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Nitrogen (N 2 ) fixation in oligotrophic surface waters is the main source of new nitrogen to the ocean 1 and has a key role in fuelling the biological carbon pump 2 . Oceanic N 2 fixation has been attributed almost exclusively to cyanobacteria, even though genes encoding nitrogenase, the enzyme that fixes N 2 into ammonia, are widespread among marine bacteria and archaea 3 – 5 . Little is known about these non-cyanobacterial N 2 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 N 2 -fixing symbiont, ‘ Candidatus Tectiglobus diatomicola’, which provides its diatom host with fixed nitrogen in return for photosynthetic carbon. The N 2 -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 N 2 -fixing rhizobia–legume symbioses on land 6 . Our results show that the rhizobia–diatom symbioses can contribute as much fixed nitrogen as can cyanobacterial N 2 fixers in the tropical North Atlantic, and that they might be responsible for N 2 fixation in the vast regions of the ocean in which cyanobacteria are too rare to account for the measured rates. A symbiosis between a diatom and a newly discovered species of alphaproteobacteria, ‘ Candidatus Tectiglobus diatomicola’, can fix nitrogen in the ocean, providing evidence that nitrogen fixers other than cyanobacteria have a key role in the marine environment.

Biogeochemical cycles are essential for the maintenance of life on Earth. However, little is known about the underlying processes, which hinders the estimation of future states of cycles in a changing environment. Microorganisms are essential in biogeochemical cycles by controlling reactions, rates, and products. This study compared bacterial community compositions by amplicon sequencing of V3-V4 regions of the 16S rRNA genes in two sediment cores (oxic vs. anoxic) from the southwestern Black Sea. To understand the interactions between bacterial diversity and environmental parameters, nutrient, organic carbon, and major seawater ions analyses were performed and used in Non-Metric Multidimensional Scaling. Dissolved oxygen was found as the main driver of community composition. However, organic carbon became one of the main drivers of differentiating bacterial diversity after oxygen consumption. While the community composition in deeper sediments were correlated with the concentrations of H2S, NH4+, PO43+, and major ions; the microbial community in the middle and upper sediments were related to more energetic molecules such as NO3-, NO2-, and dFe, SO42-, and TOC. Results show that decreases in oxygen concentration in the water column would change the bacterial community composition through anaerobic metabolisms, producing greenhouse gases.This study suggests that climate change and anthropogenic effects related to the oxygen and carbon cycle will directly affect bacterial communities. The results suggest that sediment bacterial communities should be considered in the climate change models. The Black Sea is a suitable habitat for further analysis to study anaerobic microbial metabolisms. This is one of the first studies investigating the sediment bacteria in the Black Sea to predict the changes in bacterial communities under the redox shift from oxic to the anoxic water column. Also, whole bacterial diversity was analyzed by amplicon sequencing in the Black Sea sediments for the first time without targeting specific groups/metabolisms. This study is also a pioneer in its contribution to the overall biodiversity record in the Turkish seas.

Nitrogen fixation is crucial for sustaining productivity in most of the open ocean. Cyanobacteria are the most prominent N2-fixers, but based on the nifH gene, a marker gene of the enzyme that fixes N2 into ammonia, non-cyanobacterial N2-fixers often predominate the N2-fixing community. Yet, the vast majority of them remain poorly characterized. In the oligotrophic South Pacific gyre, we found that most nifH gene sequences belonged to non-cyanobacterial N2-fixers that dominated the waters with measurable N2 fixation rates. Approximately two thirds of the non-cyanobacterial sequences affiliated with the group “Marine 1” which also contains the recently identified diatom symbiont Ca. Tectiglobus diatomicola, a heterotrophic bacterium belonging to the order Rhizobiales, and its closest relative, Ca. Tectiglobus profundi. Using fluorescence in situ hybridization and electron microscopy, we found that Ca. Tectiglobus-diatom symbioses were present throughout the gyre. These diatom symbioses were also present in samples devoid of nifH from Ca. T. diatomicola and Ca. T. profundi indicating that other members of the “Marine 1” group are also diatom symbionts. At least two morphologically distinct diatoms harbored Ca. Tectiglobus symbionts, revealing a so far unknown diversity in hosts for these rhizobial N2-fixers. Single-cell activity measurements showed that Ca. Tectiglobus-diatom symbioses actively fixed nitrogen and could account for up to 40% of the N2 fixation in the South Pacific gyre. Given the size of the largest oceanic biome and the abundance of Ca. Tectiglobus-related nifH genes in other ocean regions, these heterotrophic N2-fixers likely play a major role in marine nitrogen cycling.