ASLO/TOS Ocean Research Conference, Honolulu, HI, February 2004, p. 28.

Diversity and vertical distributions of nitrogen-fixing bacteria in the oligotrophic North Pacific Ocean

M. J. Church¹, B. D. Jenkins¹, D. M. Karl², S. M. Short¹, E. O. Omoregie¹, J. P. Zehr¹

¹University of California Santa Cruz, Santa Cruz, USA

²University of Hawaii, Honolulu, USA

The diversity of upper ocean nitrogen fixing bacteria was examined using RT-PCR amplification and sequencing of the dinitrogenase gene (nifH) at Station ALOHA in the oligotrophic North Pacific Ocean. Expression of nifH by upper ocean plankton revealed that diverse diazotrophic bacteria, includng various members of the cyanobacteria and proteobacteria, might influence nutrient cycling and productivity in this ecosystem. The vertical distributions of two unicellular nifH cyanobacteria and a Cluster III nifH phylotype (presumably anaerobes) were evaluated using quantitative PCR. The gene abundances of all three nifH phylotypes investigated had distinct vertical profiles; the unicellular cyanobacteria accounted for approximately 200 nifH gene copies ml^-1 in high irradiance, nitrate-depleted upper ocean, while the Cluster III nifH phylotype accounted for approximately 100 nifH copies ml^-1. The combined abundances of the unicellular nifH cyanobacterial phylotypes were equivalent to roughly 5% of the depth integrated (0-100m) Synechococcus abundance at station ALOHA. These results reveal that diverse diazotrophic plankton play important biogeochemical and ecological roles in oligotrophic ocean ecosystems.