ABSTRACT
Transient-state experiments with the obligately autotrophic Thiobacillus sp. strain W5 revealed that sulfide oxidation proceeds in two physiological phases, (i) the sulfate-producing phase and (ii) the sulfur- and sulfate-producing phase, after which sulfide toxicity occurs. Specific sulfur-producing characteristics were independent of the growth rate. Sulfur formation was shown to occur when the maximum oxidative capacity of the culture was approached. In order to be able to oxidize increasing amounts of sulfide, the organism has to convert part of the sulfide to sulfur (HS(sup-)(symbl)S(sup0) + H(sup+) + 2e(sup-)) instead of sulfate (HS(sup-) + 4H(inf2)O(symbl)SO(inf4)(sup2-) + 9 H(sup+) + 8e(sup-)), thereby keeping the electron flux constant. Measurements of the in vivo degree of reduction of the cytochrome pool as a function of increasing sulfide supply suggested a redox-related down-regulation of the sulfur oxidation rate. Comparison of the sulfur-producing properties of Thiobacillus sp. strain W5 and Thiobacillus neapolitanus showed that the former has twice the maximum specific sulfide-oxidizing capacity of the latter (3.6 versus 1.9 (mu)mol/mg of protein/min). Their maximum specific oxygen uptake rates were very similar. Significant mechanistic differences in sulfur production between the high-sulfur-producing Thiobacillus sp. strain W5 and the moderate-sulfur-producing species T. neapolitanus were not observed. The limited sulfide-oxidizing capacity of T. neapolitanus appears to be the reason that it can convert only 50% of the incoming sulfide to elemental sulfur.
