Aerobic Methanotrophy and Nitrification: Processes and Connections

Abstract

Ammonia and methane are structurally similar molecules. Not surprisingly therefore, microorganisms that use methane as a sole energy source (methanotrophs) and microorganisms that use ammonia as a sole energy source (ammonia oxidisers or nitrifiers) share many similarities. They have several key enzymes in common, most especially the ammonia monooxygenase/particulate methane monooxygenase enzyme family. The two groups are proposed to have a common evolutionary history. They occupy similar ecological niches, and compete for nitrogen. Enzymatically, nitrifiers are capable of methane oxidation, and methanotrophs are capable of nitrification. Microbial ecologists have attempted to find specific inhibitors for either group in order to study their respective roles in the environment. The contribution of ammonia oxidisers to methanotrophy in natural systems appears to be very minor, however methanotrophs may sometimes have important roles in the nitrogen cycle.

Key Concepts:

• Some bacteria and archaea are capable of using methane or ammonia as energy sources.

• Methanotrophs and ammonia oxidisers are each highly specialised to living on their particular substrate.

• Methanotrophs and ammonia oxidisers have several key enzymes in common, and may share a common evolutionary history.

• Both groups must cope with toxic by-products of ammonia oxidation.

• Methanotrophs may be important in the environmental nitrogen cycle, but ammonia oxidisers do not affect the methane cycle.

• Ammonia oxidising archaea appear to outcompete ammonia oxidising bacteria under ammonia-limiting and acidic conditions.