Global Biogeochemical Cycles Energetic costs of calcification under ocean acidification

Anthropogenic ocean acidification threatens to negatively impact marine organisms that precipitate calcium carbonate skeletons. Past geological events, such as the Permian-Triassic Mass Extinction, together with modern experiments generally support these concerns. However, the physiological costs of producing a calcium carbonate skeleton under different acidification scenarios remain poorly understood. Here we present an idealized mathematical model to quantify whole-skeleton costs, concluding that they rise only modestly (up to ∼10%) under acidification expected for 2100. The modest magnitude of this effect reflects in part the low energetic cost of inorganic, calcium carbonate relative to the proteinaceous organic matrix component of skeletons. Our analysis does, however, point to an important kinetic constraint that depends on seawater carbonate chemistry, and we hypothesize that the impact of acidification is more likely to cause extinctions within groups where the timescale of larval development is tightly constrained. The cheapness of carbonate skeletons compared to organic materials also helps explain the widespread evolutionary convergence upon calcification within the metazoa. Plain Language Summary Human activity continues to raise atmospheric levels of carbon dioxide, a gas that tends to increase the acidity of the world’s oceans. Numerous marine species, such as corals and many types of shellfish, must manufacture skeletons of calcium carbonate, a mineral that is susceptible to corrosion in acidified seawater. This mineral is shaped into intricate and unique structures by way of an organic matrix that the organism must also generate. It remains poorly understood how much more energy organisms will need to expend in order to continue making their skeletons as ocean acidification continues. In this work, we use a simple mathematical approach to model the dependence of the calcification process to seawater chemistry. We find that the organic component of the skeleton is typically more costly than the calcium carbonate, mineral component. Therefore, the effect of acidification is somewhat damped in organisms possessing a more organic-rich skeleton. Owing to the relatively low sensitivity, we conclude that larval stages, when organisms are under much tighter constraints, are more critical for determining the impact of acidification upon a given group of organisms.