Ocean acidification increases photosynthate translocation in a coral–dinoflagellates symbiosis

This study has examined the effect of an increased seawater  pCO2 on the rates of photosynthesis 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 for  a large part of the carbon acquisition in tropical coral species and  is therefore an important process that may determine their survival  under climate change scenarios. Nubbins of S. pistillata  were maintained for six months under two pHs (8.1 and 7.2). 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 7.2  presented a lower areal symbiont concentration, lower areal rates of  gross photosynthesis, and lower carbon incorporation rates compared  to nubbins maintained at pH 8.1, therefore suggesting that the total  carbon acquisition was lower in this first set of nubbins. However,  the total percentage of carbon translocated to the host, as well as  the amount of carbon translocated per symbiont cell was  significantly higher under pH 7.2 than under pH 8.1 (70% at pH  7.2 versus 60% at pH 8.1), so 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 against 1.8 μg C cm−2 h−1),  explaining the overall decrease in symbiont concentration at low  pH. In the long-term, this decrease might have important  consequences for the survival of corals under an acidification  stress.

Tremblay P., Fine M., Maguer J. F., Grover R. & Ferrier-Pagès C., 2013. Ocean acidification increases photosynthate translocation in a coral–dinoflagellates symbiosis. Biogeosciences Discussions 10: 83-109. Article.

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