Catalyzes the dehydrogenation of (R)-lactate (D-lactate) to pyruvate. Is likely involved in the utilization of D-lactate as a sole source for both carbon and electrons for dissimilatory sulfate reduction. Cannot use L-lactate as substrate, and NAD(+), horse cytochrome c, methylene blue or dimethylnaphthoquinone as acceptors. Active in vitro with artificial electron acceptors such as 2,6-dichlorophenolindophenol (DCPIP); the physiological acceptor is not known, but potential acceptors include cytochromes or quinones. (R)-lactate + A = AH2 + pyruvate Binds 1 FAD non-covalently per subunit. Binds 1 Zn(2+) ion per subunit. Optimum pH is 8.0. Optimum temperature is 90 degrees Celsius. Extracellular part of the protein faces the S-layer. Constitutively expressed. Belongs to the FAD-binding oxidoreductase/transferase type 4 family.