Microbial life on Earth occupies an astonishingly wide range of habitats. Microbes proliferate in hydrothermal vents at the bottom of the ocean, in the intensely cold and dry valleys of Antarctica, in acid mine drainage streams laden with toxic metals, and in anoxic sediments in lakes, rivers, and the bottom of the sea. They occur in vast numbers in environments as diverse as the nutrient-poor waters of the open ocean and the guts of animals. The sources of carbon, nitrogen, phosphorous, sulfur, and energy for building biomass vary widely in these environments. Microbes have evolved an incredibly diverse range of strategies that take advantage of the resources available in particular environmental niches. Some, termed autotrophs (“self-feeding”), can synthesize all organic compounds needed for cell growth from CO2, H2, and inorganic sources of the phosphorous, sulfur, and trace metals they need. Others, termed heterotrophs (“other”-feeding), derive carbon and energy from organic molecules synthesized by other life forms. Figure 3.1 summarizes these two approaches to metabolism. Building biomass requires energy. Consequently, living organisms must harness energy from the environment and store it in a chemical or physical form. Energy can be harnessed either from light or from oxidation of electron-rich compounds. Some microbes oxidize organic molecules such as sugars, while others oxidize inorganic species such as H2, Fe(II), or H2S. Electrons derived from oxidation of organic or inorganic compounds can be passed to a variety of electron acceptors, including O2, NO− 3, and SO4 −2. Such coupled oxidation and reduction reactions are termed “redox processes.” During the thermodynamically downhill passage of electrons from the initial electron donor to the final electron acceptor, energy is stored in the form of electrochemical gradients of ions (usually H+, but sometimes Na+) across lipid bilayers. These electrochemical gradients are subsequently used to drive energy-requiring processes such as movement, uptake of nutrients, and, most importantly, synthesis of ATP (see Figure 3.2). The chemical energy stored in ATP is used to drive hundreds of other energy-requiring processes.