| Summary: | In the recent discussion how biotic systems may react to raised carbon dioxide partial pressure (pCO2) and temperatures in the marine realm, substantial research is devoted to calcifying organisms such as stony corals, whereas the antagonistic process – biologically induced dissolution via bioerosion – is largely being neglected. As opposed to skeletal growth, bioerosion by chemical means can be expected 78 to be facilitated under the more acidic environment in a high-CO2 world. In order to elucidate the combined effects of ocean acidification and global warming on bioerosion, the zooxanthellate sponge Cliona orientalis, one of the most abundant and detrimental bioeroders at Australia’s Great Barrier Reef, was exposed to lowered as well as elevated levels of both pCO2 and temperature. Our results show a significant enforcement of the sponges’ bioerosion capacity with increasing pCO2 (decreasing pH), whereas temperature had comparatively little effect. This finding implies that tropical reef ecosystems are facing the combined effect of weakened coral calcification and accelerated bioerosion, resulting in critical pressure on the fragile balance between biogenic carbonate build-up and degradation.
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