Regional distributions of nitrogen-fixing bacteria in the Pacific Ocean

We evaluated the regional distributions of six nitrogen (N2)-fixing bacteria in the North Pacific Ocean using quantitative polymerase chain reaction amplification of planktonic nifH genes. Samples were collected on four oceanographic research cruises between March 2002 and May 2005 that spanned a latitudinal range from 12uS and 54uN between 152uW and 170uW. Samples were collected throughout the upper ocean (,200 m) in the northern regions of the South Pacific Subtropical Gyre (SPSG), equatorial waters, the North Pacific Subtropical Gyre (NPSG), the North Pacific Transitional Zone (NPTZ), and within the Pacific Sub Arctic Gyre (PSAG). There were distinct spatial gradients in concentrations of nutrients, chlorophyll, and the abundances of N2-fixing bacteria within the various oceanic biomes. In general, nifH-containing bacteria were most abundant in the midregions of the NPSG (latitudes between ,14uN and 29uN), where unicellular cyanobacterial phylotypes dominated nifH gene abundances. The abundances of all nifH-containing groups declined within the northern and southern regions of NPSG. Although nifH-containing groups were detectable in the northern regions of the SPSG, throughout the equatorial waters, and within the NPTZ, gene copy abundances of most groups were lower in these regions than those found the in the NPSG. In the NPSG, surface water abundances of the various nifH phylotypes examined ranged from ,50 copies L21 to ,105 nifH copies L21. Overall, the abundances of an uncultivated, presumed unicellular nifH sequence-type (termed Group A) were the most abundant and widely distributed of the phylotypes examined. Our results indicate that the distributions of N2-fixing plankton were largely restricted to the subtropical regions of the North and South Pacific Oceans. In large areas of the world’s oceans, the activities of nitrogen (N2)-fixing microorganisms (diazotrophs) provide an important source of nitrogen to the marine ecosystem. Planktonic diazotrophs provide a major fraction of the exogenous nitrogen supporting both productivity and carbon export in many tropical and subtropical ocean environments (Capone and Carpenter 1982; Michaels et al. 1996; Karl et al. 2002). Time-series measurements of particulate nitrogen export in the oligotrophic North Pacific Ocean indicate that N2-fixing microorganisms contribute more than half of the new nitrogen supporting particulate matter export, rivaling or surpassing the contribution by nitrate in supporting new production in this ecosystem (Karl et al. 1997; Dore et al. 2002). The biological reduction of N2 gas to ammonia (NH3) is catalyzed by the enzyme nitrogenase. Nitrogenase consists of two conserved proteins: an iron (Fe) containing dinitrogenase reductase (or Fe protein), encoded by the nifH gene, and a molybdenum iron (MoFe) dinitrogenase (or MoFe protein) that is encoded by the nifDK genes. Developments of molecular-based techniques to interrogate nifH gene sequences in marine environments have provided unique tools for assessing the diversity of organisms with the genetic capacity for N2 fixation (Zehr and McReynolds 1989; Zehr et al. 1996). Much of our understanding of the ecology and biogeochemistry of oceanic diazotrophy has derived from studies on the filamentous cyanobacterium Trichodesmium spp., a cosmopolitan cyanobacterium in tropical and subtropical marine systems, including oligotrophic regions throughout the Atlantic and Pacific Oceans (Capone and Carpenter 1982; Carpenter and Romans 1991; Letelier and Karl 1996), the Red Sea (Post et al. 2002), and the Arabian Sea (Capone et al. 1998). However, use of the nifH gene as a genetic marker for N2-fixing plankton has identified greater diversity among the N2-fixing marine microorganisms than previously recognized. In particular, several 1 Corresponding author ([email protected]).