AME 19:29-36 (1999)  -  doi:10.3354/ame019029

Ubiquity of heterotrophic diazotrophs in marine microbial mats

Julie B. Olson1,*, R. Wayne Litaker2, Hans W. Paerl1,**

1Institute of Marine Sciences, University of North Carolina at Chapel Hill, Morehead City, North Carolina 28557, USA
2Program in Molecular Biology and Biotechnology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA
*Present address: Harbor Branch Oceanographic Institution, Fort Pierce, Florida 34946, USA
**Addressee for correspondence. E-mail:

ABSTRACT: Cyanobacteria are a dominant structural component of N2-fixing microbial mats in diverse marine ecosystems. As a result, much of the measured N2 fixation activity has been attributed to the cyanobacterial component. Until recently, the contribution of heterotrophic N2 fixers has received much less attention. In this study, the presence, diversity, and ubiquity of heterotrophic N2 fixers were investigated in 3 cyanobacteria (Microcoleus)-dominated, intertidal microbial mats obtained from Tomales Bay, California, Sippewissett Salt Marsh, Massachusetts, and Bird Shoal, North Carolina. Using PCR techniques, a diverse array of heretofore uncharacterized heterotrophic nifH (gene encoding the Fe-protein subunit of nitrogenase) sequences were found in these geographically disparate microbial mats, suggesting that heterotrophic diazotrophs may play a larger role in N2 fixation dynamics than previously thought. Phylogenetically similar heterotrophic diazotrophic sequences were obtained from the 3 sites, potentially indicating that similar heterotrophic N2-fixing communities are responsible for this process at diverse locations.


KEY WORDS: Microbial mat · Heterotrophic bacteria · N2 fixation · Cyanobacteria


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