Microbial electron transport in the deep subsurface

The deep ocean may be one of the largest microbial habitats on the planet. Hence, high hydrostatic pressure is a feature of microbial life. We know very little about the deep biosphere because simulating deep ocean conditions in the laboratory whilst simultaneously monitoring microbial processes is difficult. Changes in pressure can inhibit some reactions, whilst simultaneously accelerating others. Assumptions about how biochemical reactions proceed under ambient conditions may lack validity in the deep biosphere. In extreme environments, microbes often exploit metabolic strategies that yield slim energetic margins. How these occur under pressure is an interesting thermodynamic puzzle. Extracellular electron transfer (EET) is a process whereby microbes respire solid substrates in their surrounding environment. For an electron to move outside of the cell, it must transit the microbial envelope through a series of membrane bound electron carriers each of which will have a unique pressure response. EET most likely evolved in the deep biosphere and therefore makes an excellent model system for studying microbial energetics in high pressure environments. In this chapter, the reader can explore the fundamentals of thermodynamics, the discovery of EET, theoretical implications of pressure effects on the relevant biochemical apparatus, and learn about a proposed system for studying the interesting phenomenon of EET under high pressure.