (which form large multi-century-old colonies and calcify relatively slowly) and four colonies of the branching Stylophora pistillata coral (which is short-lived and deposits its skeleton rather rapidly) from a reef at the northern tip of the Red Sea. They grew fragments of these corals in 1,000-liter tanks through which they pumped Gulf of Eilat seawater adjusted to be in equilibrium with air of three different CO2 concentrations (385, 1,904, and 3,970 ppm), which led to corresponding pH values of 8.09, 7.49, and 7.19 and corresponding aragonite saturation state (Ωarag) values of 3.99, 1.25, and 0.65. After an incubation period of six months for S. pistillata and seven months for the Porites corals, several fragments were sampled and analyzed for a number of different coral properties. Then, 14 months from the start of the experiment, fragments of each coral species from each CO2 treatment were analyzed for zooxanthellae cell density, chlorophyll a concentration, and host protein concentration.
In the words of the seven scientists who conducted the study, “following 14 months incubation under reduced pH conditions, all coral fragments survived and added new skeletal calcium carbonate, despite Ωarag values as low as 1.25 and 0.65.” This was done, however, at a reduced rate of calcification compared to fragments growing in the normal pH treatment with a Ωarag value of 3.99. Yet in spite of this reduction in skeletal growth, they report, “tissue biomass (measured by protein concentration) was found to be higher in both species after 14 months of growth under increased CO2.” And they further note the same phenomenon had been seen by Fine and Tchernov (2007), who, as they describe it, “reported a dramatic increase (orders of magnitude larger than the present study) in protein concentration following incubation of scleractinian Mediterranean corals (Oculina patagonica and Madracis pharencis) under reduced pH,” stating “these findings imply tissue thickening in response to exposure to high CO2.” Also, in a somewhat analogous situation, Krief et al. report “a decrease in zooxanthellae cell density with decreasing pH was recorded in both species,” but they state “this trend was accompanied by an increase in chlorophyll concentration per cell at the highest CO2 level.”
In discussing their findings, the Israeli, French, and U.K. researchers write, “the inverse response of skeleton deposition and tissue biomass to changing CO2 conditions is consistent with the hypothesis that calcification stimulates zooxanthellae photosynthesis by enhancing CO2 concentration within the coelenteron (McConnaughey and Whelan, 1997),” and they write, “since calcification is an energy-consuming process . . a coral polyp that spends less energy on skeletal growth can instead allocate the energy to tissue biomass,” citing Anthony et al. (2002) and Houlbreque et al. (2004). Thus, they suggest “while reduced calcification rates have traditionally been investigated as a proxy of coral response to environmental stresses, tissue thickness and protein concentrations are a more sensitive indicator of the health of a colony,” citing Houlbreque et al. (2004) in this regard as well.
In concluding their paper, Krief et al. state “the long acclimation time of this study allowed the coral colonies to reach a steady state in terms of their physiological responses to elevated CO2,” and “the deposition of skeleton in seawater with Ωarag < 1 demonstrates the ability of both species to calcify by modifying internal pH toward more alkaline conditions.” As a result, they further state “the physiological response to higher CO2/lower pH conditions was significant, but less extreme than reported in previous experiments,” suggesting “scleractinian coral species will be able to acclimate to a high CO2 ocean even if changes in seawater pH are faster and more dramatic than predicted.”
In further examining the complexities of this issue, Jury et al. (2010) write as background for their analysis, “physiological data and models of coral calcification indicate that corals utilize a combination of seawater bicarbonate and (mainly) respiratory CO2 for calcification, not seawater carbonate,” but “a number of investigators are attributing observed negative effects of experimental seawater acidification by CO2 or hydrochloric acid additions to a reduction in seawater carbonate ion concentration and thus aragonite saturation state.” Thus, they state there is “a discrepancy between the physiological and geochemical views of coral biomineralization.” In addition, they report “not all calcifying organisms respond negatively to decreased pH or saturation state,” and they state, “together, these discrepancies suggest that other physiological mechanisms, such as a direct effect of reduced pH on calcium or bicarbonate ion transport and/or variable ability to regulate internal pH, are responsible for the variability in reported experimental effects of acidification on calcification.”
In an effort to shed more light on this conundrum, Jury et al. performed incubations with the coral Madracis auretenra (= Madracis mirabilis sensu Wells, 1973) in modified seawater chemistries, where, as they describe it, “carbonate parameters were manipulated to isolate the effects of each parameter more effectively than in previous studies, with a total of six different chemistries.” Their results indicated among-treatment differences “were highly significant,” and “the corals responded strongly to variation in bicarbonate concentration, but not consistently to carbonate concentration, aragonite saturation state or pH.” They found, for example, that “corals calcified at normal or elevated rates under low pH (7.6–7.8) when the sea water bicarbonate concentrations were above 1800 µM,” and, conversely, “corals incubated at normal pH had low calcification rates if the bicarbonate concentration was lowered.”
Jury et al. conclude, “coral responses to ocean acidification are more diverse than currently thought,” and they question “the reliability of using carbonate concentration or aragonite saturation state as the sole predictor of the effects of ocean acidification on coral calcification.” They state, “if we truly wish to decipher the response of coral calcification to ocean acidification, a firmer grasp of the biological component of biomineralization is paramount.”
Lastly, Ries et al. (2010) investigated the impact of CO2-induced ocean acidification on “the temperate scleractinian coral Oculina arbuscula by rearing colonies for 60 days in experimental seawaters bubbled with air-CO2 gas mixtures of 409, 606, 903 and 2,856 ppm CO2, yielding average aragonite saturation states (ΩA) of 2.6, 2.3, 1.6 and 0.8.” These operations indicated that “following the initial acclimation phase, survivorship in each experimental treatment was 100 percent,” and, in regard to the corals’ rates of calcification and linear extension, “no significant difference was detected relative to the control treatment (ΩA = 2.6) for corals reared under ΩA of 2.3 and 1.6.” The latter values correspond to pH reductions from current conditions of 0.08 and 0.26, respectively. It is enlightening to note the 0.26 pH reduction is approximately twice the maximum reduction derived from the analysis of Tans (2009) that would likely result from the burning of all fossil fuels in the crust of the Earth.
Given the above findings, the three researchers, in their words, “propose that the apparent insensitivity of calcification and linear extension within O. arbuscula to reductions in ΩA from 2.6 to 1.6 reflects the corals’ ability to manipulate the carbonate chemistry at their site of calcification.” And it would further appear that ability should serve the corals well as the pH of the ocean declines in the future.
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