Abstract
Nitrogen fixation is crucial for sustaining productivity in most of the open ocean. Cyanobacteria are the most prominent N(2)-fixers, but based on the nifH gene, a marker gene of the enzyme that fixes N(2) into ammonia, non-cyanobacterial N(2)-fixers often predominate the N(2)-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 N(2)-fixers that dominated the waters with measurable N(2) 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 N(2)-fixers. Single-cell activity measurements showed that Ca. Tectiglobus-diatom symbioses actively fixed nitrogen and could account for up to 40% of the N(2) 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 N(2)-fixers likely play a major role in marine nitrogen cycling.