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Applied and Environmental Microbiology, September 2005, p. 5362-5370, Vol. 71, No. 9
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.9.5362-5370.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Temporal Patterns of Nitrogenase Gene (nifH) Expression in the Oligotrophic North Pacific Ocean

Ocean Sciences Department, University of California at Santa Cruz, Earth and Marine Sciences Building, Santa Cruz, California 95064,¹ Department of Oceanography, University of Hawaii, Honolulu, Hawaii 96822²

Received 12 November 2004/ Accepted 4 April 2005

Dinitrogen (N₂)-fixing microorganisms (diazotrophs) play important roles in ocean biogeochemistry and plankton productivity. In this study, we examined the presence and expression of specific planktonic nitrogenase genes (nifH) in the upper ocean (0 to 175 m) at Station ALOHA in the oligotrophic North Pacific Ocean. Clone libraries constructed from reverse-transcribed PCR-amplified mRNA revealed six unique phylotypes. Five of the nifH phylotypes grouped with sequences from unicellular and filamentous cyanobacteria, and one of the phylotypes clustered with -proteobacteria. The cyanobacterial nifH phylotypes retrieved included two sequence types that phylogenetically grouped with unicellular cyanobacteria (termed groups A and B), several sequences closely related (97 to 99%) to Trichodesmium spp. and Katagnymene spiralis, and two previously unreported phylotypes clustering with heterocyst-forming nifH cyanobacteria. Temporal patterns of nifH expression were evaluated using reverse-transcribed quantitative PCR amplification of nifH gene transcripts. The filamentous and presumed unicellular group A cyanobacterial phylotypes exhibited elevated nifH transcription during the day, while members of the group B (closely related to Crocosphaera watsonii) unicellular phylotype displayed greater nifH transcription at night. In situ nifH expression by all of the cyanobacterial phylotypes exhibited pronounced diel periodicity. The -proteobacterial phylotype had low transcript abundance and did not exhibit a clear diurnal periodicity in nifH expression. The temporal separation of nifH expression by the various phylotypes suggests that open ocean diazotrophic cyanobacteria have unique in situ physiological responses to daily fluctuations of light in the upper ocean.

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