Dissolved Organic Matter Impacts Nitrogen Fixation in the Arctic Ocean

Climate change-driven sea ice loss, increased primary production, and nitrogen depletion are reshaping biogeochemistry in the Arctic Ocean (AO). These changes potentially increase the importance of nitrogen fixation, the conversion of dinitrogen gas into bioavailable ammonium, carried out by microorganisms called diazotrophs. However, it remains unknown how climate change-driven changes, such as increased primary production and dissolved organic matter (DOM) from riverine discharge, will affect nitrogen fixation. We show nitrogen fixation rates along with the composition of present and transcriptionally active diazotroph communities across depths and contrasting hydrographic regimes in the Pacific sector of the AO. Nitrogen fixation was widespread but generally low (<1 nmol N L−1 d−1). UCYN-A2, the nitroplast of a haptophyte, accounted for most of the present and expressed nitrogenase (nifH) genes. Experiments revealed that additions of labile DOM (monosaccharides, pyruvate, and acetate), filtered chlorophyll a-rich seawater (a proxy for autochthonous DOM) or 4-hydroxybenzoic acid (a proxy for terrestrial-derived DOM) stimulated nitrogen fixation at some stations, with responses largely depending on in situ conditions. UCYN-A1 and UCYN-A2 nitroplasts were the most responsive across DOM treatments, and elevated nifH gene transcription suggests the haptophyte-nitroplast symbioses can exploit diverse DOM sources. Our findings indicate that climate change, through its impact on DOM dynamics and primary production, may affect nitrogen fixation by particularly UCYN-A and thereby influence biogeochemistry and productivity in the AO.