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

Heterotrophic diazotrophs, which rely on dissolved organic matter (DOM) for nitrogen fixation, contribute minimally to overall N2 fixation activity in the New Caledonian lagoon compared to photosynthetic cyanobacterial diazotrophs. A study using the photosynthesis inhibitor DCMU revealed that heterotrophic diazotrophs accounted for less than 10% of total N2 fixation, with their activity increasing near shore where labile DOM was more abundant. Experimental additions of carbohydrates (DOC), amino acids (DON), and phosphonates (DOP) showed significant increases in nifH gene expression and bulk N2 fixation rates only under DOP treatment, suggesting a nuanced relationship between DOM composition and diazotroph activity. The research highlights the dominance of cyanobacterial diazotrophs like Trichodesmium while emphasizing the need for further exploration of heterotrophic diazotrophs in response to rising anthropogenic DOM inputs.

Benavides, M., Martias, C., Elifantz, H., Berman-Frank, I., Dupouy, C., & Bonnet, S. (2018). Dissolved Organic Matter Influences N2 Fixation in the New Caledonian Lagoon (Western Tropical South Pacific). Frontiers in Marine Science, 5. https://doi.org/10.3389/fmars.2018.00089