The deep biosphere of the subseafloor igneous crust

Abstract

The igneous portion of the subseafloor crust is considered to be the largest potential microbial habitat on Earth; thus, it is somewhat of a paradox that our knowledge regarding its abundance, diversity and ecology is sparse, close to non-existent. This is mainly due to issues involved in sampling live species, and therefore much of our present knowledge of the deep biosphere is based on a fossil record. However, drilling and sampling techniques are constantly being developed to facilitate sampling of live microorganisms, and recent molecular studies show a positive progress towards better recovery and less contamination. Here we discuss the subseafloor igneous crust as a microbial habitat, its physical and geochemical prerequisites to support life and what type of life that could sustain in such an extreme environment. We also discuss what the fossil record, and the few successful molecular studies, tells us regarding what type of microorganisms exist in the deep subseafloor settings. It appears as if the igneous crust is more diverse than previously expected consisting of both prokaryotes and eukaryotes in close interplay with each other and their physical environment. As our knowledge increases so does the questions, and hopefully future technique development can provide us with an increased understanding of this deep, hidden world.