Page 42 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 2. The diversity of methane cycling microorganisms
Methane is the most reduced one-carbon compound, and it plays a key role in the global carbon cycle. Many processes in a wide variety of ecosystems control the global CH4 budget (Heilig 1994; Kirschke et al. 2013; Dean et al. 2018). The majority of CH4 released into the atmosphere (70%–80%) is of biogenic origin, and most, if not all, biogenic CH4 is produced by methanogenic archaea within the phylum Euryarchaeota (Conrad 1996, 2009).
Figure 1. Conceptual illustration of CH4 production and consumption prior to atmospheric release; all microbial conversion processes are shown in italics. Complex organic matter is degraded by microorganisms in anoxic environments by a multistep process including hydrolysis, fermentation, homoacetogenesis, and syntrophic acetate oxidation, leading to CO2 and CH4 as end products. The CH4 diffuses upward through the soil/sediment layer where it can be oxidized by (an)aerobic methanotrophs. Here, a theoretical distribution of available electron acceptors based on their electron potential is shown; the extent of the electron acceptor zones will vary between different environments. Part of this CH4 can eventually reach the atmosphere. Some of the CH4 can be released via plants or ebullition; note that these pathways of CH4 release can also occur in aquatic environments (not shown). The vegetation represents vascular plants, which provide a direct pathway of vertical CH4 release via aerenchyma. Picture taken from Dean et al. (2018).
40