Thermoacidophile

A thermoacidophile is an extremophilic microorganism that is both thermophilic and acidophilic; i.e., it can grow under conditions of high temperature and low pH.^[1] The large majority of thermoacidophiles are archaea (particularly the Thermoproteota and "Euryarchaeota") or bacteria, though occasional eukaryotic examples have been reported.^[2][3] Thermoacidophiles can be found in hot springs and solfataric environments, within deep sea vents, or in other environments of geothermal activity.^[1]: 602 They also occur in polluted environments, such as in acid mine drainage.^[4]

Biotopes that favor thermoacidophiles can be found both on land and in the sea, where the mineral composition of the water typically consists of highly reduced compounds such as various sulfides, and highly oxidized sulfates. The conversion of reduced sulfides to oxidized sulfates leads to a production of protons, lowering the pH^[1] of the surrounding environment. While reduced sulfides are generally considered to be reactive, their conversion to their oxidized counterpart by abiotic natural processes (reacting with things that aren’t living organisms) is relatively low. This fact emphasizes the importance of bio-oxidizers (i.e. thermoacidophiles) in constructing and maintaining this ecological niche.^[5] Most of the microbes in these harsh environments are chemolithoautotrophs^[6] (they gain electrons from pre-formed inorganic compounds, and use carbon dioxide as a carbon source), which have evolved specific adaptations to inhabit and grow in such selective environments. Archaea are unique in their ability to thrive in these environments, as many bacterial and eukaryotic organisms are limited to tolerance of such acidic (pH < 3.5), thermal (T> 65 °C) environments and don’t demonstrate sustained thermoacidophilicity.^[6] However, the genome of a thermoacidophilic eukaryote, the red algae Galdieria sulphuraria, revealed that its environmental adaptations likely originated from horizontal gene transfer from thermoacidophilic archaea and bacteria.^[2]

An apparent tradeoff has been described between adaptation to high temperature and low pH; relatively few examples are known that are tolerant of the extremes of both environments (pH < 2, growth temperature > 80 °C).^[1] Adaptations that allow them to survive in these harsh environments include proton pumps and buffering strategies, epigenetic modifications of the chromosome, and altered membrane structures. Many thermoacidophilic archaea have aerobic or microaerophilic metabolism,^[1]: 602 although obligately anaerobic examples (e.g. the Acidilobales) have also been identified.^[6]