Ubiquity of heterotrophic diazotrophs in marine microbial mats

Cyanobacteria are a dominant structural component of N-2-fixing microbial m ats in diverse marine ecosystems. As a result, much of the measured N-2 fix ation activity has been attributed to the cyanobacterial component. Until r ecently, the contribution of heterotrophic N-2 fixers has received much les s attention. In this study. the presence, diversity, and ubiquity of hetero trophic N-2 fixers were investigated in 3 cyanobacteria (Microcoleus)-domin ated, intertidal microbial mats obtained from Tomales Bay, California. Sipp ewissett Salt Marsh, Massachusetts. and Bird Shoal, North Carolina. Using P CR techniques, a diverse array of heretofore uncharacterized heterotrophic nifH (gene encoding the Fe-protein subunit of nitrogenase) sequences were f ound in these geographically disparate microbial mats, suggesting that hete rotrophic diazotrophs may play a larger role in N-2 fixation dynamics than previously thought. Phylogenetically similar heterotrophic diazotrophic seq uences were obtained from the 3 sites, potentially indicating that similar heterotrophic N-2-fixing communities are responsible for this process at di verse locations.