The bioenergetic response of the extremely thermoacidophilic archaeon Metallosphaera sedula to thermal and nutritional stresses was examined. Continuous cultures (pH 2.0, 70°C, and dilution rate of 0.05 h-1) in which the levels of Casamino Acids and ferrous iron in growth media were reduced by a step change of 25 to 50% resulted in higher levels of several proteins, including a 62-kDa protein immunologically related to the molecular chaperone designated thermophilic factor 55 in Sulfolobus shibatae (J. D. Trent, J. Osipiuk, and T. Pinkau, J. Bacteriol. 172:1478-1484, 1990), on sodium dodecyl sulfate-polyacrylamide gels. The 62-kDa protein was also noted at elevated levels in cells that had been shifted from 70 to either 80 or 85°C. The proton motive force (Δp), transmembrane pH (ΔpH), and membrane potential (ΔΨ) were determined for samples obtained from continuous cultures (pH 2.0, 70°C, and dilution rate of 0.05 h-1) and incubated under nutritionally and/or thermally stressed and unstressed conditions. At 70°C under optimal growth conditions, M. sedula was typically found to have a Ap of approximately -190 to -200 mV, the result of an intracellular pH of 5.4 (extracellular pH, 2.0) and a ΔΨ of +40 to +50 mV (positive inside). After cells had been shifted to either 80 or 85°C, ΔΨ decreased to nearly 0 mV and internal pH approached 4.0 within 4 h of the shift; respiratory activity, as evidenced by iron speciation in parallel temperature-shifted cultures on iron pyrite, had ceased by this point. If cultures shifted from 70 to 80°C were shifted back to 70°C after 4 h, cells were able to regain pyrite oxidation capacity and internal pH increased to nearly normal levels after 13 h. However, Δψ remained close to 0 mV, possibly the result of enhanced ionic exchange with media upon thermal damage to cell membranes. Further, when M. sedula was subjected to an intermediate temperature shift from 73 to 79°C, an increase in pyrite dissolution (ferric iron levels doubled) over that of the unshifted control at 73°C was noted. The improvement in leaching was attributed to the synergistic effect of chemical and biological factors. As such, periodic exposure to higher temperatures, followed by a suitable recovery period, may provide a basis for improving bioleaching rates of acidophilic chemolithotrophs.
All Science Journal Classification (ASJC) codes
- Food Science
- Applied Microbiology and Biotechnology