Effect of growth conditions on thiocyanate degradation and emission of carbonyl sulfide by Thiobacillus thioparus THI115

Abstract

Thiobacillus thioparus THI115 utilizes thiocyanate as its sole source of energy for growth. Carbonyl sulfide (COS) was found as an intermediate in the headspace of a sealed flask during the degradation of thiocyanate, and ammonia and sulfate were produced as the final products which slightly decreased the medium pH. In a pH-controlled TC medium, the highest degradation activity for thiocyanate was obtained at pH 6.2 and the activity was inhibited at pH 6.0. When whole gases in both liquid medium and headspace were removed from the culture vessel by forced aeration, neither growth of T. thioparus THI115 nor the degradation of thiocyanate could be observed. On the other hand, thiocyanate was degraded after a lag period of about 150 h when only the gases in the headspace were removed. These results suggest that the entire COS produced was easy to diffuse outside the cells and that the evolved COS was re-incorporated to be metabolized by cells. Despite the fact that COS is toxic to organisms, thiocyanate degradation by T. thioparus THI115 was tolerated when a total of 500 μmol of COS, which is about 17 times higher than the lethal dose of 50 for 90 min in rats, was added to the culture. T. thioparus THI115 degraded up to 60 mM thiocyanate in 130 h when supplied with sufficient oxygen and keeping the medium pH over 6.2.