Ubiquity of heterotrophic diazotrophs in marine microbial mats

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.