Abstract
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.
Highlights
The earliest evidence of life captured in lithified microbial mats 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
The La Brava spring is fed continuously by alkaline groundwater leached through calc-alkaline volcanic rock[44]
These lakes likely comprised diverse microbial habitats several 100 million years before the Great Oxidation Event (GOE), some anoxygenic phototrophic[28,47], others possibly supporting some level of oxygenic photosynthesis[17,47,48,49]
Summary
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. 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. Based on evidence from early Archean microbialites, hydrogen, ferrous iron, reduced sulfur, nitrite, or possibly organic molecules were possible electron donors for anoxygenic photosynthesis[22] It was not until the discovery of arsenotrophy in modern systems of Mono Basin, California[23,24,25] that the use of As(III) for photosynthesis in the Precambrian was proposed as well[26]. The antiquity of this metabolism is shown by phylogenetic evidence in the deeply rooted Crenarchaeota[29]
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