Involved in the degradation of 3,6-anhydro-L-galactose, which is the major monomeric sugar of red macroalgae. Catalyzes the fourth step of the pathway, the reduction of 3-deoxy-D-glycero-2,5-hexodiulosonate (L-DDGal) to 2-dehydro-3-deoxy-D-gluconate (KDG). 2-dehydro-3-deoxy-D-gluconate + NAD(+) = 3-deoxy-D-glycero-2,5-hexodiulosonate + H(+) + NADH Belongs to the short-chain dehydrogenases/reductases (SDR) family.