The respiratory system of Sulfolobus acidocaldarius, a thermoacidophilic archaebacterium

The respiratory properties, cellular ATP content and absorption difference spectra of Sulfolobus acidocaldarius (DSM 639) have been investigated. In contrast to earlier postulates regarding Thermoplasma acidophilum [(1984) Syst. Appl. Microbiol. 5, 30‐40], S. acidocaldarius seemed to depend energetically on respiration‐coupled phosphorylation. Its ATP content strictly depended on respiratory activity. Its membrane is capable of proton pumping and presumably contains a branched electron transport system. The latter is composed of at least 2 types of cytochromes, an a+‐ and presumably an o‐type, while c‐type cytochromes could not be detected. It appears possible that one of the terminal oxidases is directly reduced by the socalled caldariellaquinone, found in the same organism [(1983) Syst. Appl. Microbiol. 4, 295‐304].