Nitrogen fixation by unicellular diazotrophic cyanobacteria in the temperate oligotrophic North Pacific Ocean

N₂ fixation has been understudied in marine environments outside of the subtropical and tropical oceans and where water temperatures are typically below 20-25°C. We identified nifH phylotypes and measured N ₂ fixation rates under ambient conditions (maximum of 19°C) in water collected 750 km off the coast of California in oligotrophic waters of the North Pacific Ocean (34°N, 129°W). Near-surface N₂ fixation rates averaged 0.25 ± 0.05 nmol N L^-1 d^-1 for 24 incubation bottles. Despite low ambient concentrations of iron (<0.1 nmol L^-1) and phosphorus (<0.3 μmol L^-1), N₂ fixation rates were unaffected by iron and phosphorus amendments. Using reverse transcription-quantitative polymerase chain reaction (RT-QPCR) methodology, we estimated transcript abundance and patterns of expression for several unicellular diazotrophs, including the group A phylotype, which showed the highest daily mRNA abundances. The N₂-fixing assemblage extended to 60-80 m depth, well below the seasonal thermocline (40 m). The calculated areal N₂ fixation rate (15 μmol N m^-2 d^-1) was small compared with estimates from other regions of the Pacific; however, the estimated fixation rate was similar to other published results, suggesting that processes other than cellular growth rate may determine the abundance of unicellular diazotrophs. Despite the low N₂ fixation rates, the new nitrogen added to the euphotic zone by N₂ fixation could account for at least 10% of new production during the study period.