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AME 38:3-14 (2005)
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Abstract
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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
*Email: mjchurch@hawaii.edu
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ABSTRACT: In large areas of the worlds 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 l1 in the high-irradiance (>700 µmol quanta m2 s1), nitrate-depleted
(<10 nmol l1) 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 l1 in the upper photic zone. The abundance of Trichodesmium spp. ranged from 1 × 103 to 7 × 103 nifH gene copies
l1 in the upper ocean, declining to <100 nifH gene copies l1 below the mixed layer (~82 m). The Cluster III nifH phylotype was the most abundant nifH phylotype in the dimly lit (<12 µmol
quanta m2 s1) 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
Full text in pdf format
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Published in AME Vol.
38, No. 1
(2005) on January 21
Print ISSN: 0948-3055; Online ISSN: 1616-1564.
Copyright © Inter-Research, Oldendorf/Luhe, 2005
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