Physiological Studies of Extremely Thermoacidophilicmicroorganisms Under Normal and Stressed Conditions

Abstract

The purpose of this research was to study thermoacidophilic physiology for the purpose of developing a new strategy for metal and sulfur biotransformation processes. Metallosphaera sedula, an extremely thermoacidophilic archaeon growing mixotrophically on organic and inorganic substrates at pH 2.0 and 74° C, was used as a model organism to address this issue. The manipulation of M. sedula's energetics under thermally and chemically challenged states led to increase in specific leaching rates. A dual-limited continuous culture, at a dilution rate of 0.04 hr, was used to provide a steady-state operation at which the cellular and sub-cellular responses were monitored and the biocatalytic efficiency of cells was evaluated. Under the thermal stress, M. sedula showed acquired thermotolerance and 'stressed-phase' growth at 81C, which is 2C is higher than its normal maximum growth temperature, and over-synthesized (~6-fold increase) a 66 kDa heat shock protein (MseHSP60), which is immunologically related to a molecular chaperone (Thermophilic Factor 55) from Sulfolobus shibatae. More importantly, however, there was a significant increase in specific iron turnover rate presumably through disruption of cellular proton network. In addition, although less dramatic when compared to thermal stress effects, similar results could be obtained when M. sedula was subjected to the chemical stress. By temporarily short-circuiting M. sedula's proton network with uncouplers, the respiration capacity of cells was disrupted, leading to the increased Fe³⁺ iron turnover rate. The work described here should propose a new approach for analyzing physiological responses under extreme growth conditions and thereby points to novel strategies for improving whole-cell catalytic efficiency of thermoacidophilic microorganisms.