Intersection of Biotic and Abiotic Sulfur Chemistry Supporting Extreme Microbial Life in Hot Acid

Microbial life on Earth exists within
wide ranges of temperature,
pressure, pH, salinity, radiation, and water activity. Extreme thermoacidophiles,
in particular, are microbes found in hot, acidic biotopes laden with
heavy metals and reduced inorganic sulfur species. As chemolithoautotrophs,
they thrive in the absence of organic carbon, instead using sulfur
and metal oxidation to fuel their bioenergetic needs, while incorporating
CO2 as a carbon source. Metal oxidation by these microbes
takes place extracellularly, mediated by membrane-associated oxidase
complexes. In contrast, sulfur oxidation involves extracellular, membrane-associated,
and cytoplasmic biotransformations, which intersect with abiotic sulfur
chemistry. This novel lifestyle has been examined in the context of
early aerobic life on this planet, but it is also interesting when
considering the prospects of life, now or previously, on other solar
bodies. Here, extreme thermoacidophily (growth at pH below 4.0, temperature
above 55 °C), a characteristic of species in the archaeal order
Sulfolobales, is considered from the perspective of sulfur chemistry,
both biotic and abiotic, as it relates to microbial bioenergetics.
Current understanding of the mechanisms involved are reviewed which
are further expanded through recent experimental results focused on
imparting sulfur oxidation capacity on a natively nonsulfur oxidizing
extremely thermoacidophilic archaeon, Sulfolobus acidocaldarius, through metabolic engineering.