Page 30 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 1. General introduction
and land use change rather than climate change, although changes in climate, population, and
land use are intrinsically linked.
1.2 Microbes and the methane cycle
Biological processes have major impacts on the global CH4 cycle. Climate change is affecting these processes, which in turn affect the CH4 budget. In this context, it is important to understand the microbial conversions of CH4 that are carried out by a complex web of microorganisms. Methanogens are responsible for the terminal step in this anaerobic food web and produce an estimated 583 Tg (range: 458–748 Tg) of CH4 per year from natural and agricultural sources (Saunois et al. 2016). Most, if not all, biogenic CH4 is produced by methanogenic archaea within the phylum Euryarchaeota. Proposed alternative pathways include CH4 production by iron-only nitrogenases (Zheng et al. 2018), CH4 release from methyl phosphonate in marine and freshwater ecosystems (Daughton, Cook and Alexander 1979; Bižić-Ionescu et al. 2018; Taenzer et al. 2020), and in situ formation of CH4 in terrestrial plants (Keppler et al. 2006).
Before CH4 reaches the atmosphere, it can be oxidized by a range of anaerobic and aerobic methanotrophs using a suite of electron acceptors, including sulphate, nitrate, nitrite, and oxidized metals. These methanotrophs include anaerobic methanotrophic (ANME) archaea, and anaerobic and aerobic methanotrophic bacteria. Therewith, microorganisms play a critical role in modulating the release of CH4 by driving a range of redox reactions that ultimately transform CH4 to CO2. For a detailed overview of the microbes involved in the methane cycle, see Chapter 2.
Environmental controls and processes of methane emission
The microbially mediated processes of CH4 production and consumption are controlled directly and indirectly by local environmental conditions. These conditions have an effect on the biological pathways and rates of CH4 production and emission. Changes to these conditions can therefore alter local microbial communities and their activities, and thus the resultant CH4 emissions. Climate change has pronounced effects on the Arctic, where large frozen ecosystems are subjected to thaw. In addition, disproportionate near-surface warming, which is also known
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