Effect of hyperbaric oxygen and carbon dioxide on heterotrophic growth of the extreme thermophile Sulfolobus acidocaldarius

Abstract

Recent interest in extremely thermophilic archaebacteria comes, in part, from a variety of potential biotechnological applications that have been proposed. These range from highly thermostable enzymes to possible novel biotransformations. Suljolobus acidocaldarius, for example, one of the most studied of the extreme thermophiles, has already been identified as playing a role in coal desulfurization and ore leaching processes. '3, Relatively little has been done thus far, however, to examine the engineering considerations associated with the growth of extremely thermophilic bacteria. Since utilization of several nutrients present as sparingly soluble dissolved gases is often characteristic of these microorganisms, gas-liquid mass transfer and sensitivity to dissolved gas tensions are important factors to be examined.It has been suggested that the use of elevated partial pressures of nutrient gases might provide a means to overcome solubility limitations on mass transfer at the temperatures at which extreme thermophiles g r o~.~,~ It is well known that, for the growth of aerobic microorganisms, the transfer of oxygen from the gas phase to the liquid phase is often rate limiting, especially when biomass concentrations and oxygen demand are high. For aerobic extreme thermophiles, although the mass transfer coefficient in a given system would be expected to increase at high temp e r a t~r e ,~ mass transfer rates may not increase because of the decrease in oxygen solubility. It is not clear the degree to which oxygen (or other gas) mass transfer limitations exist for microorganisms growing at elevated temperatures. Also, there is almost no information available that quantifies the sensitivity of extreme thermophiles to dissolved gas tension^.^'^ In this study, the aerobic, extremely thermophilic archaebacterium, Suljolobus acidocaldarius, was cultured heterotrophically in an autoclave bioreactor under elevated partial pressures of CO, and air with hyperbaric helium used to obtain the desired incubation pressure. Of interest was the effect of elevated nutrient gas partial pressures on growth