Fermentation and Anaerobic Respiration

In the absence of oxygen, microbes conserve energy through fermentation, which balances redox internally, or through anaerobic respiration, which uses alternative terminal electron acceptors.

Definition

Fermentation is an anaerobic, electron-balanced catabolism that conserves energy by substrate-level phosphorylation without an external electron acceptor, while anaerobic respiration conserves energy through electron transport to an acceptor other than oxygen.

Scope

This topic covers the principles of fermentation and its major pathways, including lactic acid, alcoholic, and mixed-acid fermentations; the diversity of anaerobic respirations using acceptors such as nitrate, sulfate, ferric iron, and carbon dioxide; and the ecological and applied significance of anaerobic metabolism. It contrasts the energetics of these strategies with aerobic respiration.

Core questions

• How do cells conserve energy when oxygen is unavailable?
• What products characterize the major fermentation pathways?
• Which alternative electron acceptors support anaerobic respiration?
• Why do anaerobic strategies typically yield less energy than aerobic respiration?

Key concepts

• Fermentation and internal redox balance
• Lactic, alcoholic, and mixed-acid fermentations
• Alternative electron acceptors
• Nitrate, sulfate, and carbonate respiration
• Energy yield of anaerobic versus aerobic metabolism

Mechanisms

In fermentation a substrate is partially oxidized and its electrons are passed to an organic intermediate, regenerating electron carriers and yielding ATP by substrate-level phosphorylation; the result is characteristic fermentation products. In anaerobic respiration, electrons flow through a transport chain to an inorganic or organic acceptor other than oxygen, generating a proton motive force; because these acceptors have lower reduction potentials than oxygen, less energy is typically conserved.

Clinical relevance

Anaerobic metabolism is central to many industrial and environmental processes, including the production of fermented foods, beverages, and biofuels, the function of anaerobic habitats such as sediments and the gut, and the global cycling of nitrogen and sulfur through respiratory reductions.

History

Louis Pasteur's nineteenth-century studies established fermentation as a biological process carried out by living microbes in the absence of air, and later work characterized the many forms of anaerobic respiration that exploit alternative electron acceptors in anoxic environments.

Key figures

• Louis Pasteur
• Sergei Winogradsky

Seminal works

• madigan2018
• willey2020

Frequently asked questions

How is fermentation different from anaerobic respiration?

Both occur without oxygen, but fermentation balances electrons internally using an organic molecule as the final acceptor and makes ATP by substrate-level phosphorylation, whereas anaerobic respiration passes electrons through a transport chain to an external acceptor such as nitrate or sulfate, generating a proton motive force.