Diazotrophs: a non-negligible source of nitrogen for the tropical coral Stylophora pistillata.

Corals are mixotrophs: they are able to fix inorganic carbon through the activity of their symbiotic dinoflagellates and to gain nitrogen from predation on plankton and uptake of dissolved organic and inorganic nutrients. They also live in close association with diverse diazotrophic communities, inhabiting their skeleton, tissue and mucus layer, which are able to fix dinitrogen (N2). The quantity of fixed N2 transferred to the corals and its distribution within coral compartments as well as the quantity of nitrogen assimilated through the ingestion of planktonic diazotrophs are still unknown. Here, we quantified nitrogen assimilation via (i) N2 fixation by symbiont diazotrophs, (ii) ingestion of cultured unicellular diazotrophs and (iii) ingestion of natural planktonic diazotrophs. We estimate that the ingestion of diazotrophs provides 0.76±0.15 µg N cm(-2) h(-1), suggesting that diazotrophs represent a non-negligible source of nitrogen for scleractinian corals.