Marine Non-Cyanobacterial Diazotrophs: Moving beyond Molecular Detection

The nitrogen input through biological N-2 fixation is essential for life in vast areas of the global ocean. The belief is that cyanobacteria are the only relevant N-2-fixing (diazotrophic) organisms. It has, however, now become evident that non-cyanobacterial diazotrophs, bacteria and archaea with ecologies fundamentally distinct from those of cyanobacteria, are widespread and occasionally fix N-2 at significant rates. The documentation of a globally relevant nitrogen input from these diazotrophs would constitute a new paradigm for research on oceanic nitrogen cycling. Here we highlight the need for combining rate measurements and molecular analyses of field samples with cultivation studies in order to clarify the ecology of non-cyanobacteria and their contribution to marine N-2 fixation on local and global scales.