Dissolved Organic Matter Influences N2 Fixation in the New Caledonian Lagoon (Western Tropical South Pacific)

Specialized prokaryotes performing biological dinitrogen (N-2) fixation (diazotrophs) provide an important source of fixed nitrogen in oligotrophic marine ecosystems such as tropical and subtropical oceans. In these waters, cyanobacterial photosynthetic diazotrophs are well known to be abundant and active, yet the role and contribution of non -cyanobacterial diazotrophs are currently unclear. The latter are not photosynthetic (here called heterotrophic) and hence require external sources of organic matter to sustain N-2 fixation. Here we added the photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) to estimate the N-2 fixation potential of heterotrophic diazotrophs as compared to autotrophic ones. Additionally, we explored the influence of dissolved organic matter (DOM) on these diazotrophs along a coast to open ocean gradient in the surface waters of a subtropical coral lagoon (New Caledonia). Total N-2 fixation (samples not amended with DCMU) ranged from 0.66 to 1.32 nmol N L-1 d(-1). The addition of DCMU reduced N-2 fixation by >90%, suggesting that the contribution of heterotrophic diazotrophs to overall N-2 fixation activity was minor in this environment. Higher contribution of heterotrophic diazotrophs occurred in stations closer to the shore and coincided with the decreasing lability of DOM, as shown by various colored DOM and fluorescent DOM (ODOM and FDOM) indices. We tested the response of diazotrophs (in terms of nifH gene expression and bulk N-2 fixation rates) upon the addition of a mix of carbohydrates (DOC treatment), amino acids (DON treatment), and phosphonates and phosphomonesters (DOP treatment). While nifH expression increased significantly in Trichodesmium exposed to the DOC treatment, bulk N-2 fixation rates increased significantly only in the DOP treatment. The lack of nifH expression by gammaproteobacteria, in any of the DOM addition treatments applied, questions the contribution of non-cyanobacterial diazotrophs to fixed nitrogen inputs in the New Caledonian lagoon. While the metabolism and ecology of heterotrophic diazotrophs is currently elusive, a deeper understanding of their ecology and relationship with DOM is needed in the light of increased DOM inputs in coastal zones due to anthropogenic pressure.