Abstract
In addition to the cyanobacterial N-2-fixers (diazotrophs), there is a high nifH gene diversity of non-cyanobacterial groups present in marine environments, yet quantitative information about these groups is scarce. N-2 fixation potential (nifH gene expression), diversity and distributions of the uncultivated diazotroph phylotype gamma-24774A11, a putative gammaproteobacterium, were investigated in the western South Pacific Ocean. gamma-24774A11 gene copies correlated positively with diazotrophic cyanobacteria, temperature, dissolved organic carbon and ambient O-2 saturation, and negatively with depth, chlorophyll a and nutrients, suggesting that carbon supply, access to light or inhibitory effects of DIN may control gamma-24774A11 abundances. Maximum nifH gene-copy abundance was 2 x 10(4) I-1, two orders of magnitude less than that for diazotrophic cyanobacteria, while the median gamma-24774A11 abundance, 8 x 10(2) I-1, was greater than that for the UCYN-A cyanobacteria, suggesting a more homogeneous distribution in surface waters. The abundance of nifH transcripts by gamma-24774A11 was greater during the night than during the day, and the transcripts generally ranged from 0-7%, but were up to 26% of all nifH transcripts at each station. The ubiquitous presence and low variability of gamma-24774A11 abundances across tropical and subtropical oceans, combined with the consistent nifH expression reported in this study, suggest that gamma-24774A11 could be one of the most important heterotrophic (or photoheterotrophic) diazotrophs and may need to be considered in future N budget estimates and models.