A respiring culture of Rhodobacter sphneroides, grown in the dark under defined aerobic conditions, produced cells capable of immediately commencing adaptation to photosynthetic growth on exposure to light and further reduction of oxygen tension. Adaptation was complete after 12 h and the bacteriochlorophyll a content increased 10-20-fold. This adaptation was performed in the presence of either H,"0 or "0,. The extracted bacteriochlorophyll a was examined by mass spectrometry to determine the origin of both the 3-acetyl and 13'-oxo oxygen atoms: both were derived from water.The derivation of the 13'-0xo group from water in R. sphaeroides indicates that the formation of isocyclic ring E from the 13-propionic acid methylester side chain of Mgz'-protoporphyrin IX monomethylester is an anaerobic process involving a hydratase. This is very different to the situation in higher plants and green algae where the formation of isocyclic ring E is an aerobic process in which the 13'-0x0 group is derived from molecular oxygen via an oxygenase.In contrast to adapting R. sphaeroides cells, the 13'-oxo group of bacteriochlorophyll a i n growing cells of the obligate aerobic chemotrophic bacterium Roseobacter denitrificans, was labelled by ''02 and is, therefore, derived from molecular oxygen like in higher plants and green algae; however, the 3-acetyl group was not labelled by "0,. Thus, while the 13'-oxo group has different origins in R. sp4aeroide.r and R . denitrificans, the 3-acetyl group arises in both bacteria by enzymic hydration of the vinyl group of a chlorophyll a derivative.