Involved in the biosynthesis of the major light-harvesting pigment bacteriochlorophyll c (BChlc), which confers a significant competitive advantage to green sulfur bacteria living at limiting red and near-infrared light intensities (PubMed:15090495). Catalyzes the methylation at the C-20 position of the cyclic tetrapyrrole chlorin of bacteriochlorophyll d (BChld) to produce bacteriochlorophyll c (BChlc) using S-adenosylmethionine (SAM) as a methyl source (PubMed:15090495, PubMed:16797589). a bacteriochlorophyllide d + S-adenosyl-L-methionine = a bacteriochlorophyllide c + H(+) + S-adenosyl-L-homocysteine Porphyrin-containing compound metabolism; bacteriochlorophyll biosynthesis (light-independent). Homodimer. Cells lacking this gene produce only bacteriochlorophyll d (BChld). Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family.