MICROBIAL MATS AND MICROBIALITES:UPSIDE-DOWN GEOCHEMISTRY AND EXTRACELLULAR ORGANIC MATTER

Microbial mats are organosedimentary biofilms that greatly impacted the geochemical and physicochemical conditions on Earth through geological time. These laminated ecosystems are formed by various geomicrobiological processes, including prokaryotic biomass production, binding and trapping of sediments, and mineral precipitation. Lithified mats, or microbialites date back in the rock record well over 3 billion years.

The interpretation of fossil microbial mats in the rock record and, consecutively, assessment of their role in altering the Earth’s geochemical environment through time is hampered by the relatively poor preservation of these organic-rich structures. However, the preservation potential can be enhanced through microbially-mediated lithification (microbially-induced organomineralization). The three key main components of microbially-mediated mineral precipitation are: 1) an "alkalinity" engine (i.e., microbial community metabolism and environmental conditions impacting the CaCO₃ saturation index); 2) a complex organic matrix comprised of exopolymeric substances (EPS); and 3) the coordination of community physiologies and sensing of environmental conditions (e.g., pH, oxygen concentration) through chemical communication, or quorum sensing. These combined geochemical-microbial activities provide conditions that allow specific microbialites to form, both on a macroscale (i.e., morphology) as well as on a microscale (i.e., shape and composition of minerals)

The cyanobacterial community, situated near the mat surface according to the ambient light conditions, provides the organic carbon for heterotrophs. All these respiring organisms (including “strict” anaerobes, such as sulfate-reducing bacteria and methanogens) display their maximum metabolic activity along the surface horizon that may lithify. Some ideas emerge how chemical communication may play a role in this, and how microbial signaling compounds may be used to detect specific environmental conditions and may allow synchronizing of intra- and interspecies metabolic activities. These recent observations and ideas are, however, merely a first step in the understanding of microbialite formation, and their potential to weather the diagenetic processes so that some of the biological signatures are preserved.