Glycollate Metabolism in the Obligate Chemolithotroph Thiobacillus neapolitanus Grown in Continuous Culture

Abstract

CO2-limited chemostat cultures of Thiobacillus neapolitanusexcreted 13·5; nmol glycollate min−1 (mg protein)−1. This confirmed the earlier finding that glycollate excretion by T. neapolitanusis dependent on the CO2/O2 ratio applied in the chemostat and thus is probably due to the oxygenase activity of d-ribulose-1,5-bisphosphate (RuBP) carboxylase. RuBP-dependent oxygen uptake and activity of phosphoglycollate phosphatase were demonstrated in cell-free extracts of T. neapolitanus. Thiobacillus neapolitanuswas able to metabolize exogenous glycollate only to a limited extent. The proportion of dry weight that was derived from [1-14C]glycollate by both CO2- and thiosulphate-limited chemostat cultures was only 0·7 % of the total. Short-term labelling experiments with [1-14C]glycollate and enzyme studies indicated that T. neapolitanuscan metabolize glycollate via two pathways. The enzymes of the glycine-serine pathway were demonstrated in cell-free extracts, but on the basis of kinetic experiments it was concluded that this pathway does not play a major role in glycollate metabolism. Malate was the major primary labelling product. Glyoxylate, formed from glycollate, may be incorporated into malate via malate synthase, which was detected in cell-free extracts of T. neapolitanus. The activities of the enzymes needed for the metabolism of glycollate were independent of growth conditions applied in the chemostat. The results are discussed in relation to the possible causes of obligate chemolithotrophy.