The impacts of human activity on the microorganisms in deep subsurface

Abstract

In the deep biosphere, there are a large number of microorganisms that are mainly bacteria and archaea. Fungi and viruses are also found in deep subsurface. These microorganisms found in the deep biosphere are functionally and phylogenetically diverse. Different from the microorganisms on the Earth surface where they use sunlight as their primary energy source, the microorganisms in deep subsurface employ the hydrogen gas and methane, which are produced mainly from geochemical interactions between water and rock, as their primary energy and/or electron sources. Human activities impact substantially on the deep subsurface environment as well as the microorganisms living there. This review focuses on the impacts of recovery of shale gas by horizontal drilling and hydraulic fracturing (or fracking), geological storage of nuclear wastes and CO2 capture and geological storage on the microorganisms in the deep biosphere. Shale gas is the natural gas locked within the black shale in deep subsurface. Horizontal drilling and fracking are widely used to recover the shale gas. Research results demonstrate that the structures and functions of microbial community differ greatly at different stages of recovery of shale gas by fracking. Results also find that viruses play crucial roles in influencing the structures and functions of microbial community during different stages of recovery of shale gas by fracking. Among the microorganisms identified, the methanogens in the black shale impact positively on the natural gas recovery by producing methane. In contrast to the methanogens, the acid-producing microorganisms impact negatively on the natural gas recovery by causing reservoir souring and corrosion of metal structures. Thus, the microorganisms in the deep subsurface play dual roles in impacting the natural gas recovery. Nuclear wastes are the materials containing or contaminated with radionuclide. Safe storage of nuclear wastes is the most important factor that determines whether nuclear industry and peaceful use of nuclear energy can be continuously developed. Currently, geological storage in deep subsurface is the only acceptable way for long-term and safe storage of the nuclear wastes whose half-life is long and radioactivity is high. For this reason, underground research laboratories (URL) are constructed to assess potential impacts of the microorganisms in the deep subsurface on the safe storage of nuclear wastes in the geological formation. The results demonstrate that complex of microbial communities reside in the geological formations planned for storage of nuclear wastes. Although structures and functions of these microbial communities vary, the sulfate-reducing bacteria found in these communities may corrode the metal structures, while gas-producing microorganisms may increase the chamber pressure of the storage structure. All of these microbial activities may damage the structure and function of the nuclear waste geological repository. In contrast, the Fe(III)-reducing microorganisms may stabilize nuclear wastes in the repository. CO2 capture and storage could slow down the global warming by reducing CO2 emissions. Research results show that certain group of bacteria live under supercritical CO2 condition in the deep subsurface. Moreover, geological storage of supercritical CO2 acidify the water in deep subsurface, which increases dissolution of rocks and minerals. Consequently, it changes the chemical environment and the structures and functions of microbial community in the deep subsurface. In turn, the responses of microbial communities to the addition of supercritical CO2 may affect the stability of CO2 storage in the geological settings. Over the past decades, our understanding of the impacts of human activity on the microorganisms in deep subsurface has been advanced significantly. A common theme emerging from the results of these studies is that the microorganisms influenced by human activity may have either positive or negative effects in recovery of shale gas and geological storage of nuclear waste and CO2.