Bacterial disproportionation of elemental sulfur is an important process
in the sulfur cycle of natural sediments and leads to the formation of
hydrogen sulfide and sulfate. The oxygen atoms in sulfate during this
anaerobic process are completely derived from water according to the
overall reaction:
4H(2)O + 4S(0) --> 3H(2)S + SO42- + 2H(+)
In the present study, stable oxygen isotope fractionation during
formation of sulfate via this reaction was experimentally investigated
for a pure culture (Desulfocapsa thiozymogenes) and an enrichment
culture (''Kuhgraben'') at 28 degreesC. Synthetic FeCO3, and FeOOH
were used as scavengers for hydrogen sulfide to keep the
disproportionation reaction exergonic and to suppress polysulfide
formation and isotope exchange between elemental sulfur and hydrogen
sulfide. Compared to water, dissolved sulfate was enriched in O-18 by +
17.4 +/- 0.1 parts per thousand (Desulfocapsa thiozymegenes) and + 16.6
+/- 0.5 parts per thousand (Kuhgraben) at cell specific sulfur
disproportionation rates of 10(-15.4) +/- (0.4) mol S degrees cell(-1)
h(-l) and 10(-14.4) +/- (0.9) mol S degrees cell(-1) h(-1) respectively.
Oxygen isotope fractionation was not influenced by the type of
iron-bearing scavenger used, corroborating earlier findings that H2S
oxidation by FeOOH yields elemental sulfur as the dominant oxidation
product. Sulfite is suggested to be formed as a metabolic intermediate
to facilitate isotope exchange with water. Due to bacterial
disproportionation, dissolved sulfate was also enriched in S-34 compared
to elemental sulfur by +11.0 to +18.4 parts per thousand (D,
thiozymogenes) and +12.7 to +-17.9 parts per thousand (Kuhgraben). FeS
was depleted in S-32 compared to elemental sulfur by -3.7 to -5.3 parts
per thousand (D. thiozymogenes). Copyright (C) 2001 Elsevier Science
Ltd.