Carbon assimilation by the autotrophic thermophilic archaebacterium Thermoproteus neutrophilus

Growth of Thermoproteus neutrophilus at 85°C was studied using an improved mineral medium with CO₂, CO₂ plus acetate, CO₂ plus propionate, or CO₂ plus succinate as carbon sources; sulfur reduction with H₂ to H₂S was the sole source of energy. None of the carbon compounds added was oxidized to CO₂. The organism grew autotrophically with a generation time of 9–14 h, up to a cell density of 0.5 g dry weight per liter (2×10⁹ cells/ml). Propionate did not stimulate, succinate slightly stimulated the growth rate. Acetate, even at low concentrations (0.5 mM), stimulated the growth rate, the generation time being shortened to 3–4 h. Acetate provided 70% of the cell carbon, which shows that Thermoproteus neutrophilus is a facultative autotroph. The path of these carbon precursors into cell compounds was studied by ¹⁴C long-term labelling and investigation of enzyme activities. Propionate could not be used as a major carbon source and was incorporated only into isoleucine, probably via the citramalate pathway. Acetate was a preferred carbon source which suppressed autotrophic CO₂ fixation: acetate grown cells exhibited an incomplete citric acid cycle in which 2-oxoglutarate dehydrogenase was present, but fumarate reductase was repressed. The succinate incorporation pattern and enzyme pattern indicated that autotrophic CO₂ fixation proceeded via a yet to be defined reductive citric acid cycle.