Experiments linking nitrogenase gene expression to nitrogen fixation in the North Pacific subtropical gyre

We examined diazotroph activity in controlled experiments containing natural populations of nitrogen (N₂)-fixing microbes collected offshore from Kane'ohe Bay, Hawai'i, and from Station ALOHA (A Long-term Oligotrophic Habitat Assessment) in the North Pacific subtropical gyre. Quantitative polymerase chain reaction and reverse transcription-polymerase chain reaction approaches showed that the abundances of the major phylotypes did not change substantially during experimental incubations. Diel phasing of nifH gene expression in the experiments was similar to that documented for natural populations. Cell-specific N₂ fixation rates, calculated as ¹⁵N₂ incorporation rate divided by specific phylotype abundance (gene copy number), varied between the experiments, but generally agreed with the abundance and levels of nifH expression of each group of diazotrophs. Phosphorus additions had little effect on the abundance, gene expression, or N₂ fixation activity of diazotrophs across the course of the 36-h experiments, indicating that other factors may limit the growth, gene expression, and activity of diazotrophs in the North Pacific subtropical gyre.