Expanding the Limits of Thermoacidophily in the Archaeon Sulfolobus solfataricus by Adaptive Evolution

Extremely thermoacidophilic Crenarchaeota belonging to the family Sulfolobales flourish in hot acidic habitats that are strongly oxidizing. The pH extremes of these habitats however, often exceed the acid tolerance of type species and strains. Here, adaptive laboratory evolution was used over a three year period to test whether such organisms harbor additional thermoacidophilic capacity. Three distinct cell lines derived from a single type species were subjected to high temperature serial passage while culture acidity was gradually increased. A 178-fold increase in thermoacidophily was achieved after 29 increments of shifted culture pH resulting in growth at pH 0.8 and 80 degrees C. These strains were named super acid resistant Crenarchaeota (SARC). Mathematical modeling using growth parameters predicted the limits of acid resistance while genome and transcriptome resequencing were conducted for insight into mechanisms responsible for the evolved trait. Among the mutations that were detected, a set of eight nonsynonymous changes may explain the heritability of increased acid resistance despite an unexpected lack of transposition. Four multi-gene components of the SARC transcriptome implicated oxidative stress as a primary challenge accompanying growth at acid extremes. These included accelerated membrane biogenesis, induction of the mer operon, and increased capacity for generation of energy and reductant.

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