About half of the planet’s nitrogen fixation occurs out at sea by prokaryote diazotrophs, yet we still have a poor understanding of which ones do what. Karlusich et al. applied a combination of image capture and nitrogenase gene (nifH) sequencing on globally gathered data to map diazotrophs sampled at different depths. Unexpected hotspots of diazotroph abundance were discovered in the Mozambique Channel and the south Pacific Ocean. However, in the Arctic Ocean, nitrogen fixers were found among ultrasmall bacterioplankton, whereas in the tropics, larger cyanobacterial symbionts and colony-dwelling species such as Trichodesmium and Richelia dominated. Although single-cell, free-living, noncyanobacterial diazotrophs were the most abundant overall, Trichodesmium dominated by sheer size; however, several species tended to co-occur in assemblies. Whether or how such assemblages interact will have to await further study.
Nat. Commun. 12, 4160 (2021).
