Diazotrophs and N-Fixation Associated With Particles in Coastal Estuarine Waters.

Putative heterotrophic bacteria carrying out N-fixation, so-called non-cyanobacterial diazotrophs (NCDs), are widely distributed in marine waters, but details of how the O-inhibited N-fixation process is promoted in the oxic water column remains ambiguous. Here we carried out two experiments with water from a eutrophic temperate fjord to examine whether low-oxygen microenvironments within particulate organic matter could be loci suitable for N-fixation. First, water enriched with natural particles or sediment showed higher N-fixation rates than bulk water, and nitrogenase genes () revealed that specific diazotrophs were affiliated with the particulate matter.

Marine Non-Cyanobacterial Diazotrophs: Moving beyond Molecular Detection.

The nitrogen input through biological N fixation is essential for life in vast areas of the global ocean. The belief is that cyanobacteria are the only relevant N-fixing (diazotrophic) organisms. It has, however, now become evident that non-cyanobacterial diazotrophs, bacteria and archaea with ecologies fundamentally distinct from those of cyanobacteria, are widespread and occasionally fix N at significant rates.

Comparative genomics reveals surprising divergence of two closely related strains of uncultivated UCYN-A cyanobacteria.

Marine planktonic cyanobacteria capable of fixing molecular nitrogen (termed 'diazotrophs') are key in biogeochemical cycling, and the nitrogen fixed is one of the major external sources of nitrogen to the open ocean. Candidatus Atelocyanobacterium thalassa (UCYN-A) is a diazotrophic cyanobacterium known for its widespread geographic distribution in tropical and subtropical oligotrophic oceans, unusually reduced genome and symbiosis with a single-celled prymnesiophyte alga. Recently a novel strain of this organism was also detected in coastal waters sampled from the Scripps Institute of Oceanography pier.

Ecogenomic sensor reveals controls on N2-fixing microorganisms in the North Pacific Ocean.

Nitrogen-fixing microorganisms (diazotrophs) are keystone species that reduce atmospheric dinitrogen (N2) gas to fixed nitrogen (N), thereby accounting for much of N-based new production annually in the oligotrophic North Pacific. However, current approaches to study N2 fixation provide relatively limited spatiotemporal sampling resolution; hence, little is known about the ecological controls on these microorganisms or the scales over which they change. In the present study, we used a drifting robotic gene sensor to obtain high-resolution data on the distributions and abundances of N2-fixing populations over small spatiotemporal scales.

Non-cyanobacterial nifH phylotypes in the North Pacific Subtropical Gyre detected by flow-cytometry cell sorting.

In contrast to cyanobacteria, the significance of bacteria and archaea in oceanic N2 fixation remains unknown, apart from the knowledge that their nitrogenase (nifH) genes are diverse, present in all oceans and at least occasionally expressed. Non-cyanobacterial nifH sequences often occur as contamination from reagents and other sources, complicating the detection and interpretation of environmental phylotypes. We amplified and sequenced partial nifH gene fragments directly from cell populations sorted by fluorescence activated cell sorting from water collected in the North Pacific Subtropical Gyre (NPSG).