The deep continental subsurface: the dark biosphere

Abstract

Although information from devoted geomicrobiological drilling studies is limited, it is clear that the results obtained so far call for a systematic exploration of the deep continental subsurface, similar to what has been accomplished in recent years by the Ocean Drilling Initiatives. In addition to devoted drillings from the surface, much of the continental subsurface data has been obtained using different subterranean Bwindows,^each with their correspondent limitations. In general, the number and diversity of microorganisms decrease with depth, and the abundance of Bacteria is superior to Archaea. Within Bacteria, the most commonly detected phyla correspond to Proteobacteria, Actinobacteria, Bacteroidetes, and Firmicutes. Within Archaea, methanogens are recurrently detected in most analyzed subsurface samples. One of the most controversial topics in the study of subsurface environments is whether the available energy source is endogenous or partly dependent on products photosynthetically generated in the subsurface. More information, at better depth resolution, is needed to build up the catalog of deep subsurface microbiota and the biologically available electron acceptors and donors.

Detection of the deep biosphere in metamorphic rocks from the Chinese continental scientific drilling

“…Notably, the microbial abundance exhibits no apparent depth dependence. This finding contrasts with a general belief that microbial abundance decreases with depth in continental subsurface (Cockell et al, 2012;Escudero et al, 2018;Itavaara et al, 2011;Kieft, 2016;McMahon & Parnell, 2014;Nyyssonen et al, 2014). Likewise, the microbial cell abundance in rock cores exhibited no statistical correlation with rock lithology, although some gneiss rocks tend to have higher biomass (Table S1), similar to the findings of a previous review study (Magnabosco et al, 2018).…”

Microbial Life in the Deep Subsurface Aquifer Illuminated by Metagenomics

“…Since sulfate-reducing members of the Firmicutes are common to the deep subsurface (e.g., Moser et al, 2005;Magnabosco et al, 2016;Jungbluth et al, 2017;Escudero et al, 2018) we searched for the presence of dissimilatory sulfate reduction pathways in all obtained Firmicutes MAGs. Surprisingly, only two MAGs, Ch2 and Ch87, whose shares in the genome were 1.61 and 0.24%, respectively, encoded the necessary set of enzymes and could be sulfate reducers.…”

Subsurface Microbial Hydrogen Cycling: Natural Occurrence and Implications for Industry

“…Hydrogen-based metabolisms are thought to have been critical to life on early Earth; however, with the advent of oxygenic photosynthesis, hydrogen-based life became confined to specialized niches [26]. Hydrogen appears to be the main source of energy for autotrophs in many subsurface environments, especially those that have no, or a limited supply of carbon from photosynthesis [48]. Even at low concentrations, various microbial metabolisms can be supported by hydrogen when a suitable terminal electron acceptor is available (Table 3).…”

“…This study, which proposed a microbial ecosystem supported by hydrogen produced from the reaction of water with iron in ferromagnesian silicates, was controversial, and there was considerable debate around a number of issues including whether the amount of hydrogen experimentally generated from these rocks, at environmentally relevant pH, was sufficient to support the biomass; whether the rocks contain sufficient reduced iron to sustain microbial activity over geologically relevant timescales; whether or not organic material is more likely the source of hydrogen; and whether reduced gases from deeper in the Earth support the microbial ecosystem in this environment [66,67,68]. It will always be difficult to determine the contribution of different hydrogen-generating mechanisms in the deep subsurface; however, support for the existence of microbial ecosystems that are primarily, if not completely dependent on abiotic hydrogen sources in various geological settings is growing [48,62,63,69,70,71], including from some of the authors arguing against Stevens and McKinley’s original paper [8]. Independence from photosynthesis, including any contributions of carbon, nitrate, sulfate, and ferric iron, would be of great significance for the understanding of life on a pre-photosynthetic Earth and the possibilities of life on other planetary bodies, but remains to be unequivocally proven.…”