By Jennifer Stearns, Michael Surette

Catabolism (the breakdown of compounds for energy conservation) can happen in different ways for different types of microorganisms. In chemoorganotrophs (organisms that derive their energy from organic compounds), there are two forms of catabolic metabolism: fermentation and respiration.

Fermentation is a form of anaerobic catabolism (in the absence of O2) where the organic substrate acts as both the electron donor and the electron acceptor. Here, ATP is made from energy-rich reaction intermediates through a process called substrate-level phosphorylation.

Respiration can occur in the presence or absence of oxygen. During aerobic respiration, O2 acts as the terminal electron acceptor and ATP is formed at the expense of the proton motive force by a process called oxidative phosphorylation. In the absence of oxygen, respiration can also happen if an O2 alternative is present to act as the terminal electron acceptor. In that case, it’s called anaerobic respiration.

Although fermentation produces less ATP than respiration does, in anoxic (oxygen-free) conditions, microbes that can use fermentation have a lot of flexibility in terms of which foods they can break down. Because of this, they’re very important to the anaerobic food chain, not to mention a great help when we want to make cheese, beer, spirits, or bread.