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Applied and Environmental Microbiology, September 2001, p. 4009-4016, Vol. 67, No. 9
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.9.4009-4016.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

nifH Sequences and Nitrogen Fixation in Type I and Type II Methanotrophs

Ann J. Auman,1,* Catherine C. Speake,1 and Mary E. Lidstrom1,2

Departments of Microbiology1 and Chemical Engineering,2 University of Washington, Seattle, Washington 98195

Received 7 February 2001/Accepted 12 June 2001

Some methane-oxidizing bacteria (methanotrophs) are known to be capable of expressing nitrogenase and utilizing N2 as a nitrogen source. However, no sequences are available for nif genes in these strains, and the known nitrogen-fixing methanotrophs are confined mainly to a few genera. The purpose of this work was to assess the nitrogen-fixing capabilities of a variety of methanotroph strains. nifH gene fragments from four type I methanotrophs and seven type II methanotrophs were PCR amplified and sequenced. Nitrogenase activity was confirmed in selected type I and type II strains by acetylene reduction. Activities ranged from 0.4 to 3.3 nmol/min/mg of protein. Sequence analysis shows that the nifH sequences from the type I and type II strains cluster with nifH sequences from other gamma proteobacteria and alpha proteobacteria, respectively. The translated nifH sequences from three Methylomonas strains show high identity (95 to 99%) to several published translated environmental nifH sequences PCR amplified from rice roots and a freshwater lake. The translated nifH sequences from the type II strains show high identity (94 to 99%) to published translated nifH sequences from a variety of environments, including rice roots, a freshwater lake, an oligotrophic ocean, and forest soil. These results provide evidence for nitrogen fixation in a broad range of methanotrophs and suggest that nitrogen-fixing methanotrophs may be widespread and important in the nitrogen cycling of many environments.

* Corresponding author. Mailing address: Department of Microbiology, University of Washington, Box 357242, Seattle, WA 98195. Phone: (206) 616-6954. Fax: (206) 616-5721. E-mail: aauman{at}u.washington.edu.

Applied and Environmental Microbiology, September 2001, p. 4009-4016, Vol. 67, No. 9
0099-2240/01/$04.00+0   DOI: 10.1128/AEM.67.9.4009-4016.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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