Rates and mechanisms of sponge bioerosion

Coral reefs are complex marine ecosystems providing valuable economic and environmental services. These ecosystems are under serious threat worldwide due to global warming and ocean acidification. Particularly calcium carbonate producing marine organisms, such as corals, will suffer from the projected decrease in pH. Because the compounded effects of ocean acidification and warming significantly reduce reef-building potential, shifts in erosional processes could have substantial consequences for the carbonate budgets of coral reefs. Bioerosion by boring sponges, especially from the genus Cliona, has been shown to increase under acidification scenarios, but the mechanisms involved are poorly understood. Here, we decouple the covariance of [H+] and [CO32-] to reveal the individual effects of these parameters on the bioerosion rates of C. caribbaea during incubations. Varying these carbonate system parameters independently from each other revealed that high [H+] alone does not induce enhanced bioerosion. Instead, Ω constitutes a crucial threshold for bioeroding sponges; values above Ω ~ 4 result in the sponge being unable to reduce Ω enough for dissolution to occur. Thus, Ω is the defining factor that governs whether and at which rate bioerosion occurs. Inspired by osteoclast bone resorption, the enzyme carbonic anhydrase II plays a central role in our hypothetical model for sponge bioerosion, which creates a framework for future study directions. By reducing the metabolic cost of both carbonate dissolution and the maintenance of ion gradients, the changes in chemistry due to ocean acidification enhance the bioerosion potential of clionaid sponges. Our findings confirm the present hypothesis that enhanced bioerosion rates are related to increased metabolic efficiency. The process of flushing is a potentially limiting factor for bioerosion rates and requires detailed, in-situ observations to reveal its underlying mechanisms. With enhanced bioerosion in future oceans alongside a decreased reef building potential of corals and calcifying algae, it seems inevitable that carbonate budgets will become increasingly negative. 

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