Catalyses the reaction in the opposite direction. This enzyme, purified from the cyanobacterium Synechococcus elongatus PCC 7942, catalyses the NAD(P)H-dependent reduction of an activated fatty acid (acyl-[acp]) to the corresponding aldehyde. Together with EC 188.8.131.52, octadecanal decarbonylase, it is involved in alkane biosynthesis. The natural substrates of the enzyme are C16 and C18 activated fatty acids. Requires Mg2+.