Continuous culture as a tool for investigating the growth physiology of heterotrophic hyperthermophiles and extreme thermoacidophiles

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Abstract
Although there is great scientific and technological interest in examining the physiology and bioenergetics of microorganisms from extreme environments, difficulties encountered in their cultivation and lack of genetic systems hampers the investigation of these issues. As such, we have adapted methods for continuous cultivation of mesophilic organisms to extremes of temperature and pH to study extremophiles. Since the risk for contamination of extremophilic continuous cultures is relatively small, long‐term, steady state experiments investigating physiological response to culture perturbations are possible. Experiments along these lines have provided insights into the significance of specific enzymes in the metabolism of particular substrates, in addition to providing a better understanding of stress response and unusual physiological characteristics of hyperthermophilic and extremely thermoacidophilic microorganisms. Several examples are provided here, including the thermal stress response of Metallosphaera sedula (Topt 74 °C) growing at pH 2·0, and the response of the heterotrophic hyperthermophiles Pyrococcus furiosus (Topt 98 °C), Thermococcus litoralis (Topt 88 °C) and T. maritima (Topt 80 °C) to changes in growth medium. Also discussed will be how the same experimental systems have been used to study exopolysaccharide production and biofilm formation by hyperthermophilic heterotrophs and facilitated the estimation of bioenergetic parameters for these organisms under a variety of growth conditions. Continuous culture, used in conjunction with genome sequence information, two‐dimensional gel electrophoresis and differential gene expression, can provide important insights into the metabolism of high temperature extremophiles.