The evolution of the sulfur cycle

There are 2 principal avenues of inquiry relevant to reconstructing the his tory of the sulfur cycle. One avenue relies on the comparison of molecular sequences derived fi om biologically essential proteins and genetic materia l. Most sequence information is available from the small subunit of the rRN A molecule and from these sequences a Tree of Life emerges providing a reco nstruction of the evolutionary relationships among organisms. Near the root of the tree are numerous bacteria(1)metabolizing sulfur species including organisms living from dissimilatory elemental sulfur reduction, dissimilato ry sulfate reduction, and anoxygenic photosynthesis. These metabolisms are likely very ancient. Many of the deep-branching bacteria of the sulfur cycl e are active at very high-temperatures (hyperthemophiles) and are commonly found in modern sulfide-rich hydrothermal systems. One can imagine a primit ive early Earth terrestrial ecosystem housed around active hydrothermal are as with anoxygenic photosynthesis producing organic matter and oxidized sul fur species. These oxidized sulfur species could have been used as electron accepters in the mineralization of organic matter, completing the carbon c ycle. The evolution of oxygenic photosynthesis provided for dramatically in creased rates of carbon production, and a much wider range of ecosystems fo r both carbon production, and carbon oxidation. Either associated with, or following, the evolution of oxygenic photosynthesis is the emergence of lin eages housing most of the bacteria of which we are familiar, including most of the bacteria of the sulfur cycle. The geologic record can provide direct evidence fbr the state of chemical o xidation of the Earth-surface, with possible indications of when specific b acterial metabolisms first occurred. We offer the following scenario for th e evolution of the Earth-surface environment based on the available geologi cal evidence. By 3.5 Ga anoxygenic photosynthesis was established and provi ded a weak source of sulfate to the global ocean with sulfate concentration s likely much less than 1 mM. In some instances locally high concentrations of sulfate could accumulate and precipitated as evaporitic sulfate mineral s. There is no compelling evidence for sulfate reduction at this time. The first evidence for sulfate reduction is found between 2.7 and 2.5 Ga, and t he first evidence for oxygen production by oxygenic photosynthesis is found at around 2.8 Ga. Even so, levels of seawater sulfate remained low, below I mM, and did not increase to >1 mM until around 2.3 Ga This increase in su lfate levels may have been promoted by a rise in atmospheric oxygen concent ration at this time. Throughout the Archean and early Proterozoic the deep oceans contained appreciable concentrations of dissolved ferrous iron, and banded iron formations (BIFs) were a common form of chemical sediment. Sulfate levels increased slowly, and by 1.8 Ga sulfate concentrations were sufficient to increase rates of sulfate reduction to greater than the deliv ery flux of iron to the oceans. Sulfide accumulated and precipitated ferrou s iron from solution. It is proposed that the oceans remained sulfide-rich until the Neoproterozoic, where renewed deposition of banded iron formation s occurred at around 0.75 Ga. It is possible that during the Neoproterozoic , decreased carbon production resulted from an ice covered "Snowball Earth" reducing rates of sulfate reduction below rates of iron delivery to the oc eans, promoting BIF formation. At around this time high carbon burial rates increased levels of atmospheric oxygen to >10 percent present-day levels, promoting the widespread oxidation of marine surface sediments and an evolu tionary radiation of sulfide oxidizing bacteria.