Chlorophyll (Chl) d, which is a major pigment in an oxygenic photosynthetic organism Acaryochloris marina, was almost fully inserted into the circularly arranged binding sites of B800 bacteriochlorophyll (BChl) a in a bacterial light-harvesting protein, LH2. The 3-formyl group in Chl d was hydrogen-bonded with the polypeptides in essentially the same manner as the 3-acetyl group in BChl a in native LH2.