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. The large majority of thermoacidophiles are archaea (particularly the Thermoproteota and "Euryarchaeota") or bacteria, though occasional eukaryotic examples have been reported. Thermoacidophiles can be found in hot springs and solfataric environments, within deep sea vents, or in other environments of geothermal activity. They also occur in polluted environments, such as in acid mine drainage.
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 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 are not living organisms) is relatively low. This fact emphasizes the importance of bio-oxidizers (i.e. thermoacidophiles) in constructing and maintaining this ecological niche. Most of the microbes in these harsh environments are chemolithoautotrophs (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 do not demonstrate sustained thermoacidophilicity. 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. 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). 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, although obligately anaerobic examples (e.g. the Acidilobales) have also been identified.
Unique biological adaptations
Chromosome structure and epigenetic modifications Most thermoacidophiles are archaeal, with Crenarchaea belonging to the Sulfolobales order serving as a model system. Many of the biological mechanisms used by Crenarchae are shared by all archaea but there are some lineage-specific differences unique to thermoacidophilic archaea. One example of the unique differences for thermoacidophilic archaea is their lack of eukaryote-like histones, which are typically involved in the packaging and reorganization of the chromosome, all Thermoplasma and Crenarchaeota lack histone like proteins. Thermoacidophillic archaea typically have single, small, circular chromosome between 1.5 and 3 Mbp in length. Chromatin proteins are used to condense and organize the genome, however this is not done with histone-orthologs as in eukaryotes or some bacteria, a large evolutionary divergence that characterizes thermoacidophilic archaea. Instead of using histones, a type of protein called nucleoid associated proteins (NAPs) are expressed, however the degree of conservation between species varies from protein to protein. These proteins are typically between 7 and 10 kDa in size, are basic, and account for up to 5% of cellular protein, making them one of the most highly expressed proteins in the cell. The functionality of NAPs can be altered via post-translational modifications (PTMs). These epigenetic modifications have significant impacts on their functionality and the fitness of thermophiles in extreme environments. Methylation, a form of PTM, is a common alteration to NAP structure. It has been linked to the thermostabilization of the proteins as well as the regulation of genes in epigenetic studies. An example of the impact of methylation can be seen in an adaptive laboratory evolution experiment, in which a strain of Sa. solfataricu developed a super-acid resistant phenotype, even though its genome had not changed from the reference sequence. Investigation revealed that the acid resistance was conferred by a difference in the methylation of Sso7d and Cren7, both Naps. This highlights how a difference in methylation can have a significant impact on the fitness of thermophiles. In addition to local structuring by NAPs, the Sulfolobales genome is also globally compartmentalized into two sub-Mbp compartments. Each compartment, often referred to as the A and B compartment respectively, is populated by specific sets of genes that vary in their level of transcription. The A compartment generally contains genes involved in essential biological processes such as the creation of metabolic proteins, which are highly expressed in the cell. The B compartment holds genes related to environmental stress responses, CRISPR-Cas clusters and fatty acid metabolism. The exact mechanism regarding the global restructuring of the genome is not currently known, however a protein known as coalescin has been found to play a critical role in the restructuring, with an inverse correlation observed between the occurrence of coalescin and the transcriptional activity of genes in the B compartment.
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