Alkalithermophiles are an exciting subset of extremophilic organisms and represent extremophiles that are adapted to two extreme conditions, i.e. to a combination of alkaline and thermobiotic growth conditions. Among the anaerobic alkalithermophiles are representatives of both Bacteria and Archaea within a wide variety of physiological types and systematic groups, although a great majority belongs to the Firmicutes. Alkaliphiles have been isolated from a variety of niches including mesobiotic and neutrophilic soils and sediments. Interestingly anaerobic isolates from mesobiotic and neutrophilic niches exhibit shorter doubling times than isolates from thermobiotic niches; some anaerobic alkalithermophiles exhibit extremely fast growth rates, i.e. doubling times as short as 10 min. Their adaptation to both high pH and high temperature draws our attention not only because they are potential sources of industrially valuable enzymes but also because of their adaptive mechanisms to external environmental parameters. They could thus function as model organisms for extraterrestrial life in some environments and for theories on the origin of life. Alkalithermophiles, as far we know, do not represent the most thermophilic nor the most alkaliphilic of micro-organisms but represent the most alkaliphilic ones among the thermophiles and vice versa. We believe that the presently known species are only the tip of the iceberg and therefore that they do not represent the true boundaries under which life can thrive in respect to high temperature in alkaline environments.

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