Modern arsenotrophic microbial mats provide an analogue for life in the anoxic Archean

The earliest evidence of life captured in lithified microbial mats (microbialites) predates the onset of oxygen production and yet, modern oxygenic mats are often studied as analogs based on their morphological similarity and their sedimentological and biogeochemical context. Despite their structural similarity to fossil microbialites, the presence of oxygen in most modern microbial mats disqualifies them as appropriate models for understanding early Earth conditions. Here we describe the geochemistry, element cycling and lithification potential of microbial mats that thrive under permanently anoxic conditions in arsenic laden, sulfidic waters feeding Laguna La Brava, a hypersaline lake in the Salar de Atacama of northern Chile. We propose that these anoxygenic, arsenosulfidic, phototrophic mats are a link to the Archean because of their distinctive metabolic adaptations to a reducing environment with extreme conditions of high UV, vast temperature fluctuations, and alkaline water inputs from combined meteoric and volcanic origin, reminiscent of early Earth.