Catalyzes the reversible hydration of fumarate to (S)-malate. Functions in the generation of fumarate for use as an anaerobic electron acceptor. To a lesser extent, also displays D-tartrate dehydratase activity, but is not able to convert (R)-malate, L-tartrate or meso-tartrate. Is required for anaerobic growth on D-tartrate. (S)-malate = fumarate + H2O (S,S)-tartrate = H2O + oxaloacetate Binds 1 [4Fe-4S] cluster. Homodimer. Is mainly expressed during anaerobic growth. Is under the control of the Fnr transcriptional regulator. Disruption of this gene seriously impairs growth under anaerobic conditions on D-tartrate when glycerol is supplied as an electron donor (D-tartrate fermentation). Cells lacking this gene also lose most (about 79%) of D-tartrate dehydratase activity. Belongs to the class-I fumarase family.