# Microbial Activity in Subterranean Ecosystems: Recent Advances
## 2 The Control of Greenhouse Gas Fluxes by Cave Microorganisms
Thus, a remarkable spatial pattern is distinguished; the highest average values of CO\({}_{2}\) concentration and the lowest CH\({}_{4}\) were found in the sectors of the lower gallery furthest from the main cave entrances (Garma A and Garma B, Figure 1b,c). Therefore, these cave maps with the contoured CO\({}_{2}\) and CH\({}_{4}\) levels reveal the importance of cave morphology in complex subterranean systems which control the gaseous composition of cave air, particularly in terms of gas variations due to the occurrence of elevation changes, multiple entrances or presence of dead-end passages. In the case of CH\({}_{4}\), its average concentration decreased drastically below 0.7 ppm from the connection of the intermediate gallery with the lower gallery and was practically null (\(<\)0.5 ppm) in the most distant sectors of the cave entrances (Figure 1c). This CH\({}_{4}\) pattern results from a decreasing percentage of mixing with the exterior and, consequently, a more effective methanotrophic activity of bacterial origin. [PERSON] et al. [42] are developing seasonal campaigns for CH\({}_{4}\) and CO\({}_{2}\) daily fluxes with continuous monitoring by a closed chamber-based gas exchange system (LI-COR Automated Soil Gas Flux System), in conjunction with a compatible Gasmet Fourier Transform Infrared (FTIR) gas analyzer and combined with \(\xi^{13}\)C geochemical tracing by cavity ring-down spectroscopy (CRDS) to understand the underlying mechanisms in cave sediments. Moreover, an autonomous piece of equipment monitored the main microenvironmental parameters of the local subsurface-soil-atmosphere system. Preliminary results showed net CO\({}_{2}\) emissions from cave sediments resulting from respiration by chemolithotrophic microorganisms. The results also revealed simultaneous net CH\({}_{4}\) uptake from cave sediments on a daily scale, with no significant level of variations along the day (Figure 2). Anaerobic oxidation of CH\({}_{4}\) coupled to nitrite reduction is produced by members of the phylum _Rokubacteria_. These bacteria have also been found in Pindal Cave [42] and in an Alpine cave [43]. Other studies combining the depletion of CH\({}_{4}\) with other GHGs (N\({}_{2}\)O and NO\({}_{2}\)) were carried out in Vapor Cave, Southeast Spain. This is a hypogenic cave formed by the upwelling of hydrothermal CO\({}_{2}\)-rich fluids in which anomalous concentrations of nitrogen oxides can be found [44]. The cave is characterized by a combination of rising warm air with large CO\({}_{2}\) outgassing and highly diluted CH\({}_{4}\) of endogenous origin. Additionally, extreme environmental conditions were observed, such as high air temperatures (38-43 \({}^{\circ}\)C) and 100% relative humidity, hypoxic conditions (17% O\({}_{2}\)), CO\({}_{2}\) concentrations that exceed 1%, 222 Rn activity with values above 50 kBq/m\({}^{3}\), and a vertical thermal gradient of 3.2 \({}^{\circ}\)C/100 m [45]. These conditions, associated with the combined effects of tectonic activity and hydrothermalism, make this cave a remarkable site for the study of uncommon or extremophilic microbial communities. In Vapor Cave, the depletion of CH\({}_{4}\) was quantified to account for more than 60% removal of the deep endogenous component of this gas [45]. Figure 2: **(a)** Detail of CH\({}_{4}\) uptake fluxes with an average of –3.15 nmol m\({}^{-2}\) s\({}^{-1}\) and **(b)** simultaneous CO\({}_{2}\) emission fluxes uptake fluxes with an average of +0.17 umol m\({}^{-2}\) s\({}^{-1}\), monitored on 17 December 2019 directly above sediments inside Pindal Cave (Astruias, Spain). The value of the diffusive flux (F) and the corresponding exponential adjustment (R\({}^{2}\)) of each measurement are indicated. [PERSON] et al. [44] collected different cave air and sediment samples from \(-\)2 to \(-\)80 m in Vapor Cave.