THE SENSITIVITY OF CALCIFICATION ENERGETICS TO OCEAN ACIDIFICATION

Anthropogenic ocean acidification has raised concerns as to the fate of marine organisms whose survival depends upon calcium carbonate skeletons. Past geological events, such as the end-Permian Mass Extinction, indicate the potential for acidification to significantly impact the diversity and structure of marine ecosystems. However, despite a wealth of evidence for the deleterious effects of acidification upon calcifying organisms, we still do not have a quantitative understanding of the physiological costs associated with producing a calcium carbonate skeleton and how this energetic cost is impacted by seawater chemistry. Here, we present an idealized mathematical model to quantify the fundamental costs of calcification as a function of seawater chemistry. In agreement with past experiemntal findings, our results suggest that the inorganic, calcum carbonate portion of skeletons actually constitutes a relatively small energetic investment when compared to the organic components. Accordingly, even under the worst IPCC5 forecasts for acidification by 2100, the absolute cost of calcification increases only modestly (up to about 10%). In contrast, we highlight the significance of acidification with respect to a decrease in the maximum attainable calcification rate, an effect felt most strongly during larval stages, where kinetic constraints dominate development. Cumulatively we lend support to the hypothesis that extinctions due to ocean acidification, both ancient and modern, play out early in ontogeny, demanding greater research focus on larval stages. Furthermore we observe that the relatively low cost of carbonate skeletons compared to pure organic material in part explains the widespread evolutionary convergence upon calcification within the metazoa.