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

Published in AME Vol. 19, No. 1 (1999) on September 6
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 1999