concept

Dark fermentation

Microbial breakdown of carbohydrate under anaerobic conditions and without light, yielding organic acids, carbon dioxide and hydrogen gas. There is a firm thermodynamic ceiling: at most four hydrogen molecules per glucose via the acetate route, and getting past it requires energy pumped in from elsewhere — the limit named after Thauer. In practice most gut communities take the butyrate route instead and collect two. This is the mechanism by which a cow's rumen evolves cubic metres of hydrogen a day, and the one industry is trying to harness for biological hydrogen production.