ABSTRACT
Geomicrobial phenomena can be studied in the eld (in situ) and in the laboratory (in vitro), in microcosms or as isolated reactions. Field study of a given geomicrobial phenomenon should ideally involve identication and enumeration of the active microorganisms and in situ measurements of their metabolic activities and growth rate. It should also involve chemical and physical identication of the substrates and nutrients, that is, the reactants (e.g., minerals [insoluble] and dissolved inorganic or organic substances) and the products that are formed in the processes surrounding metabolism and growth. Furthermore, it should involve measurement of the overall rate at which the process occurs and assessment of the impact of different environmental factors on it. In
practice, however, it may happen that a suspected geomicrobial process is no longer operating at a given site but took place in the geologic past. In such instances, the role of the microorganisms in the process has to be reconstructed from microscopic observations (e.g., searching for biosignatures or microfossils associated with the starting materials, if still present, and especially the products of the process). It may also be reconstructed from geochemical observations such as biomarker (ngerprint) compounds in sedimentary rock, which indicate the past existence of an organism or a group of organisms that could have been the geochemical agents responsible. If applicable, evidence of isotopic fractionation of a key element relevant to the geomicrobial process should be sought.
