Physiological characterization of a key enzyme involved in gaseous COS assimilation in the filamentous fungus Trichoderma harzianum

Filamentous fungi are among the primary organisms responsible for the biological deterioration of wood, including wood decay and surface contamination. Therefore, understanding their physiological mechanisms is important for developing effective wood preservation strategies. Most physiological studies on filamentous fungi have focused on the metabolism of major elements, such as carbon in the form of carbohydrates as an energy source and nitrogen as a relatively abundant cellular component. In contrast, trace nutrient elements such as sulphur have received far less attention, despite their physiological importance. As a result, the acquisition pathways and metabolic mechanisms of sulphur in fungi remain poorly understood. Recently, we discovered that the filamentous fungus Trichoderma harzianum isolated from soil is capable of assimilating gaseous carbonyl sulphide (COS), which is the most abundant gaseous sulfur compound in the atmosphere, as a sulphur source. In addition, we identified a COS hydrolase (COSase) from this fungus that catalyses the hydrolysis of COS (COS + H₂O → CO₂ + H₂S). Previous studies have indicated a possible involvement of COSase in COS assimilation, but conclusive evidence has yet to be provided. In this study, we aimed to clarify the role of COSase in COS metabolism. Accordingly, we evaluated the contribution of COSase to COS metabolism by analysing its gene expression and generating gene-targeted mutants. The gene expression analysis showed that COSase expression changed over time following COS exposure, and the COSase knockout strains completely lost COS-degrading activity and were affected in growth, indicating that COSase is a key enzyme in COS metabolism in T. harzianum.