Seawater acidification rapidly changes the isotopic signature of coral tissue during a short term transplantation experiment.

It is well known that ocean acidification decreases coral calcification and thus threatens tropical coral reefs. However, the impact of acidification on the trophic ecology of corals has received less attention, despite the fact that feeding is a major process for coral calcification. During the TARA-Pacific expedition, we have investigated the potential food sources and the importance of auto- and heterotrophy in coral species living under common and more acidified environments. For this purpose, Palau archipelago has been chosen as a natural laboratory. The main islands are surrounded by a coral reef barrier and form a labyrinth of small bays and semi-enclosed lagoons. While most of the area has a normal pH of 8.1-8.2, several bays are characterized by low pH (7.84-7.95), equivalent to conditions predicted to occur in the tropical Pacific ocean by the end of this century. Three coral genus have been selected (Pocillopora, Acropora and Porites) for a transplantation experiment from an acidic site to a control site and vice versa, during 10 days, and then sampled for the analysis of the carbon and nitrogen isotopic signature of the coral tissues and algal symbionts. In parallel, samples of seawater and plankton were collected and analyzed for their isotopic signature.
For each genus, del13C values of the tissues were more negative for corals living in the acidic zone compared to the control zone (-20.3 vs -15.7 per mil for Pocillopora, -15.7 vs -11.1 per mil for Acropora, and -15.2 vs -13.5 per milfor Porites). The transplanted corals had intermediate del13Ctis values (-17.1 per mil for Pocillopora and -13.8 per mil for Acropora). The del13C values of the algal symbionts followed the del13Ctis, with no fractionation between the two compartments, indicating a high autotrophic behavior. Except for Pocillopora (for which no differences were observed), del15Ntis values of corals in the acidic zone were higher than for corals living in control condition (4.1 vs 3.8 per mil for Acropora and 3.4 vs 2.6 per mil for Porites). These results can be explained by a higher level of 15N- and 13C enriched particulate matter in the acidified site and a different isotopic signature of the inorganic sources. Overall, this experiment shows that the coral tissue can rapidly trace the changes in seawater chemistry.