The most common sulfur compounds are and various sulfates, , and sulfites, , Sulfur is found in living organisms mainly in disulfide -S-S- cross-links between chains in proteins. Photosynthesis in blue-green algae and all higher plants uses water as the source of hydrogen for synthesis, and liberates oxygen. Photosynthesis by purple and green sulfur bacteria depends on a supply of instead, and yields sulfate ions as a by-product. Other bacteria, the Desulfovibrio, use sulfate to oxidize their foods instead of nitrate or oxygen. Green plants and animals share an oxygen cycle in which photosyntbesis oxidizes to in the process of storing energy in sugar molecules, and respiration reduces back to when energy is extracted from organic molecules. In a similar fashion, the sulfur bacteria and Desulfovibrio share a sulfur cycle, in which bacterial photosynthesis oxidizes to , and bacterial respiration reduces back down to . In both cases, the driving energy for the cycle is the radiation absorbed from sunlight. This sulfur cycle probably arose early in the Earth's history, and represents a process that did not become universal in the sense that water-using photosynthesis and oxygen respiration did. We will come back to these energy cycles and the evolution of metabolic processes in Chapters 23 and 26. Nitrate respiration, wherever it occurs in bacteria, always is an alternative to oxygen respiration, which the bacteria prefer if oxygen is available. Other than molecular oxygen, nitrate, and sulfate, no other chemical substance seems to have been used as an oxidant of foods in living organisms, at least not successfully enough to permit descendants of the organisms to survive to the present day.