In Chapter 25, we discussed how the cycling of carbon by plants and animals is linked to their production and consumption of oxygen. But can carbon cycle through the deep waters of the Black Sea, within black muds beneath swamps and marshlands, or in other habitats where oxygen is limited or absent? Can biological communities even survive without oxygen? The answer, emphatically, is yes. Indeed, life existed on Earth for more than a billion years before oxygen-rich habitats first appeared on our planet.

The seemingly alien oxygen-poor environments on today’s Earth are populated by organisms that neither produce nor consume oxygen, yet are able to cycle carbon. The organisms responsible for this expanded cycle share one fundamental feature: They have prokaryotic cell organization. Prokaryotes, which include bacteria and archaeons, lack a nucleus (Chapter 5). They inhabit the full range of environments present on Earth, including those rich in oxygen, but their presence in habitats where plants and animals cannot live suggests that some of these organisms have biological features quite unlike those of our familiar world. We’ll look at the carbon cycle again later in this chapter, but first we discuss Bacteria and Archaea, the prokaryotic domains of microscopic organisms so critical to the cycling of carbon and other elements essential to life. We examine their diversity and how they have evolved through time.