Photosynthate translocation increases in response to low seawater pH in a coral–dinoflagellate symbiosis
This study has examined the effect of low seawater pH values (induced by an increased CO 2 partial pressure) on the rates of photosynthesis, as well as on the carbon budget and carbon translocation in the scleractinian coral species Stylophora pistillata, using a new model based on 13C labelling of the photosynthetic products. Symbiont photosynthesis contributes to a large part of the carbon acquisition in tropical coral species, and it is thus important to know how environmental changes affect this carbon acquisition and allocation. For this purpose, nubbins of S. pistillata were maintained for six months at two pH Ts (8.1 and 7.2, by bubbling seawater with CO 2). The lowest pH value was used to tackle how seawater pH impacts the carbon budget of a scleractinian coral. Rates of photosynthesis and respiration of the symbiotic association and of isolated symbionts were assessed at each pH. The fate of 13C photosynthates was then followed in the symbionts and the coral host for 48 h. Nubbins maintained at pH T 7.2 presented a lower areal symbiont concentration, and lower areal rates of gross photosynthesis and carbon incorporation compared to nubbins maintained at pH T 8.1. The total carbon acquisition was thus lower under low pH. However, the total percentage of carbon translocated to the host as well as the amount of carbon translocated per symbiont cell were significantly higher under pH T 7.2 than under pH T 8.1 (70% at pH T 7.2 vs. 60% at pH T 8.1), such that the total amount of photosynthetic carbon received by the coral host was equivalent under both pHs (5.5 to 6.1 μg C cm −2 h −1). Although the carbon budget of the host was unchanged, symbionts acquired less carbon for their own needs (0.6 compared to 1.8 μg C cm −2 h −1), explaining the overall decrease in symbiont concentration at low pH. In the long term, such decrease in symbiont concentration might severely affect the carbon budget of the symbiotic association.