Microbialite

Microbialite (microbial + ite) is a rock or benthic sedimentary deposit made of carbonate mud (particle diameter < 5 μm) that is formed with the mediation of microbial activity. The constituent carbonate mud is a type of automicrite, or autigenic carbonate mud, and therefore it precipitates in situ instead of being transported and deposited.

Been formed in situ, a microbialite can be seen as a type of boundstone where reef builders are microbes, and precipitation of carbonate is biotically induced instead of forming tests, shells or skeletons.

Bacteria can precipitate carbonate both in shallow and in deep water (except for cyanobacyeria) and so microbialites can form regardless of the sun linght.

Classification

Microbialites can have three different fabrics: (INSERT PLOT)

• Stromatolitic: microbialite layered, laminated or agglutinated to form a stromatolite.
• Thrombolitic: microbialite with a clotted peloidal fabric if observed with a petrographic microscope. The density of peloids is variable. At the scale of the hand sample, the rock shows a dendritic fabric, and can be named thrombolite.
• Leiolitic: a microbialite with no layering nor clotted peloidal fabric. It is only made of a dense automicrite.

Microbes

Microbes that precipitate carbonate to built microbialites are mostly procariota, which include bacteria and archaea. The best known carbonate-producing bacteria are cyanobacteria that are organized into colonies that form filaments. Archaea are often estremophiles and thus live in remote environments where other microbes cannot live, such as white smokers at the bottom of the oceans.

Eucariota microbes, instead, produce less carbonate than prokariota.