Aerobic Methanotrophy and Nitrification: Processes and Connections Lisa Y Stein, University of Alberta, Edmonton, Alberta, Canada Re ´ al Roy, University of Victoria, Victoria, British Columbia, Canada Peter F Dunfield, University of Calgary, Calgary, Alberta, Canada This article is dedicated to the memory of Roger Knowles. 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 espe- cially the ammonia monooxygenase/particulate methane monooxygenase enzyme family. The two groups are pro- posed to have a common evolutionary history. They occupy similar ecological niches, and compete for nitro- gen. Enzymatically, nitrifiers are capable of methane oxidation, and methanotrophs are capable of nitrifi- cation. Microbial ecologists have attempted to find spe- cific 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. Introduction Methane (CH 4 ) and ammonia (NH 3 ) are highly reduced molecules and therefore suitable growth substrates for microbes. They can be oxidised either aerobically or anaerobically to yield energy. The anaerobic processes are only recently yielding up their secrets, particularly methane oxidation coupled to sulfate-reduction and ammonia oxi- dation coupled to nitrite-reduction. However, the focus of this review is on aerobic processes. The first step in aerobic methane or ammonia oxidation is the introduction of a single oxygen atom from O 2 into the respective molecules via a monooxygenase enzyme, yielding methanol (CH 3 OH) from CH 4 and hydroxylamine (NH 2 OH) or possibly alternate N-oxides from NH 3 (Figure 1). Energy released in the monooxygenation is not conserved, in fact the microbes expend reducing power to reduce the second oxygen atom from O 2 . However, methanol and hydro- xylamine are further oxidised in processes coupled to energy generation. See also: Chemolithotrophy; Nitrifi- cation; The Metabolism of Anammox Similarities between bacterial methanotrophs and nitri- fiers have long been evident. Both grow obligately on their particular substrate (NH 3 for nitrifiers and CH 4 for methanotrophs), but are capable of cooxidising the alter- native substrate. Both contain intracellular membrane systems, inhabit oxic/anoxic interface environments, rely on monooxygenase reactions catalysed by ammonia NO Cell biomass N 2 O CHOOH CO 2 NO 2 - NH 3 HNO? NH 2 OH CH 3 OH CH 4 HCHO ? ? ? d b a a a a e b c c f i g h Figure 1 Overlapping pathways for ammonia and methane oxidation. Black lines denote bacterial pathways; grey lines denote putative pathways for ammonia-oxidising thaumarchaea. Enzymes catalysing each process are: a, ammonia/methane monooxygenase; b, hydroxylamine oxidoreductase; c, nitrite reductase; d, nitric oxide reductase; e, methanol dehydrogenase; f, formaldehyde dehydrogenase; g, formate dehydrogenase; h, enzymes of the serine or ribulose monophosphate pathway and i, enzymes of the Calvin–Benson–Bassham cycle. The question marks in the thaumarchaeal pathway denote uncertainty regarding the intermediate produced by ammonia monooxygenase, the enzyme that oxidises this intermediate to nitrite, and enzymes that form NO/N 2 O from the intermediate of ammonia oxidation or from reduction of nitrite as measured by Santoro et al. (2011). The dashed line denotes a possible role of NO in ammonia-oxidation (Schleper and Nicol, 2010). Advanced article Article Contents . Introduction . Methanotrophs . Ammonia Oxidisers . Linkages between Methane and Ammonia Oxidisers . Conclusion . Acknowledgements Online posting date: 16 th April 2012 eLS subject area: Microbiology How to cite: Stein, Lisa Y; Roy, Re ´al; and Dunfield, Peter F (April 2012) Aerobic Methanotrophy and Nitrification: Processes and Connections. In: eLS. John Wiley & Sons, Ltd: Chichester. DOI: 10.1002/9780470015902.a0022213 eLS & 2012, John Wiley & Sons, Ltd. www.els.net 1