Role of pH in the metabolism of Thiobacillus thiooxidans

The pH for the maximal growth of Thiobacillus thiooxidans was found to be 2.0 to 3.0. The cultures failed to grow when the pH was maintained at 1.0 even up to 390 hours. Such cultures resumed growth after changing the pH to 3.0. It was shown that the cells of T. thiooxidans can consume oxygen and oxidize freshly added 35Sofor at least eight hours at pH 1.0. These observations disproved the hypothesis of pH-dependent steps in the initial stages of the sulfur oxidation process. Cultures pre-adjusted to pH 1.0 failed to fix 14CO2, but 14CO2 fixing cultures continued to fix 14CO2 for nearly half an hour at approximately the same rate on changing the pH from 2.0 to 1.0. This suggests that components of the CO2 fixing system 'become unavailable at pH 1.0 since cell-free extracts With added AT.P and Ribose-5-phosphate fix CO2 at pH 1.2. The assimilation of 2-l4C-glycerol ceases when the pH of the culture is changed from 2.0 to 1.0. This suggests that at pH 7.0 energy is not available for synthetic processes. The intracellular ATP pool decreases suddenly as a result of changing pH from 2.0 to 1.0. This implicates .ATP as one of the limiting substrates of the CO2 fixation system at pH 1.0 and perhaps also for glycerol assimilation. The decreased pool size of ATP and continued oxidation of sulfur at pH 1.0 indicates the uncoupling of phosphorylation from sulfur oxidation. This uncoupling is explained in terms of pH-dependent conformational changes in the energy producing sites, probably the membranes. It is concluded that To thiooxidans requires an acidic environment to maintain an integrated energy generating system. Addition of 10^-3 M. pyruvic acid to cultures of T. thiooxidans at pH 2.3 resulted in its rapid intracellular accumulation and in the cessation of sulfur oxidation, C02 fixation and oxygen consumption; at pH 7.0 pyruvate neither inhibited oxygen consumption nor accumulated appreciably intracellularly. Pyruvate did not affect the CO2 fixation in cell-free system at pH 7.0. The above data is explained by the intracellular accumulation of pyruvic acid through passive permeation along with the concomitant decrease in the intracellular pH. Hence, it is concluded that pyruvic acid is an inhibitor for T. thiooxidans not because of special features associated with the intermediary metabolism of the organism but because it renders the intracellular environment acidic. Extracellular acidic pH is necessary to maintain an integrated energy generating system in T. thiooxidans, and the prevailing low pH-values in cultures are responsible for the inhibitory action of pyruvate; the cell is permeable to pyruvic acid as an undissociated molecule.