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metabolic themes of respiration, fermentation, and photosynthesis are, thus, impressive, but prokaryotic organisms have evolved yet another way of growing that is completely unknown in eukaryotes: chemosynthesis . Like photosynthetic organisms, chemosynthetic microbes get their carbon from CO 2 , but they harvest energy from chemical reactions rather than sunlight. Oxygen or nitrate (or, less commonly, sulfate, oxidized iron, or manganese) is combined with hydrogen gas, methane, or reduced forms of iron, sulfur, and nitrogen in ways that allow the cell to capture the energy released by the reaction. Methanogenic prokaryotes are of particular evolutionary and ecological interest; these tiny cells can gain energy from the reaction of hydrogen gas and carbon dioxide to produce methane.
    The metabolic pathways of prokaryotes sustain the chemical cycles that maintain Earth as a habitable planet. Take carbon dioxide, for example. Volcanoes supply CO 2 to the oceans and atmosphere, but photosynthesis removes it at a far faster clip. So much faster, in fact, that photosynthetic organisms could strip the present-day atmosphere of its CO 2 in little more than a decade. They don’t, of course, principally because respiration, in essence, runs the photosynthetic reaction backward. While photosynthetic organisms react CO 2 and water to produce sugar and oxygen, respiring creatures (including you, as you read this sentence) react sugar with oxygen, giving off water and carbon dioxide. Together, photosynthesis and respiration cycle carbon through the biosphere, sustaining life and maintaining the environment through time.
    It is easy to envision a simple carbon cycle in which cyanobacteria fix CO 2 into organic matter and supply oxygen to the environment, while respiring bacteria do the reverse, consuming oxygen and regenerating CO 2 . Plants and algae would do just as well as cyanobacteria, and protozoa, fungi, and animals could substitute for bacterial respirers—the prokaryotes and eukaryotes are functionally equivalent. But let’s let some cells sink to the seafloor and become buried in oxygen-depleted sediments. Now the limitations of eukaryotic metabolism become clear—reactions that do not use oxygen ( anaerobic reactions) are needed to complete the carbon cycle. In modern seafloor sediments, sulfate reduction and respiration using iron and manganese are just as important as aerobic respiration in recycling organic matter. More generally, wherever carbon passes through oxygen-free environments, bacteria are essential to the carbon cycle; eukaryotes are everywhere optional.
    The fundamental importance of prokaryotes extends to other biologically important elements, as well. Indeed, in the biogeochemical cycles of sulfur and nitrogen, all the principal metabolic pathways that cycle these elements are prokaryotic. Consider, in particular, nitrogen, an essential element required for the formation of proteins, nucleic acids, and other biological compounds. We live our lives bathed in nitrogen gas. (Air is about 80 percent N 2 by volume.) But this vast repository of nitrogen is not biologically available to us; like other animals, we obtain the nitrogen we need by eating other organisms. As it turns out, nitrogen gas is no more available to cattle or corn than it is to humans. Plants can take up ammonium (NH 4 + ) or nitrate from the soil, but how do these compounds get there in the first place? Ammonium is released as dead cells decay; nitrate, in turn, is produced by bacteria that oxidize ammonium. In oxygen-rich habitats, the resulting nitrate is available to plants (or, in aquatic ecosystems, algae and cyanobacteria), but in waterlogged soil or other environments where O 2 is depleted, other bacteria use nitrate for respiration, returning nitrogen to the atmospheric pool of N 2 . (Much of the nitrate spread across fields as fertilizer is lost in this way.)
    So, we haven’t solved our problem. The ammonium and nitrate in soil and