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