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Sulfur is a key element in plant growth and development, especially in the production of cysteine and methionine. Sulfur is found in the form of sulfur ions, mineral sulfate and sulfide gas.
34 S is an important tracer of pollution sources and ecosystem processes (Krouse et al 1984, Mitchell et al 1998, Mayer et al 2010). It can be used to differentiate atmospheric sulfur from geologic sulfur sources or to investigate the nature of S imports to soil microbes in acid rain-impacted forests.
Phytoplankton, algae, and microbial communities can reduce S to sulfate and sulfide by either anaerobic or aerobic processes. Anaerobic reduction of S to sulfide and sulfate (MSR) strongly fractionates 34S/32S, whereas oxidative processes such as photosynthesis or microbial respiration typically result in minimal isotopic fractionation.
Biomass and its products can be highly enriched in 34S relative to S inputs from primary S sources (Mitchell et al 1998, Mayer & Rouse 1986). Relatively high 34S enrichments have been reported for herbivore muscle meat and milk, as well as in hair, hooves, and horn.
However, a fully committed pathway that consumes sulfate molecules would inhibit the isotope fractionation associated with S incorporation in the biomass. Consequently, the isotopic composition of Cys and Met should be quite similar to that of absorbed sulfate, while SAhC and SMM should have a large isotopic offset, particularly if they are produced through the SMM cycle (Giner et al 2012). Because 34S can also be depleted by inorganic processes, a largely unreduced S source such as animal manure can be enriched with 34S.