The deep continental subsurface: the dark biosphere.

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 "windows," 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.