# Microbial Activity in Subterranean Ecosystems: Recent Advances
## 2 The Control of Greenhouse Gas Fluxes by Cave Microorganisms
They conducted a field mesocosm experiment to test whether or not microbial methanotrophy has the potential to act as a daily sink for CH\({}_{4}\) in two fairly well-ventilated Vietnamese caves with low Rn concentrations (75-115 Bq/m\({}^{3}\)), temperatures of 19-21 \({}^{\circ}\)C, and relative humidity ranging between 85 and 95%, depending on the airflow and location within the cave. The data suggested that biological processes have the potential to deplete atmospheric levels of CH\({}_{4}\) (~2 ppmv) via methanotrophy on a daily basis, as an 87% reduction in CH\({}_{4}\) concentrations was observed. It appears that CH\({}_{4}\) depletion is a seasonal phenomenon, as reported by several authors. [PERSON] et al. [34] found significant seasonal and even daily variations in the gas composition of cave air, which involves the exchange of large amounts of other GHGs, in addition to CO\({}_{2}\)(g), with the lower troposphere. [PERSON] et al. [40] performed a continuous 3-year record of CH\({}_{4}\) and other trace gases in an Australian cave and found a seasonal cycle of extreme CH\({}_{4}\) depletion, from ambient ~1775 ppb to near zero during summer and to ~800 ppb in winter. [PERSON] et al. [41] found methanotrophic bacteria from the families _Methylococcaee_ (_Gammaproteobacteria_) and _Methylocytaceae_ (_Alphaproteobacteria_) in 67% of the samples collected in Nerja Cave, Spain. In a recent innovative research, [PERSON] et al. [42] confirmed that microbial action in caves plays a crucial role in the processes of production, consumption and storage of GHGs (CO\({}_{2}\) and CH\({}_{4}\)) and largely determines the strong variations of these major GHGs in natural underground ecosystems. This study was developed in three Spanish caves (Pindal, Castanar de Ibor and La Garma) as a first approach to systematically characterize the role of cave sediments in the production and transport of CO\({}_{2}\) and CH\({}_{4}\) in the subterranean environment. Monitoring and sampling for more than two years in La Garma Cave showed that during the stages with greater ventilation, air circulates daily and there is a continual contribution of external air to the cave, which has lower CO\({}_{2}\) content and CH\({}_{4}\) levels close to the atmospheric background. Therefore, CH\({}_{4}\) depletion rises with slight changes in CO\({}_{2}\). Conversely, in stages with a low ventilation rate, CO\({}_{2}\) reaches high concentrations in the cave because air exchange with the external atmosphere is negligible. Thus, the removed CH\({}_{4}\) is not rapidly replenished. As a result, CH\({}_{4}\) depletion rate tends to become negligible as the CO\({}_{2}\) content of cave air rises (Figure 1a). Figure 1b,c shows the spatial distribution of air CO\({}_{2}\) and CH\({}_{4}\) concentrations, respectively, in the air from La Garma Cave. Data for each contour map correspond to mean values from a set of bimonthly spot air samplings, conducted from October 2014 to July 2017, in a pre-established network of 11 points covering up to three levels of cave passages along an altitude gradient. The average CO\({}_{2}\) and CH\({}_{4}\) concentrations of cave air were 894 and 0.65 ppm, respectively. Both GHGs depend on the rate of cave air exchange with the local atmosphere, which is controlled by climate-driven processes (primarily advection), and it is a very good indicator of the levels of  
Figure 1: (**a**) Monthly co-variations in the concentrations of carbon dioxide (CO\({}_{2}\)) and methane (CH\({}_{4}\)) in La Garma Cave (Cantabria, northern Spain), a dynamically ventilated cave. (**b**) Spatial distribution of average concentrations of CO\({}_{2}\). (**c**) Spatial distribution of average concentrations of CH\({}_{4}\). Data from October 2014 to July 2017.  
matter and energy exchange with the exterior, showing the isolated areas and those with a prevailing connection with the exterior.