Effects of Nitrate on Hydrogenogenic Carbon Monoxide Oxidation in Parageobacillus thermoglucosidasius.

Parageobacillus thermoglucosidasius is a thermophilic facultative anaerobe capable of hydrogenogenic carbon monoxide (CO) oxidation utilising nickel-containing CO dehydrogenase (Ni-CODH) and energy-converting hydrogenase (ECH). Nitrates have been reported to exert promoting or inhibitory effects on the growth of CO oxidizers and acetogens, and these contradictory outcomes obscure the relationship between nitrate and CO oxidation. In this study, we analysed the effects of nitrate on hydrogenogenic CO oxidation and growth in P. thermoglucosidasius NBRC 107763 using wild-type and codh- and/or ech-disrupted strains. The results demonstrated that the addition of 50 mM nitrate suppressed hydrogenogenic CO oxidation while promoting hydrogen-oxidising nitrate reduction and rapid cell growth, resulting in a 2.3-fold higher OD than the control. Assays using cell lysates showed that 10 μM nitrate suppressed CO oxidation below the detection limit without affecting hydrogen production, indicating that nitrate affects the CO-oxidising function. These findings imply that CO oxidation in P. thermoglucosidasius is primarily coupled to proton reduction, and deactivated during nitrate respiration. Therefore, hydrogenogenic CO oxidation serves as an auxiliary energy-obtaining mechanism, functioning in the absence of alternative electron acceptors such as nitrate. This study enhances our understanding of CO-dependent energy generation and highlights the supplemental use of CO in P. thermoglucosidasius.