The ability of microorganisms to thrive under oxygen-free conditions in subsurface
environments relies on the enzymatic reduction of oxidized elements, such as sulfate,
ferric iron, or CO2, coupled to the oxidation of inorganic or organic compounds. A broad
phylogenetic and functional diversity of microorganisms from subsurface environments has
been described using isolation-based and advanced molecular ecological techniques. The
physiological groups reviewed here comprise iron-, manganese-, and nitrate-reducing
microorganisms. In the context of recent findings also the potential of chlorate and
perchlorate [jointly termed (per)chlorate] reduction in oil reservoirs will be discussed.
Special attention is given to elevated temperatures that are predominant in the deep
subsurface. Microbial reduction of (per)chlorate is a thermodynamically favorable redox
process, also at high temperature. However, knowledge about (per)chlorate reduction
at elevated temperatures is still scarce and restricted to members of the Firmicutes
and the archaeon Archaeoglobus fulgidus. By analyzing the diversity and phylogenetic
distribution of functional genes in (meta)genome databases and combining this knowledge
with extrapolations to earlier-made physiological observations we speculate on the potential
of (per)chlorate reduction in the subsurface and more precisely oil fields. In addition, the
application of (per)chlorate for bioremediation, souring control, and microbial enhanced oil
recovery are addressed.