The invention relates to improvements in the production of alcohols by microbial fermentation, particularly to production of alcohols by microbial fermentation of substrates comprising CO. It more particularly relates to the provision of an improved fermentation media, comprising nickel, to a fermentation system such that one or more micro-organisms convert a substrate comprising CO to one or more alcohols, such as ethanol. In particular embodiments, a microbial culture is provided with at least 10 μM nickel, such that CO uptake by the microbial culture increases and ethanol productivity improves.