Let’s think further about prokaryotic metabolism. We saw that some photosynthetic and chemosynthetic bacteria oxidize reduced sulfur compounds like H₂S to sulfate (SO₄^2–), whereas other heterotrophic bacteria reduce sulfate to H₂S. This is reminiscent of the carbon cycle outlined in Chapter 25, where plants generate oxygen and animals consume it. Metabolisms at work where oxygen is absent can also complete the carbon cycle, and they do so by coupling the cycling of carbon to the cycling of other elements—in this case, sulfur. Once again, we see complementary metabolic pathways. The expanded carbon cycle of Bacteria and Archaea promotes the cycling of sulfur, nitrogen, and other biologically important elements.