Microbial mats, stromatolites and the rise of oxygen in the Precambrian atmosphere

Microbial mats are stratified communities that develop within the environmental microgradients established at the interfaces of water and solid substrates (Cohen, 1989). Stromatolites, the lithified remains of layered accumulations of microbial mats, occur in rocks as old as 3.5 Ga (Lowe, 1980; Walter et al., 1980). These lithified microbial communities represent the most ancient, widespread ecosystems known, and it is useful to explore their role in the accumulation of free oxygen in the ancient atmosphere.