The Role of Sulfate Reduction in Stromatolites and Microbial Mats: Ancient and Modern Perspectives

Abstract

Sulfate reduction is an evolutionarily ancient process and sulfate-reducing microorganisms were likely key members of Precambrian stromatolite communities, as they are in modern photosynthetic microbial mats. Some of the highest rates of sulfate reduction ever measured have been observed in hypersaline microbial mats, supporting the view that sulfate respiration is a dominant carbon mineralization process in these communities. Sulfate consumption and the alkalinity that results from carbon utilization have also been linked to carbonate precipitation in lithified mats. Diverse groups of sulfate-reducing bacteria (SRB), primarily members of the Deltaproteobacteria, have been found to live in stratified zones in microbial mats, some localized near the surface despite high levels of oxygenic photosynthesis by cyanobacteria. Culture studies have shown that some SRB can switch to aerobic metabolism under microaerophlic conditions; however, it is not known how SRB tolerate the very high levels found in situ. Possible strategies involve aggregation and diel migration. Recent application of technologies such as nanoSIMS and metagenomics to mats have enabled ultra fine-scale mapping of sulfate reduction activity and have broadened our understanding of how sulfur metabolism fits into the broader picture of microbial diversity and functionality.