Effects of pCO2 on the interaction between an excavating sponge, Cliona varians, and a hermatypic coral, Porites furcata

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Rising dissolved pCO2 is a mounting threat to coral reef ecosystems. While the biological and physiological impacts of increased pCO2 are well documented for many hermatypic corals, the potential effects on bioerosion processes remain largely unknown. Increases in pCO2 are likely to modify the direct interactions between corals and bioeroders, such as excavating sponges, with broad implications for the balance between biologically mediated deposition and erosion of carbonate in reef communities. This study investigated the effects of three levels of CO2 (present-day, mid-century and end-of-century projections) on the direct interaction between a bioeroding sponge, Cliona varians, and a Caribbean coral, Porites furcata. Increased pCO2 concentrations had no effect on the attachment rates of C. varians to the corals, and we observed no significant impact of pCO2 on the survival of either the coral or sponges. However, exposure to end-of-century levels of CO2-dosing (~750 μatm) reduced calcification in P. furcata and led to a significant increase in sponge-mediated erosion of P. furcata. These findings demonstrate that pCO2 can enhance erosional efficiency without impacting survival or competitive vigor in these two species. While few studies have considered the influence of pCO2 on the competitive outcomes of interactions between corals and other reef organisms, our study suggests that assessing the impacts of changing pCO2 on species interactions is crucial to adequately predict ecosystem-level responses in the future.