Camps, Mercedes, Benavides, Mar, Lema, Kimberley A., Bourne, David G., Grosso, Olivier, and Bonnet, Sophie (2016) Released coral mucus does not enhance planktonic N2 fixation rates. Aquatic Microbial Ecology, 77 (1). pp. 51-63.

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Abstract

Dinitrogen (N2) fixation by prokaryotic microorganisms provides bioavailable nitrogenin oligotrophic marine ecosystems such as coral reefs. The widespread though largelyunknown heterotrophic diazotrophs require dissolved organic matter (DOM) for their nutrition. Incoral reef ecosystems, DOM-rich mucus released by corals potentially sustains heterotrophic diazotrophicgrowth and activity. In this study, we investigated the effect of coral mucus on planktonicN2 fixation in in situ experiments over a seasonal cycle within a New Caledonian lagoon, aswell as in in vitro experiments in which the effect of mucus was monitored for 72 h. During thefield experiment, N2 fixation rates ranged between 0.12 and 7.90 nmol N l−1 d−1. Despite the highestN2 fixation being found after mucus release, no significant difference was measured betweenthe seawater surrounding the coral before and after mucus release. Similarly, the addition ofmucus during the in vitro experiment enhanced N2 fixation rates 1.5-fold, but this increase was notsignificantly different from the control. The abundance of 2 dominant populations of diazotrophsassociated with corals and their surrounding seawater environments (unicellular cyanobacteriaand rhizobia) found within pure mucus samples was on average 18-fold higher than in the surroundingseawater in the summer period and 400-fold higher in the winter. Our results suggestthat although coral mucus does not influence planktonic N2 fixation, the release of large numbersof diazotrophic cells associated with the mucus likely influences the abundance and diversity ofdiazotroph populations within the lagoon waters.

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