Shell-Shocked: The Potential Effects of Ocean Acidification on Calcifying Marine Organisms
By Ashley Jones,Mariel Hathaway, and Elizabeth Mays
Abstract
Ocean acidification is a well-documented phenomenon that is considered a great threat to calcifying marine organisms. In this process, dissolved carbon dioxide reacts with seawater, forming an acid that then reacts with calcium carbonate shells of a variety of organisms. As CO2 concentrations increase in the atmosphere, more is dissolved in the ocean, leading to increased acidification over time.
In our experiment, we treated shells of five different calcifying species (snails, coralline algae, coral, oysters, and limpets) in treatments of pH 8.1(standard seawater), 7.0, 6.1, and 5.1. We found significant variability in percent mass loss between species. However, we also found no significant difference in the amount of mass loss and the pH, although loss of mass in coral and decreasing pH were weakly positively correlated, while snails lost most mass in higher pH solutions.
Although our data suggests that calcifying organisms do not lose more mass in lower pH solutions, we were limited by our use of uninhabited shells and dead coral and coralline algae. Living organisms may respond differently to decreasing pH. Furthermore, our treatments isolated pH as a variable, while there are other conditions, such as aragonite concentrations, that also affect calcifying organisms. Nonetheless, many of the species studied showed loss of mass even in the seawater solution, making clear the danger of acidification of the world’s oceans.
Introduction
The harmful effects of anthropogenic activities upon ocean ecosystems have become increasingly evident within the last decade. Perhaps the most critical threat is the elevation of carbon dioxide, which has resulted not only in global warming,but alsoin increased acidification of seawater. As the partial pressure of oceanic CO2 increases (hypercapnia), pH drops due to a series of complex chemical reactions (Fabry et al. 2008). The current pH of surface ocean water is8.1, demonstrating a ten-fold increase in acidity since the beginning of the industrial revolution (Ries et al. 2009). Carbon dioxide concentrations are higher today than they have been in 650,000 years, and continue to rise at a rate of 0.5% (Royal Society, 2005; Fabry et al., 2008). The ocean, which serves as the world’s largest carbon sink, will receive the majority of the excess CO2. When carbon dioxide reacts with seawater, calcium carbonate (CaCO3)is reduced via the reaction CO2 + CO32- + H2O = 2HCO3-1 : (Fabry et al. 2008). The reaction increases the concentrations of H2CO3-, HCO32- and H+, and decreases the concentration of CO32- , thereby lowering pH (pH= 2log[H+]). The IPCC predicts surface ocean pH could decrease by an additional 0.3–0.4 pH units by the end of this century. (Caldeira & Wickett 2005). Such an outcome could be disastrous for organisms that construct their shells and skeletons from calcium carbonate, including organisms producing aragonite, low-Mg calcite, and high-Mg calcite forms of CaCO3. (Ries et al. 2009).
A broad range of marine organisms (including foraminifera, corals, mollusks, echinoderms, and crustaceans) secrete calcium carbonate as an exterior shell or skeleton, a trait that was likely developed for protection (Fabry et al. 2008). CaCO3 is deposited in organic layers that protect the skeleton; algae precipitates CaCO3 within cortical tissue, corals nucleate aragonite beneath several layers of epithelial tissue, and mollusks cover their shells with periostracum (Ries et al. 2009). Prior studies have indicated that increased ocean acidification reduces calcification and may even cause dissolution of the existing shell within these organisms (Ries et al. 2009). Most research has focused on tropical corals; however, more information is needed to predict potential effects on other calcifiers, especially those in temperate-zone areas (Royal Society 2005).
Our study focused on the amount of CaCO3 dissolution in five species: tropical coral, coralline algae, oysters, limpets, and small snails. We analyzed the differences in amount of protective layer lost between species and between pH treatments. We expected to find the highest occurrence of dissolution in the most acidic treatment, with little or no dissolution in standard seawater. Moreover, we predict that rate of dissolution will vary according to species.
