Coral Reef Calcifiers Buffer their Responses to Ocean Acidification Using Both Bicarbonate and Carbonate

The highest known marine biodiversity is hosted by the calcium carbonate framework of corals and calcifying algae that form rich and dynamic coral reefs. Coral reefs are threatened by ocean acidification caused by an increase in dissolved carbon dioxide in the water, and this in turn decreases carbonate ion concentrations and increases bicarbonate ion concentrations. In this study, Comeau et al. evaluated the roles of carbonate and bicarbonate in the calcification of coral and crustose coralline algae in both light and dark conditions. They found that coral can maintain present-day calcification rates in the light if the decrease in carbonate concentrations is compensated for by an increase in bicarbonate concentration but this was not sufficient for calcification in the dark. Though crustose coralline algae were able to calcify using bicarbonate, it was not sufficient to compensate for the decrease in carbonate that is a result of ocean acidification. The results of this study show that the response of tropical coral reef communities to ocean acidification might be less dramatic than previously predicted due to the ability of calcifying organisms to utilize bicarbonate while it is light, but the decrease in the ability to calcify in the dark will undoubtedly result in an overall reduction in the calcifying ability of coral reefs in increasingly acidic oceans.—Dawn Barlow
                  Comeau, S., Carpenter, R., Edmunds, P., 2013. Coral reef calcifiers buffer their response to ocean acidification using both bicarbonate and carbonate. Proceedings of the Royal Society B: Biological Sciences 280, 20122374

This experiment conducted by Comeau et al. was unique in that instead of simply increasing the acidity of the water that the calcifying organisms were in, they addressed the effects of varying concentrations of carbonate and bicarbonate ions. The carbonate chemistry of the water used in this lab experiment was manipulated using carbon dioxide-equilibrated air, and varying carbon dioxide treatments were created by bubbling ambient air, carbon dioxide-enriched air, and carbon dioxide-depleted air into tanks of seawater. One coral species, Porites rus, and one crustose coralline algae species, Hydrolithon onkodes, were used as study species of calcifying organisms for this experiment. Buoyant weight measurements were used to evaluate calcification over the two-week period in which this study was conducted, and the alkalinity anomaly technique was used to examine short-term calcification in light and dark conditions.
The methods used by Comeau et al. allowed them to examine the relative concentrations of carbonate and bicarbonate and the calcifying ability of coral and crustose coralline algae in both light and dark. They found that for coral, calcification in the light was affected by both carbonate and bicarbonate concentrations, and in the dark by carbonate but not bicarbonate. For crustose coralline algae, light and dark calcification were affected by both carbonate and bicarbonate concentrations. The implications of the fact that the coral was better able to calcify in the presence of bicarbonate in the light are consistent with other studies, which couple calcification with photosynthesis and demonstrate that photosynthesis can, under some conditions, be stimulated by additions of bicarbonate. Carbonate is important for calcification, and bicarbonate is important for both calcification and photosynthesis. However, the way that ocean acidification works is that increasing bicarbonate means decreasing carbonate, and so the fact that bicarbonate increases calcification in the light is offset by the fact that overall calcification is decreasing due to the declining amount of carbonate ions in the water.

These results provide more insight into the rate at which reef-building organisms can calcify in increasingly acidic ocean waters. The fact that there appears to be a positive correlation between bicarbonate and calcification in the light shows that the future ability of coral reefs to calcify may not be quite as dire as previously thought. But ultimately the decreasing concentrations of carbonate that come with increased acidity mean that the increased ability to calcify in the light will be offset and the reef-building organisms will struggle to calcify as ocean acidification continues to be an issue in the future.

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