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

Abstract

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 N2 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.