AME 38:3-14 (2005)  -  doi:10.3354/ame038003

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

Matthew J. Church1,3,*, Bethany D. Jenkins1, David M. Karl2, Jonathan P. Zehr1

1Ocean Sciences Department, University of California at Santa Cruz, Earth and Marine Sciences Building, Santa Cruz,California 95064, USA
2Department of Oceanography, University of Hawaii, Honolulu, Hawaii 96822, USA
3Present address: Department of Oceanography, University of Hawaii, Honolulu, Hawaii 96822, USA

ABSTRACT: In large areas of the world’s oceans, biological dinitrogen (N2) 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 N2-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 (<200 m) of 3 cyanobacterial nifH phylotypes and 1 previously uncharacterized Cluster III nifH phylotype were evaluated. The nifH-containing cyanobacteria included sequence-types similar to Trichodesmium spp. and 2 nifH sequences closely related to unicellular cyanobacteria (termed Groups A and B). The Group A cyanobacteria (most closely related to Cyanothece sp.) were the most abundant of all phylotypes examined, comprising 2 × 105 nifH gene copies l-1 in the high-irradiance (>700 µmol quanta m-2 s-1), nitrate-depleted (<10 nmol l-1) upper-ocean waters. Group B cyanobacterial phylotypes (most closely related to Crocosphaera watsonii) demonstrated a depth distribution similar to Group A, but Group B nifH abundance was considerably lower, averaging 2 × 103 nifH gene copies l-1 in the upper photic zone. The abundance of Trichodesmium spp. ranged from 1 × 103 to 7 × 103 nifH gene copies l-1 in the upper ocean, declining to <100 nifH gene copies l-1 below the mixed layer (~82 m). The Cluster III nifH phylotype was the most abundant nifH phylotype in the dimly lit (<12 µmol quanta m-2 s-1) lower photic zone (>100 m). These results revealed differences in the depth distributions of N2-fixing plankton at Stn ALOHA, and suggest that unicellular diazotrophs comprise a significant component of plankton biomass in this oligotrophic marine ecosystem.

KEY WORDS: Nitrogen fixation · Bacterial diversity · Quantitative PCR · Hawaii Ocean · Time-series

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