BIOENERGETIC RESPONSE OF THE EXTREME THERMOACIDOPHILE METALLOSPHAERA-SEDULA TO THERMAL AND NUTRITIONAL STRESSES

The bioenergetic response of the extremely thermoacidophilic archaeon Metallosphaera sedula to thermal and nutritional stresses was examined . Continuous cultures (pH 2.0, 70 degrees C, and dilution rate of 0.05 h(-1)) in which the levels of Casamino Acids and ferrous iron in grow th media were reduced by a step change of 25 to 50% resulted in higher levels of several proteins, including a 62-kDa protein immunologicall y related to the molecular chaperone designated thermophilic factor 55 in Sulfolobus shibatae (J, D. Trent, J, Osipiuk, and T. Pinkau, J, Ba cteriol, 172:1478-1484, 1990), on sodium dodecyl sulfate-polyacrylamid e gels. The 62-kDa protein was also noted at elevated levels in cells that had been shifted from 70 to either 80 or 85 degrees C. The proton motive force (Delta p), transmembrane pH (Delta pH), and membrane pot ential (Delta psi) were determined for samples obtained from continuou s cultures (pH 2.0, 70 degrees C, and dilution rate of 0.05 h(-1)) and incubated under nutritionally and/or thermally stressed and unstresse d conditions, At 70 degrees C under optimal growth conditions, M. sedu la was typically found to have a Delta p of approximately -190 to -200 mV, the result of an intracellular pH of 5.4 (extracellular pH, 2.0) and a Delta psi, of +40 to +50 mV (positive inside), After cells had b een shifted to either 80 or 85 degrees C, Delta psi decreased to nearl y 0 mV acid internal pH approached 4.0 within 4 h of the shift; respir atory activity, as evidenced by iron speciation in parallel temperatur e-shifted cultures on iron pyrite, had ceased by this point, If cultur es shifted from 70 to 80 degrees C were shifted back to 70 degrees C a fter 4 h, cells were able to regain pyrite oxidation capacity and inte rnal pH increased to nearly normal levels after 13 h. However, Delta p si remained close to 0 mV, possibly the result of enhanced ionic excha nge with media upon thermal damage to cell membranes, Further, when M. sedula was subjected to an intermediate temperature shift from 73 to 79 degrees C, an increase in pyrite dissolution (ferric iron levels do ubled) over that of the unshifted control at 73 degrees C was noted. T he 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 b asis for improving bioleaching rates of acidophilic chemolithotrophs.