Sulfur-Enhanced Fumarate Metabolism in Sulfurospirillum tamanense: Evidence for Cryptic Sulfur Cycling

A previously unexplored form of cryptic sulfur cycling is described for Sulfurospirillum tamanense, wherein fumarate and elemental sulfur alternately act as electron donor and acceptor, cycling through transient sulfide intermediates, coupling with fumarate dismutation. In a system initially containing fumarate and elemental sulfur, three processes can occur simultaneously: (1) oxidation of fumarate coupled with dissimilatory sulfur reduction and the formation of sulfide; (2) subsequent oxidation of sulfide during fumarate reduction; (3) fumarate dismutation. Sulfide concentration remains stable throughout, suggesting a dynamic equilibrium between fumarate-dependent sulfur reduction and fumarate-dependent sulfide oxidation, while succinate and acetate are the end products. This cycling mechanism increases cell yield several times compared to growth via fumarate dismutation through improved redox balance maintenance and increased ATP yield via multiple electron transfer steps.