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Aquatic Microbial Ecology

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AME 61:235-247 (2010)  -  DOI:

Nitrogenase genes in non-cyanobacterial plankton: prevalence, diversity and regulation in marine waters

Lasse Riemann1,3,*, Hanna Farnelid1, Grieg F. Steward2

1Department of Natural Sciences, Linnaeus University, 39182, Kalmar, Sweden
2Department of Oceanography, School of Ocean & Earth Science & Technology, University of Hawaii, Honolulu, Hawaii 96822, USA
3Present address: Marine Biological Laboratory, University of Copenhagen, 3000 Helsingør, Denmark

ABSTRACT: Marine waters are generally considered to be nitrogen (N) limited and are therefore favourable environments for diazotrophs, i.e. organisms converting atmospheric N2 into ammonium or nitrogen oxides available for growth. In some regions, this import of N supports up to half of the primary productivity. Diazotrophic Cyanobacteria appear to be the major contributors to marine N2 fixation in surface waters, whereas the contribution of heterotrophic or chemoautotrophic diazotrophs to this process is usually regarded inconsequential. Culture-independent studies reveal that non-cyanobacterial diazotrophs are diverse, widely distributed, and actively expressing the nitrogenase gene in marine and estuarine environments. The detection of nifH genes and nifH transcripts, even in N-replete marine waters, suggests that N2 fixation is an ecologically important process throughout the oceans. Because this process is highly sensitive to and inhibited by molecular oxygen (O2), diazotrophy requires efficient scavenging of intracellular O2 or growth in environments with low ambient O2 concentration. Particles with interior low-O2 micro-zones and oceanic oxygen minimum zones are just 2 potential habitats suitable for N2 fixation by non-cyanobacterial diazotrophs. Our ignorance about the regulation of N2 fixation by non-Cyanobacteria in their natural marine environments currently prevents an evaluation of their importance in marine N cycling and budgets. A review of the molecular data on distribution and expression of nifH genes in non-Cyanobacteria suggests that further study of the role of these Bacteria in N cycling at local, regional and global scales is needed.

KEY WORDS: N2 fixation · Nitrogen fixation · Non-Cyanobacteria · nifH · Nitrogenase

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Cite this article as: Riemann L, Farnelid H, Steward GF (2010) Nitrogenase genes in non-cyanobacterial plankton: prevalence, diversity and regulation in marine waters. Aquat Microb Ecol 61:235-247.

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