The contribution of coral-associated denitrification and N2 fixation to reef N-cycling

Nitrogen (N) is a key regulator of biological productivity on coral reefs. A multitude of studies have characterised and quantified pelagic and benthic N cycling pathways in these systems. There are however still large knowledge gaps regarding how microbial processes within coral organisms influence local and ecosystem level N availability. For example, while N₂ fixation in coral microbiomes is known to be a ubiquitous and non-negligible source of N to corals, the presence and relevance of other microbially mediated N-cycling pathways, such as denitrification, are yet to be determined. Here we conducted a series of isotope tracer incubation experiments on wild coral fragments and adjacent substrates to quantify the contribution of the coral microbiome to N cycling at both the organism and ecosystem scale. We present the first unequivocal evidence of denitrification in tropical coral microbiomes from across the Great Barrier Reef. However, measured rates of gaseous N loss through denitrification (1.53 µmol N m^-2 d^-1) were approximately 2 orders of magnitude lower than those of N₂ fixation (172.82 µmol N m^-2 d^-1), signifying that 1) coral microbiome denitrification is insignificant at the organism scale, and 2) corals are a net bioavailable N source to reef systems. Whilst highly variable, N₂ fixation on dead coral fragments with epiphyte colonisation, and sediment-associated denitrification rates, were considerably greater than those of live corals. This suggests that the bioavailable N contribution of live corals is of limited importance in reef N-cycling. More importantly, our results demonstrate that mass coral mortality will provide additional rubble habitat for filamentous diazotroph colonization, significantly altering reef N biogeochemical cycling.