Abstract: Provided is a method of anaerobically fermenting a gaseous substrate to form a liquid product, the method comprising: (a) introducing the gaseous substrate into a bio-reactor, the gaseous substrate comprising at least one of the following constituents: carbon monoxide, carbon dioxide, and hydrogen, (b) the bio-reactor comprising a fermentation broth therein, the fermentation broth containing at least two types of microorganisms, one type comprising at least one fermenting species, and the other type comprising at least one competing species; (c) introducing at least one type of ionophore into the reactor, the ionophore having selectivity for preferentially inhibiting the at least one competing species from growing and/or producing an undesired product; and (d) allowing the gaseous substrate to ferment by exposure to the at least one fermenting species, to produce the liquid product and a system for doing the same.