Of Microbes and Minerals: Biogeochemistry of Microbialites through Geologic Time
Prof Pieter T. Visscher
Center for Integrative Geosciences, University of Connecticut, Storrs, CT
Department of Marine Sciences, University of Connecticut, Groton, CT
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 biomass production, binding and trapping of sediments, and mineral precipitation. Lithified mats, or microbialites date back over 3 billion years in the rock record.
The interpretation of fossil microbial mats in the rock record and, consecutively, assessment of their potential role in the alteration of Earth’s geochemical environment through time is hampered by the poor preservation of these organic-rich structures. The preservation potential, however, can be enhanced through microbially-mediated lithification. The three key components of microbially- mediated mineral precipitation are: 1) the “alkalinity” engine (i.e., microbial community metabolism and environmental conditions impacting the calcium (or magnesium) carbonate saturation index); 2) the 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)
While mineral shape and composition may be a function of the EPS properties and therefore has the potential to reflect a specific signature of the microbial community, it is unresolved how, for example, continuous laminae vs. clotted fabrics form. The cyanobacterial community, situated near the 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 a 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.
Biography
Pieter Visscher is director of the Center for Integrative Geosciences and Professor of Organic Geochemistry in the Department of Marine Sciences at the University of Connecticut, USA. He is a Fulbright Scholar, obtained a MS Cum Laude and a PhD from the University of Groningen, The Netherlands, was post-doc in Marine and Atmospheric Chemistry at the University of Miami, and research associate at the US Geological Survey in Menlo Park, CA. He joined UConn in 1994, has written 100 peer-reviewed publications and obtained over $15M in extramural grant support throughout his career. He is director of the Geomicrobiology Laboratory where he combines organic and inorganic geochemistry with microbiology to address environment and geologic questions. Visscher is founding member of NASA’s Astrobiology Institute and member of the Australian Centre for Astrobiology.