Methods and Materials
We collected samples of washed-up coral, coralline algae, and various mollusk shells from the low tidal area of Los Piqueros Beach in Ecuador. We selected twenty representatives of each species; selectingcomplete shells whenever possible in order to obtain the best representation of the intact organisms. After rinsing the samples, we measured the initial masses of each to the nearest centigram. We then randomly divided the 20 samples of each species into 4 different pH treatments, with 5 representatives of each species per treatment. Our selected pH’s to test were A) 8.1: the pH of standard seawater, B) neutral pH: 7.0, C) 6.1: at whichcoralline red algae are thought to facilitate their own CaCO3 precipitation by proton regulation (Ries et.al 2009), and D) 5.1: representing a moderate acid. We obtained solutions of each pH to the nearest .01 unit by varying concentrations of vinegar and sodium chloride in distilled water. The samples then sat in sealed containers for 10 days, after which they were re-massed to estimate their loss of calcium carbonate.
From this data, we calculated the proportion of initial mass lost by each individual, and obtained species-specific averages for each treatment. We compared the average proportions of mass lost between species using a one-way analysis of variance. We then compared the overall proportions of mass lost per treatment using a one-way ANOVA. To analyze each species’ response to the various treatments, we calculated five separate F-statistics using ANOVA. We then calculated separate regression equations for each of the five species.
Results and Discussion
Our results indicate significant variability in the percentage of mass lost depending on species (ANOVA, df=4, F= 42.48, p<.0001, Figure 1). Mass lost ranged from .01% to 11.28%, with the greatest decrease in mass by the coralline algae and coral. Limpets and snails showed the least change; and some individuals in acidic treatments even maintained or gained mass. Prior studies have indicated species variability in their rates of calcification; and found an increase in net dissolution of temperate corals and oysters, but net calcification by coralline red algae and limpets in acidic conditions (Ries et al. 2009). However, these studies have been with live organisms that undergo photosynthesis and continued calcification, so it is interesting to note a parallel situation in their shells which have ceased building additional shell layers.
Our results suggest that loss of calcium carbonate is dependent more on the species of organism than on the treatment it undergoes. Our samples indicated calcium carbonate dissolution under basic seawater conditions, as well as at a neutral pH and in acid. The differences in overall percentages of mass lost between pHs was shown to be non-significant (ANOVA, df=3, F=0.3 and p=0.825315, Figure 2). There was no significant correlation between an decrease in pH and an increase in mass of calcium carbonate loss by calcifiers as a group.
Associations between amount of mass lost and a change in acidity(note, not necessarily a decrease) and could only be observed in two of the five species. Limpets, oysters, and coralline algae did not vary significantly in their mass lost at various pH´s (ANOVA, df=3, p = 0.914, p=0.801, p = 0.853, respectfully). Corals demonstrated a weak relationship between mass lost and a change in acidity (ANOVA, df=3, p = 0.1). However, upon calculating a regression equation, we found only a weak linear correlation between the increase in acidity and the percentage of mass lost (R² = 0.0307, p= 0.4354). In fact, corals demonstrated a greater loss of calcium carbonate in neutral than in acidic conditions (Figure 3).We also observed that corals with the highest percentage of mass lost appeared grayer and gave off a foul odor.
Snails also showed a weak association between pH and final mass (ANOVA, df=3, p=0.1058), and actually tended to increase in mass as pH decreased. Snails gained between 1.5 and 2 percent of their initial masses when in treatments of pH 5.1 and 6.1, but all samples under seawater conditions underwent dissolution. The relationship between the increase in acidity and the increase in mass was significant (linear regression,R² = 0.218, p=0.0284). This result is surprising; snail shells have previously been noted to exhibit the least resilience among mollusks given their lack of organic covering over the outer shell (Ries et al. 2009). (However it is important to note that snail shells that gained mass only increased by 0.01 to 0.02 grams, which may also be a result of equipment error).
