Modern Microbialites: A Lesson for the 3.5-Billion-Year Record of Microbe-Sediment Interactions?

Mineral deposits resulting from organomineralization (microbially–induced and microbially-influenced mineralization) are called microbialites. Most microbialites can be classified into one of three main categories based on their macroscopic features: stromatolites, showing a laminated macrofabric, thrombolites, displaying a clotted macrofabric, and leiolites, without well defined macrofabric. Microbially-induced mineralization appears near the beginning of Earth's history, as the microbial communities thriving in the Precambrian ocean orchestrated the precipitation of calcium carbonate to form laminated microbialites. These microbialites were most likely produced by microbial biofilm communities, or microbial mats.

Contemporary microbial mats, which contain copious amounts of exopolymeric substances (EPS) are characterized by high metabolic rates, and coupled to this, rapid cycling of major elements on micrometer scales. The activity of the mat communities has changed the geochemical conditions on micro- and macro-scales: On a sub-mm scale within the mat ecosystem, this has led to the formation of resilient biofilms, at the scale of the entire Earth's biosphere this has resulted in a major shift in redox conditions, the consequences of which are well-known.

The interpretation of fossil microbial mats and their potential role in the alteration of the geochemical environment is challenging because of the poor preservation of the organic material (e.g., EPS) in mats. However, the preservation in the fossil record can be enhanced through lithification. Carbonate precipitation mediated by microbial processes is one of the most important mechanisms, which increases the conservation potential. Key components of the microbially-mediated mineralization process are the “alkalinity” engine (microbial metabolism and environmental conditions impacting the calcium carbonate saturation index) and the organic matrix comprised of EPS.

This presentation reviews organomineralization, focusing on the specific role of microbes, the EPS matrix and chemical communication in carbonate mineral formation. Examples of modern aquatic (freshwater, marine and hypersaline) microbialites will be discussed.