The evolution of the sulfur cycle

ABSTRACT. There are 2 principal avenues of inquiry relevant to reconstruct-ing the history of the sulfur cycle. One avenue relies on the comparison of molecular sequences derived from biologically essential proteins and genetic material. Most sequence information is available from the small subunit of the rRNA molecule and from these sequences a Tree of Life emerges providing a reconstruction of the evolutionary relationships among organisms. Near the root of the tree are numerous bacteria1metabolizing sulfur species including organ-isms living from dissimilatory elemental sulfur reduction, dissimilatory sulfate reduction, and anoxygenic photosynthesis. These metabolisms are likely very ancient. Many of the deep-branching bacteria of the sulfur cycle are active at very high temperatures (hyperthermophiles) and are commonly found in modern sulfide-rich hydrothermal systems. One can imagine a primitive early Earth terrestrial ecosystem housed around active hydrothermal areas with anoxygenic photosynthesis producing organic matter and oxidized sulfur species. These oxidized sulfur species could have been used as electron acceptors in the mineral-