Our bodies are mostly carbon, oxygen, and hydrogen, but we also contain about 0.2% sulfur by weight. Sulfur is a component of the amino acids cysteine and methionine and therefore is present in many proteins (Chapter 2). Sulfur is also present in iron–sulfur clusters that are key components of enzymes fundamental to electron transfer (Chapter 6) and in some vitamins. Where do we get the sulfur we need? From the food we eat—protein in steak or fish provides a rich source of sulfur. Where do cows and fish get the sulfur they need? Like us, from the food they eat. Food chains don’t go on forever, so somewhere in the system, some organisms must take up inorganic sulfur from the environment. Primary producers, organisms that reduce CO₂ to form carbohydrates, fill this role.

Plants are the dominant primary producers on land. Plants take up sulfate (SO₄^2–) ions from the soil and reduce them within their cells to hydrogen sulfide (H₂S) that can be incorporated into cysteine and other biomolecules (Fig. 26.10). This process is called assimilation. Algae and photosynthetic bacteria do much the same thing in lakes, rivers, and oceans. Why don’t primary producers take up H₂S directly? There are two answers to this question. First, H₂S is rapidly oxidized in the presence of oxygen and so does not occur in environments where oxygenic photosynthesis is common. Second, H₂S is generally toxic to eukaryotic organisms, so plants and algae do not thrive where it is abundant. (The H₂S produced within eukaryotic cells has a short lifetime and is restricted to intracellular sites distant from those that are vulnerable to its toxic effects.)

We’ve now seen half the sulfur cycle, the conversion of sulfate to H₂S within cells. How did sulfate molecules get into the soil in the first place? After death, fungi and bacteria decompose cells, returning carbon, sulfur, and other compounds to the environment. Reduced sulfur compounds released from decomposing cells are oxidized by bacteria and archaeons, completing the cycle. These microbes are chemoautotrophs that obtain energy by oxidizing H₂S or photosynthesizers that use H₂S as the electron donor. In addition to being taken up by plants, the sulfate produced by these processes is consumed by heterotrophic bacteria living in oxygen-free environments found in soil, sediments, and occasionally in lakes and seawater. Sulfate rather than oxygen is used as the final electron acceptor in respiration and is reduced to H₂S. In fact, most of the sulfur cycled biologically is used to drive energy metabolism in oxygen-poor environments, not to build proteins.

Note that eukaryotes use neither H₂S for photo- (or chemo-) synthesis nor sulfate in respiration. Thus, the biological sulfur cycle is completed by bacteria and archaeons alone.