Sulphur metabolism in Thiorhodaceae. III. Storage and turnover of thiosulphate sulphur inThiocapsa floridana andChromatium species

Thiocapsa floridana strain 1711 andChromatium strains 1211 and 1611 utilize sulphide, thiosulphate, and elementary sulphur as electron donors for growth; sulphite can be used only byChromatium strain 1611. In contrast to the other strains, thiosulphate utilization inChromatium strain 1211 is inducible and not constitutive: thiosulphate is consumed only after an induction period of about 20 hours. The turnover rate of different sulphur compounds is controlled by the CO₂ fixation rate. Using differently labeled³⁵S thiosulphates in short term experiments in a special stirred cuvette, it was shown that the maximum amount of stored intracellular sulphur depends on the strain as well as on the experimental conditions like pH and thiosulphate concentration. WhileChromatium strain 1211 showed a maximum storage of only 10% from sulphane-labeled thiosulphate at pH 6.7, and of 25.7% at pH 6.2,Thiocapsa floridana accumulated 75–90% of the radioactivity into the cells at pH 6.7. While in theChromatium strains the labeling of the cells remained at a constant level until all thiosulphate was consumed, inThiocapsa floridana a defined peak of radioactivity storage was obtained, followed by a steady but 3–4 times slower rate of excretion. With sulphonelabeled thiosulphate no significant accumulation of radioactivity occurred in the cells. During dark-incubation ofThiocapsa floridana (free of intracellular sulphur) in phosphate buffer, pH 6.5, with thiosulphate a production of sulphide could be measured while sulphite was not detected; no sulphide was produced by disrupted cells under the same conditions. The results obtained withThiocapsa floridana strongly support the concept of an initial cleavage of thiosulphate. The present observations do not allow a decision concerning the enzymatic mechanism of the cleavage itself.