Thiobacillus denitriJcans was cultured chemolithotrophically under aerobic and anaerobic conditions in a chemostat with thiosulphate, nitrate or nitrite as limiting nutrient. Estimations of growth yields and maintenance coefficients showed that T. denitriJcans grew more efficiently than other thiobacilli both aerobically and anaerobically. Relative growth yield data enabled the probable amounts of ATP generated during thiosulphate-limited aerobic growth and nitrate-limited anaerobic growth on thiosulphate to be calculated as, respectively, 6 to 7 and 4 to 5 mol ATP formed per mol thiosulphate oxidized. The energy available from tetrathionate oxidation was almost twice that from thiosulphate.

Thiobacillus denitrijicans grew exclusively aerobically in thiosulphate-limited chemostat culture at all dissolved oxygen concentrations between 12 and 216 ,UM. Nitrate reduction did not occur in aerobic cultures and nitrate and nitrite reductases only reached high levels under complete anaerobiosis. Growth yield was greatest [11.50 g dry wt (mol thicsulphate oxidized)-l] at the lowest dissolved oxygen concentration (12 ,UM) and decreased at higher dissolved oxygen concentrations, indicating oxygen to be a growth-inhibitory substrate; the anaerobic yield was only 77 yo of the maximum aerobic yield (all tested at a dilution rate of 0.08 h-l) in agreement with thermodynamic calculations. Low activities of thiosulphateoxidizing enzyme and sulphur-oxidizing enzyme were detected in aerobic cultures, but activities were even lower in anaerobic cultures. Efficient energy coupling mechanisms with respect to sulphur and thiosulphate oxidations are indicated.

Three strains of Gram-negative, obligately aerobic, moderately thermophilic, CO-utilizing bacteria have been isolated by enrichment at 45-55 "C in mineral medium under CO, H2 and air. The properties of the strains, designated C2, S1 and FE, suggest that they should be included in a single species that we have named Pseudomonas thermocarboxydovorans. The isolates can grow on CO and a limited range of organic and amino acids; neither sugars nor H2/C02 support growth and €I2 is not oxidized. Strain C2 contains a soluble, inducible C0:phenazine ethosulphate oxidoreductase, lacks hydrogenase and assimilates CO via ribulose bisphosphate carboxylase. The organism apparently contains a mechanism for assimilating phosphoglycollate. Pseudomonas carboxydovorans, P . carboxydoflava, P . carboxydohydrogena, P . gazotropha,Cornomonas compransoris and Achromobacter carboxydus, can utilize CO as sole carbon and energy source (Kim & Hegeman, 1983;Meyer & Schlegel, 1983). These bacteria are Gramnegative, mostly mesophilic and, with the exception of A . carboxydus, able to grow autotrophically with hydrogen (H,) and carbon dioxide (CO,). This paper describes the isolation, characterization and growth of three strains of a novel thermophilic species of carboxydobacterium.Abbreviations: API 20E, API ZYM and API 5OCH refer to the bacteriological test strips supplied by API Laboratory Products, Grafton Way, Basingstoke, Hants, UK ; PES, phenazine ethosulphate; PHB, poly-Qhydroxybutyrate; DCPIP, 2,6-dichlorophenolindophenol.