Nitrogenase genes in non-cyanobacterial plankton: prevalence, diversity and regulation in marine waters

Marine waters are generally considered to be nitrogen (N) limited and are therefore favourable environments for diazotrophs, i.e. organisms converting atmospheric N-2 into ammonium or nitrogen oxides available for growth. In some regions, this import of N supports up to half of the primary productivity. Diazotrophic Cyanobacteria appear to be the major contributors to marine N-2 fixation in surface waters, whereas the contribution of heterotrophic or chemoautotrophic diazotrophs to this process is usually regarded inconsequential. Culture-independent studies reveal that non-cyanobacterial diazotrophs are diverse, widely distributed, and actively expressing the nitrogenase gene in marine and estuarine environments. The detection of nifH genes and nifH transcripts, even in N-replete marine waters, suggests that N-2 fixation is an ecologically important process throughout the oceans. Because this process is highly sensitive to and inhibited by molecular oxygen (O-2), diazotrophy requires efficient scavenging of intracellular O-2 or growth in environments with low ambient O-2 concentration. Particles with interior low-O-2 micro-zones and oceanic oxygen minimum zones are just 2 potential habitats suitable for N-2 fixation by non-cyanobacterial diazotrophs. Our ignorance about the regulation of N-2 fixation by non-Cyanobacteria in their natural marine environments currently prevents an evaluation of their importance in marine N cycling and budgets. A review of the molecular data on distribution and expression of nifH genes in non-Cyanobacteria suggests that further study of the role of these Bacteria in N cycling at local, regional and global scales is needed.