Figure 2.

Predicted (this study) and experimentally determined (previously reported) rate-fractionation relationships during DSR. (a) Reported range of S isotope fractionation from natural environments (data compiled in Sim et al., 2011a) and (b) rate-fractionation relationships determined in pure culture experiments with DMSS-1 (Sim et al., 2011a, 2011b, 2012), D. vulgaris Hildenborough (Leavitt et al., 2013), D. desulfuricans (Chambers et al., 1975) and A. fulgidus (Habicht et al., 2005) grown on organic substrates. The magnitude of S isotope fractionation increases with decreasing csSRR. The black vertical lines indicate the possible range of S isotope fraction allowed by the respective electron carriers (determined in this study). The full range of environmental and experimental fractionation is reproducible with electron carriers with modestly negative reduction potentials such as for example, menaquinone (MK). Ferredoxin (Fd) as a direct electron donor would limit the maximum fractionation to <22‰, which is inconsistent with reported experimental data at low csSRR. (c) Predicted rate-fractionation relationship with menaquinone (solid line) and ferredoxin (dashed line) as electron carrier vs. experimental data (markers) from experiments with D. vulgaris Hildenborough (Leavitt et al., 2013). Cyt c3, cytochrome c3; Fla, flavodoxin; Rb, rubredoxin; Rd, rubrerythrin.