Ocean acidification and warming decrease calcification in the crustose coralline alga Hydrolithon onkodes and increase susceptibility to grazing

Anthropogenic CO 2 emissions have exacerbated two environmental stressors, global climate warming and ocean acidification (OA), that have serious implications for marine ecosystems. Coral reefs are vulnerable to climate change yet few studies have explored the potential for interactive effects of warming temperature and OA on an important coral reef calcifier, crustose coralline algae (CCA). Coralline algae serve many important ecosystem functions on coral reefs and are one of the most sensitive organisms to ocean acidification. We investigated the effects of elevated pCO 2 and temperature on calcification of Hydrolithon onkodes, an important species of reef-building coralline algae, and the subsequent effects on susceptibility to grazing by sea urchins. H. onkodes was exposed to a fully factorial combination of pCO 2 (420, 530, 830μatm) and temperature (26, 29°C) treatments, and calcification was measured by the change in buoyant weight after 21days of treatment exposure. Temperature and pCO 2 had a significant interactive effect on net calcification of H. onkodes that was driven by the increased calcification response to moderately elevated pCO 2. We demonstrate that the CCA calcification response was variable and non-linear, and that there was a trend for highest calcification at ambient temperature. H. onkodes then was exposed to grazing by the sea urchin Echinothrix diadema, and grazing was quantified by the change in CCA buoyant weight from grazing trials. E. diadema removed 60% more CaCO 3 from H. onkodes grown at high temperature and high pCO 2 than at ambient temperature and low pCO 2. The increased susceptibility to grazing in the high pCO 2 treatment is among the first evidence indicating the potential for cascading effects of OA and temperature on coral reef organisms and their ecological interactions. © 2012 Elsevier B.V.