Extremophilicity, or the capability to thrive in environmental conditions
considered extreme is generally determined from the human perspective. From that
point of view, organisms adapted to scarce, or even the absence of molecular oxygen,
can be considered as one of the extremophiles, i.e., anaerobes. In this chapter, various
aspects of anaerobic microorganisms are addressed, including their different taxa, their
phylogenetic distribution, and the environments from where they have been isolated.
Since prokaryotic taxonomy is a dynamic process, here we have emphasized the
organisms that are validly placed in taxa and have cultured representatives. In this
section, Archaea and Bacteria - the two domains are separately discussed. Similar
separation is also maintained while discussing mechanisms of adaptation, as far as
possible. Since these two domains share certain properties, the subsequent sections are
not separated between these two domains.
Keywords: Anaerobic microorganisms, Anaerobic bacteria, Anaerobic archaea, ATP synthase, Biogeochemical cycle, Chemoorganotrophs, Extremophiles, Facultative anaerobes, Obligate anaerobes, Hydrothermal vent, Microaerophiles, Proteobacteria.