Dynamics of diazotroph particle colonization in the Arctic Ocean.

Global warming is causing sea ice retreat and intensifying algal blooms in the Arctic Ocean, in turn increasing nitrogen limitation in surface waters. Dinitrogen fixation by diazotrophic microorganisms, usually favored in low reactive nitrogen systems, may become an increasingly important source of nitrogen in the Arctic. Previous studies have shown that non-cyanobacterial diazotrophs are dominant in the Arctic Ocean. Lacking a photosynthetic apparatus, non-cyanobacterial diazotrophs may utilize organic particles as carbon- and energy-rich niches. However, cyanobacterial diazotrophs may also form particles by aggregation. To further understand diazotroph-particle associations, here we study the chemotactic behavior and colonization dynamics of diazotrophs on model organic particles using a modified chemotaxis assay. Artificial organic particles (agarose, alginate) were incubated with surface seawater communities from four contrasted stations in the Barents Sea, and their DNA was sequenced targeting nifH and 16S rRNA genes after 2, 36, and 72 h of incubation. Our results show that diazotroph groups have selective colonization behaviors, with Gammaproteobacteria members preferentially colonizing alginate particles derived from brown algae, a form of organic matter becoming more common in the Arctic as it warms up. We also observe niche partitioning among microbial groups, with diazotrophs colonizing nitrogen-poor, carbon-rich particles earlier than non-diazotrophic prokaryotes. As Arctic warming proceeds, increased algal blooms may expand the niches for particle-associated diazotrophs, whose dinitrogen fixation supports phytoplankton growth and primary productivity.