Glacial meltwater and seasonality influence community composition of diazotrophs in Arctic coastal and open waters

The Arctic Ocean is particularly affected by climate change with unknown consequences for primary productivity. Diazotrophs-prokaryotes capable of converting atmospheric nitrogen to ammonia-have been detected in the often nitrogen-limited Arctic Ocean but distribution and community composition dynamics are largely unknown. We performed amplicon sequencing of the diazotroph marker gene nifH from glacial rivers, coastal, and open ocean regions and identified regionally distinct Arctic communities. Proteobacterial diazotrophs dominated all seasons, epi- to mesopelagic depths and rivers to open waters and, surprisingly, Cyanobacteria were only sporadically identified in coastal and freshwaters. The upstream environment of glacial rivers influenced diazotroph diversity, and in marine samples putative anaerobic sulphate-reducers showed seasonal succession with highest prevalence in summer to polar night. Betaproteobacteria (Burkholderiales, Nitrosomonadales, and Rhodocyclales) were typically found in rivers and freshwater-influenced waters, and Delta- (Desulfuromonadales, Desulfobacterales, and Desulfovibrionales) and Gammaproteobacteria in marine waters. The identified community composition dynamics, likely driven by runoff, inorganic nutrients, particulate organic carbon, and seasonality, imply diazotrophy a phenotype of ecological relevance with expected responsiveness to ongoing climate change. Our study largely expands baseline knowledge of Arctic diazotrophs-a prerequisite to understand underpinning of nitrogen fixation-and supports nitrogen fixation as a contributor of new nitrogen in the rapidly changing Arctic Ocean. Community composition and diversity of nitrogen fixing microorganisms (diazotrophs) vary across Arctic coastal and open waters, influenced by glacial meltwater, seasonal biological activity, and communities are dominated by noncyanobacterial diazotrophs.

Automated summary

The Arctic Ocean is being significantly impacted by climate change, and the effect on primary productivity remains uncertain. Diazotrophs, which are capable of converting atmospheric nitrogen to ammonia, have been detected in the nitrogen-limited Arctic Ocean, but their distribution and community composition dynamics are not well understood. This study sequenced the diazotroph marker gene nifH from various regions in the Arctic Ocean and identified distinct regional communities. The study found that proteobacterial diazotrophs dominated all seasons and depths, while Cyanobacteria were only sporadically present in coastal and freshwaters. The composition and diversity of diazotrophs in the Arctic are influenced by glacial meltwater, seasonal activity, and the dominance of noncyanobacterial diazotrophs.