Metabolic versatility of a novel N -fixing Alphaproteobacterium isolated from a marine oxygen minimum zone.

The N -fixing (diazotrophic) community in marine ecosystems is dominated by non-cyanobacterial microorganisms. Yet, very little is known about their identity, function and ecological relevance due to a lack of cultured representatives. Here we report a novel heterotrophic diazotroph isolated from the oxygen minimum zone (OMZ) off Peru.

The small unicellular diazotrophic symbiont, UCYN-A, is a key player in the marine nitrogen cycle.

Microbial dinitrogen (N) fixation, the nitrogenase enzyme-catalysed reduction of N gas into biologically available ammonia, is the main source of new nitrogen (N) in the ocean. For more than 50 years, oceanic N fixation has mainly been attributed to the activity of the colonial cyanobacterium Trichodesmium. Other smaller N-fixing microorganisms (diazotrophs)-in particular the unicellular cyanobacteria group A (UCYN-A)-are, however, abundant enough to potentially contribute significantly to N fixation in the surface waters of the oceans.

High rates of microbial dinitrogen fixation and sulfate reduction associated with the Mediterranean seagrass Posidonia oceanica.

Seagrass meadows of Posidonia oceanica represent hotspots of productivity in the oligotrophic Mediterranean Sea. The lack of dissolved inorganic nitrogen (DIN) in the seawater suggests that the N-demand of these meadows might be in part supported by microbial dinitrogen (N) fixation. However, currently there are no direct N fixation measurements available for this productive marine macrophyte.

Simple approach for the preparation of (15-15)N2-enriched water for nitrogen fixation assessments: evaluation, application and recommendations.

Recent findings revealed that the commonly used (15)N2 tracer assay for the determination of dinitrogen (N2) fixation can underestimate the activity of aquatic N2-fixing organisms. Therefore, a modification to the method using pre-prepared (15-15)N2-enriched water was proposed. Here, we present a rigorous assessment and outline a simple procedure for the preparation of (15-15)N2-enriched water.

Aphotic N2 fixation in the Eastern Tropical South Pacific Ocean.

We examined rates of N2 fixation from the surface to 2000 m depth in the Eastern Tropical South Pacific (ETSP) during El Niño (2010) and La Niña (2011). Replicated vertical profiles performed under oxygen-free conditions show that N2 fixation takes place both in euphotic and aphotic waters, with rates reaching 155 to 509 µmol N m(-2) d(-1) in 2010 and 24±14 to 118±87 µmol N m(-2) d(-1) in 2011. In the aphotic layers, volumetric N2 fixation rates were relatively low (<1.