Intriguing size distribution of the uncultured and globally widespread marine non-cyanobacterial diazotroph Gamma-A

Non-cyanobacterial diazotrophs (NCDs) have recently emerged as potentially important contributors to marine nitrogen fixation. One of the most widely distributed NCDs is Gamma-A, yet information about its autecology is still scarce and solely relies on the PCR-based detection of its nitrogenase (nifH) gene in seawater, since previous metagenomic surveys targeting free-living planktonic size fractions (<3 mu m) have not detected it. Here, we explore the diversity, biogeography, size-distribution, and nitrogenase gene expression of Gamma-A across four larger planktonic size-fractions (0.8-5, 5-20, 20-180, and 180-2000 mu m) using metagenomes and metatranscriptomes from theTaraOceans. We detected a single variant of a complete Gamma-AnifHgene along with other nitrogenase-related genes (nifKDT) within a metatranscriptomic-based contig of the Marine Atlas of Tara Ocean Unigenes. Gamma-A was detected in tropical and subtropical oceanic regions across all the size-fractions. However, the highest gene and transcript abundances were found in the 0.8-5 and 5-20 mu m size-fractions at the surface, whereas abundances at the deep chlorophyll maximum were lower and similar across all size-fractions. The ubiquitous presence of active Gamma-A in large planktonic size-fractions suggests a filamentous or particle-attached lifestyle and places its potential to fix nitrogen in larger planktonic compartments.

Automated summary

Gamma-A, a widely distributed non-cyanobacterial diazotroph, was found to be present and active in different size fractions of plankton through marine metagenomic and metatranscriptomic studies, indicating its potential for nitrogen fixation.