# Microbial Activity in Subterranean Ecosystems: Recent Advances
## 2 The Control of Greenhouse Gas Fluxes by Cave Microorganisms  
Global changes in the Earth's climate and its relationship to the increasing concentration of greenhouse gases (GHGs) in the atmosphere has received special attention since the last quarter of the 20 th century. [PERSON] and [PERSON] [22] reported that the major sources for atmospheric methane (CH\({}_{4}\)) budget derive from the natural processes in the biosphere (modern microbial activity) and from fossil, radiocarbon-free CH\({}_{4}\) emission, estimated at approximately 20% of atmospheric CH\({}_{4}\), which is due to and mediated by anthropogenic activity. However, this estimation is higher than the estimates from statistical data of CH\({}_{4}\) emission from fossil fuel and related anthropogenic sources. For these authors, geologic sources are more than enough to provide the amount of CH\({}_{4}\) required to account for the suspected missing source of fossil CH\({}_{4}\). In addition, [PERSON] and [PERSON] [23] distinguished between biotic and abiotic CH\({}_{4}\), the latter produced in magmatic processes (volcano and high-temperature active hydrothermal vents) and postmagmatic processes at lower temperatures (gas-water-rock interactions). A better understanding of the carbon cycle in the Earth-climate system is nowadays a crucial knowledge gap. The main research efforts are focused on identifying and characterizing all possible sources, reservoirs, and sinks of GHGs, mainly CO\({}_{2}\) and CH\({}_{4}\), in order to more accurately calculate the budgets, especially in the carbon cycle [24]. This issue is critical to understand the effects of changes in the carbon cycle on Earth's climate, and to assess the level of effort required in order to adapt and mitigate climate change. The interactions between geological, microbiological, and chemical processes are responsible for the physical-chemical properties of the atmosphere and especially for changes in its composition. Caves and other shallow vadose environments are populated by methanotrophic microorganisms and thus represent a CH\({}_{4}\) sink. This subterranean CH\({}_{4}\) sink is largely overlooked in the scientific literature. Understanding how cave microbiomes influence the systems in which they inhabit is proving to be an exceptional research challenge [25]. Methane is consumed from the atmosphere by methanotrophs in forests, grasslands, paddy, and other unsaturated soils, which represent the major terrestrial sinks. Environmental CH\({}_{4}\) oxidation by bacteria is mainly carried out by _Gammaproteobacteria_, _Alphaproteobacteria_, and _Verrucomicrobia_[26], though there is also recent evidence for methanothopy in _Rokubacteria_[27]. The presence of methanotrophic bacteria in caves has been widely studied in Movie Cave, Romania, by using isolation techniques, \({}^{13}\)CH\({}_{4}\)-labelling, and \({}^{13}\)C-DNA analysis, and the significant importance to the ecosystem development and primary productivity has been remarked upon [28; 29; 30; 31]. Evidence of the occurrence of methanotrophs has also been found in other caves [10; 32; 33]. However, in these studies the microorganisms were not related with the sink of GHGs in caves. Specific studies, both on the environmental-driven controls on microbial activity and, in turn, on the microbial role in composition changes of natural subterranean ecosystems, constitute a new research area of the highest potential with a pool of questions to solve. The starting hypothesis was that the subterranean microbiome plays a significant ecological role in the biogeochemical processes controlling the composition of the underground atmosphere, as well as in the availability of nutrients for the rest of the ecosystem's biota. [PERSON] et al. [34] evidenced for the first time that cave ecosystems act as effective natural sinks of atmospheric CH\({}_{4}\) on seasonal and daily scales and this phenomenon may thus be relevant on a global scale in terms of its contribution to the global balance of GHGs.