**2. Biogeochemical cycle of CH4**

Global biogeochemical cycles are mainly driven by microorganisms that feed on base compounds of carbon (C), such as CH4 or carbon dioxide (CO2) [5]. The CH4 is the most abundant hydrocarbon in the atmosphere [6]. Due to its absorption characteristics, CH4 manifests positive radiative forcing, being a GHG that contributes to the regulation of temperature on the surface of the planet. It is believed that CH4 is responsible for 17% of global warming [7], taking into account the indirect chemical reactions of this gas with aerosols. The Global Warming Potential (GWP) of CH4 is estimated to be 25 times higher than the GWP of CO2 [8, 9] on a 100-year horizon.

The interest in estimates of CH4 emission in tropical forests has grown in recent years, particularly in wetlands such as the Amazon basin [10–15] and Pantanal [16–18]. This is due to the fact that the largest natural sources of CH4 are wetlands [19], contributing with 177–284 Tg CH4 per year [7]. Humid areas are the largest and most uncertain sources of CH4 to the atmosphere [20]. Remote sensing techniques employing visible, infrared and microwave observations offer varying degrees of success in providing quantitative estimates of wetlands and inundation

### *Methane, Microbes and Models in Amazonian Floodplains: State of the Art and Perspectives DOI: http://dx.doi.org/10.5772/intechopen.90247*

extent and monitoring natural and anthropogenic variations in these environments [21]. Another factor that may contribute to this uncertainty is the interannual variability of the water column associated to lakes and rivers, which directly influence the wetlands linked to them.

Wetlands have high C sequestration and store capacity, which justify the growing interest in studying the production and consumption of this gas in these ecosystems. The C sequestration refers to the removal of CO2 from the atmosphere, transfer, and accumulation of that gas in the flooded areas as soil organic matter. That is, the sequestration of C in wetlands is related to the photosynthetic removal of CO2 by producing organisms and its conversion into cellulose and other forms of C, and subsequently the transformation of waste into soil organic matter [22]. This ability to act on the C cycling, in addition to all other ecosystem services performed by those environments, makes them critical components in understanding local, regional, and global C stocks, capable of influencing the balance of CO2, CH4, and other GHG.

Floodplains are defined as environments that are seasonally flooded or saturated due to rising groundwater or surface water and remain like that for a certain period of the year or throughout the year [3]. According to Junk et al. [3], flooding of plains along rivers tends to occur as a single annual flood pulse that lasts months. In these plains, flooding can also lead to an increase in allochthonous inputs of C, making them essential to the food web and interesting to the scientific community.

The CH4 is produced mainly by microorganisms belonging to the domain *Archaea* in the final stage of organic matter fermentation in anaerobic environments [23], which play a crucial role in the biodegradation of organic matter [24]. However, only a fraction of the produced CH4 is emitted into the atmosphere. Microorganisms that oxidize CH4, which are known as methanotrophic bacteria use the other part. There is no consensus in the literature on the percentage of CH4 assimilated by these microorganisms. There are estimates that 10–100% of the CH4 produced by anaerobic microorganisms are oxidized into CO2 before reaching the atmosphere [25].

Part of the current understanding of the dynamics of CH4 in wetlands is based on the premise that most of the oxidation of CH4 occurs under aerobic conditions. However, recent studies indicate the action of several other electron acceptors (alternative to sulfate under aerobic conditions) in the anaerobic oxidation of CH4, including nitrate, nitrite, iron, and manganese [5, 26–32]. Studies also point to humic substances acting as a terminal acceptor for electrons in tropical flood areas [33]. In previous studies [32, 34], when attempting to justify the predominance of academic papers addressing the oxidation of CH4 exclusively by aerobic means, taking into account the fact that sulfate has been, for a long time, the only electron acceptor involved in the oxidation of CH4 in anoxic environments, the concentration of sulfate is generally too low in freshwater environments to play a role in the anaerobic oxidation of CH4. The contribution of anaerobic oxidation of CH4 to the methanotrophic processes is not fully elucidated, but the increasing number of papers validating the information shows that this mechanism seems to be more common than previously thought.

In turn, methanogenesis occurs when energetically favorable electron acceptors such as oxygen, nitrate, sulfate, and iron are absent or have been depleted [35]. In the absence of oxygen, the complete decomposition of complex organic compounds requires syntrophic system interactions in individual steps in the global process [36]. A sequential action involves hydrolysis, acidogenesis, acetogenesis, and methanogenesis steps [37]. Therefore, the many microbial guilds involved in those processes include hydrolytic, syntrophic fermentative, acetogenic, and methanogenic microorganisms.

Bacteria and fungi are responsible for breaking down complex molecules during hydrolysis, such as polysaccharides, proteins, and their forming units (amino acids, fatty acids, and alcohols) [38]. In the acidogenesis stage, fermentative microorganisms convert simple substrates into volatile fatty acids (VFA) (e.g., acetate, propionate, and butyrate), alcohols (e.g., ethanol and butanol), H2, and CO2 [39]. In acetogenesis, the VFA and alcohols produced, such as propionate, butyrate, and ethanol, are converted into acetate, H2, and CO2 by acetogenic bacteria [39]. Finally, methanogens convert acetate, H2/CO2, formate, and methylated compounds into CH4.
