The Deep Subseafloor and Biosignatures

A critical issue in astrobiology is “where to look for present or past life?” and which types of environments could be relevant, i.e. environments associated with high probabilities to (have) support(ed) life and preserve(d) biosignatures. Due both to the large reservoir it represents and to its protective effect against harmful surface conditions, for example radiation, oxidation, the subsurface is of considerable interest in astrobiology. On Earth, living microorganisms have been documented buried in the subsurface up to depths of several kilometers, demonstrating that the deep subsurface can be inhabited by complex microbial communities for millions of years and offering astrobiologists the possibility to better understand how life could be supported, and what kind of biosignatures could be expected, in the subsurface of other planetary bodies. In this chapter we present general trends in the microbial ecology of deep subsurface environments and their peculiar conditions, with a focus on sedimentary microbial ecosystems. We provide a case study of the Canterbury Basin subseafloor as an analogue, subsurface ecosystem on extraterrestrial planetary bodies, and discuss analytical methods for studying microbial lifestyles and preservation in that ecosystem.