The following definition of geomicrobiology will provide a proper context for the discussion in this essay. Geomicrobiology is a study of the role that microbes have played in the geologic past from the time of their first appearance on the planet Earth about 4 eons ago to the present, and the role they are playing today and are likely to play in the future in some of the processes that are of fundamental importance to geology. The discussion will be restricted to current geomicrobial activities because being able to observe them directly, we know most about them. Geomicrobial activities in the geologic past have been deduced from the detection in the geologic record of (1) microbial fossils that morphologically resemble present-day microorganisms of geologic significance and (2) relevant biomarkers. Past geomicrobial activities have also been inferred from present-day geomicrobial activities that occur under conditions similar to those presumed to have existed in the geologic past. Molecular phylogeny is providing information that supports inferences about ancient geomicrobial activity.
Although geomicrobial agents that are presently recognized include members of the domains Bacteria (Eubacteria) and Archaea in the Prokaryota and members of Algae, Protozoa and Fungi in the Eukaryota, the following discussion will emphasize mainly geomicrobial activities of members of the Bacteria, Archaea and Fungi.
Types of geomicrobial activities
Geomicrobial activities play a role in (1) mineral formation, (2) mineral degradation, (3) the cycling of organic and inorganic matter, (4) chemical and isotopic fractionation and (5) fossil-fuel genesis and degradation.
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