Vertical distributions of nitrogen-fixing phylotypes at Stn Aloha in the oligotrophic North Pacific Ocean

In large areas of the world's oceans, biological dinitrogen (N 2 ) fixation supports a significant fraction of ecosystem productivity; to date, however, there is little information on the abundances of specific diazotrophs in the ocean. In this study, the vertical distributions of several different groups of N 2 -fixing bacteria were examined using quantitative polymerase chain reaction (QPCR) amplification of group-specific dinitrogenase reductase (nifH) genes from Stn ALOHA in the subtropical North Pacific Ocean. Depth distributions ( 700 μmol quanta m -2 s -1 ), nitrate-depleted ( 100 m). These results revealed differences in the depth distributions of N 2 -fixing plankton at Stn ALOHA, and suggest that unicellular diazotrophs comprise a significant component of plankton biomass in this oligotrophic marine ecosystem.