Released coral mucus does not enhance planktonic N2 fixation rates

Dinitrogen (N-2) fixation by prokaryotic microorganisms provides bioavailable nitrogen in oligotrophic marine ecosystems such as coral reefs. The widespread though largely unknown heterotrophic diazotrophs require dissolved organic matter (DOM) for their nutrition. In coral reef ecosystems, DOM-rich mucus released by corals potentially sustains heterotrophic diazotrophic growth and activity. In this study, we investigated the effect of coral mucus on planktonic N-2 fixation in in situ experiments over a seasonal cycle within a New Caledonian lagoon, as well as in in vitro experiments in which the effect of mucus was monitored for 72 h. During the field experiment, N-2 fixation rates ranged between 0.12 and 7.90 nmol N1(-1) d(-1). Despite the highest N-2 fixation being found after mucus release, no significant difference was measured between the seawater surrounding the coral before and after mucus release. Similarly, the addition of mucus during the in vitro experiment enhanced N-2 fixation rates 1.5-fold, but this increase was not significantly different from the control. The abundance of 2 dominant populations of diazotrophs associated with corals and their surrounding seawater environments (unicellular cyanobacteria and rhizobia) found within pure mucus samples was on average 18-fold higher than in the surrounding seawater in the summer period and 400-fold higher in the winter. Our results suggest that although coral mucus does not influence planktonic N-2 fixation, the release of large numbers of diazotrophic cells associated with the mucus likely influences the abundance and diversity of diazotroph populations within the lagoon waters.