Marchant, Hannah K.

AbstractNitrogen (N2) fixation in oligotrophic surface waters is the main source of new nitrogen to the ocean1 and has a key role in fuelling the biological carbon pump2. Oceanic N2 fixation has been attributed almost exclusively to cyanobacteria, even though genes encoding nitrogenase, the enzyme that fixes N2 into ammonia, are widespread among marine bacteria and archaea3–5. Little is known about these non-cyanobacterial N2 fixers, and direct proof that they can fix nitrogen in the ocean has so far been lacking. Here we report the discovery of a non-cyanobacterial N2-fixing symbiont, ‘Candidatus Tectiglobus diatomicola’, which provides its diatom host with fixed nitrogen in return for photosynthetic carbon. The N2-fixing symbiont belongs to the order Rhizobiales and its association with a unicellular diatom expands the known hosts for this order beyond the well-known N2-fixing rhizobia–legume symbioses on land6. Our results show that the rhizobia–diatom symbioses can contribute as much fixed nitrogen as can cyanobacterial N2 fixers in the tropical North Atlantic, and that they might be responsible for N2 fixation in the vast regions of the ocean in which cyanobacteria are too rare to account for the measured rates.

Summary Methanotrophic bacteria represent an important biological filter regulating methane emissions into the atmosphere. Planktonic methanotrophic communities in freshwater lakes are typically dominated by aerobic gamma‐proteobacteria, with a contribution from alpha‐proteobacterial methanotrophs and the NC10 bacteria. The NC10 clade encompasses methanotrophs related to ‘ Candidatus Methylomirabilis oxyfera’, which oxidize methane using a unique pathway of denitrification that tentatively produces N 2 and O 2 from nitric oxide (NO). Here, we describe a new species of the NC10 clade, ‘ Ca . Methylomirabilis limnetica’, which dominated the planktonic microbial community in the anoxic depths of the deep stratified Lake Zug in two consecutive years, comprising up to 27% of the total bacterial population. Gene transcripts assigned to ‘ Ca . M. limnetica’ constituted up to one third of all metatranscriptomic sequences in situ . The reconstructed genome encoded a complete pathway for methane oxidation, and an incomplete denitrification pathway, including two putative nitric oxide dismutase genes. The genome of ‘ Ca . M. limnetica’ exhibited features possibly related to genome streamlining (i.e. less redundancy of key metabolic genes) and adaptation to its planktonic habitat (i.e. gas vesicle genes). We speculate that ‘ Ca . M. limnetica’ temporarily bloomed in the lake during non‐steady‐state conditions suggesting a niche for NC10 bacteria in the lacustrine methane and nitrogen cycle.