Upon removing the shells from their respective treatments, we measured the pH of each solution, and found that every solution had returned to a slightly basic pH between 7.5 and 8.1, perhaps due to the elevated levels of calcium carbonate from dissolution. This indicates that an increase in acidity may in fact be buffered by the corresponding release of calcium carbonate. It may be possible that the snail shells uptake dissolved CaCO3, and thus increase in mass.
Conclusions
Despite concern that increasing ocean acidity will negatively impact calcifying organisms, our study suggests that effects of a changing pH might not be as catastrophic as anticipated. Current research has estimated that the ocean will decrease in pH by another .0.3 to 0.4 units by the end of the century, causing seawater to fall to 7.6 to 7.9 (Shaw et al. 2012).This is expected to cause the loss of 55% of coral reef habitats, which would become dominated by algae (Fabry et al. 2008).The fishing industry is projected to lose over six billion dollars in revenue due to the impact that global climate change will have on mollusks (Narita 2008). On the other hand, should coral and mollusks retain their calcium carbonate as they have in our experiment, a small change in pH might not affect marine life on such a large scale.
It is important to note that our study does not consider the effects of a change in pH on living organisms, nor the natural ocean setting where other chemical compounds are present. We must also take into consideration additional limitations to mollusk and coral survival, including aragonite concentrations, which have been found to be a more important determinant of calcification in coral reefs(Shaw et al. 2012). Due to high levels of aragonite in the tropics, these zones are the least affected by decreased calcification (Fabry et al. 2008). Despite the lower threat and apparent resilience of calcifiers to increased ocean acidity, it is still crucial to monitor climate change as it affects the Earth.
Figures
Figure 1.The percentage of mass lost in calcium-containing structures of marine organisms at various pH treatments varies significantly depending on species (ANOVA, df=4, p<0.001). Coralline algae and coral are most susceptible to dissolution, losing between 3.5 to 7.5% of their original mass. Percentage of mass lost by coral and coralline algae is significantly greater than the mass lost by species within the phylum Mollusca.
Figure 2The percentage of mass lost by calcifying marine organisms cannot be attributed to acidity, with no significant differences shown in the dissolution between organisms at an acidic, neutral, and basic pHs (ANOVA, df = 4, p=0.825). The shells of the experimental organisms demonstrated a slight decrease in mass under all treatments.
Figure 3Corals demonstrated a weak relationship between total calcium carbonate lost and decreasing pH; thus we cannot conclude a lower pH increases percentage of mass lost(ANOVA, df=3, p= 0.1). No linear correlation exists between increase in acidity and the percentage of mass lost, with corals demonstratingthe greatest loss at neutral pH rather than in acidic conditions (R² = 0.0307, p= 0.4354).
Figure 4Snails show a weak association between pH and final mass after 10 days of exposure to various pH treatments (ANOVA, df=3, p=0.1058). Snail shells show a positive response to increased acidity, and tend to gain or retain mass as pH decreases (R² = 0.218, p=0.0284).
Literature Cited
Caldeira, K., & Wickett, M. 2005. Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean.J. Geophys.Res.,110.
Fabry, V., Seibel, B., Feely, R. & Orr, J. 2008.Impacts of ocean acidification on marine fauna and ecosystem processes.ICES Journal of Marine Science. 65: 414–432.
Narita, D., Rehdanz. K. & Tol, R. 2012. Economic costs of ocean acidification: a look into the impacts on global shellfish production. Climatic Change.113: 1049–1063.
Ries, J., Cohen, A.McCorkle, D. December 2009. Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification. GEOLOGY, 37; 12: 1131–1134.
Royal Society. 2005. Ocean acidification due to increasing atmospheric carbon dioxide. Policy Document 12/05, The Royal Society, London. 60 pp.
Shaw, E., McNeil, B.,Tilbrook, B. 2012. Impacts of ocean acidification in naturally variable coral reef flat ecosystems, J. Geophys. Res.,117.