Ultrastructure of Acaryochloris marina, an oxyphotobacterium containing mainly chlorophyll d

We present a detailed investigation of the ultrastructure of the chlorophyl l aid-containing unicellular oxyphotobacterium Acaryochloris marina, combin ing light and transmission electron microscopy and showing freeze fractures of this organism for the first time. The cells were 1.8-2.1 mu mx1.5-1.7 m u m in size. The cell envelope consisted of a peptidoglycan layer of approx imately 10 nm thickness combined with an outer membrane. Cell division was intermediate between the constrictive and the septum type. The nucleoplasm, which contained several carboxysomes, was surrounded by 7-11 concentricall y arranged thylakoids, which were predominantly stacked, with the exception of distinct areas where phycobiliproteins were located. The thylakoids wer e perforated by channel-like structures connecting the central and peripher al portions of the cytoplasm and not yet observed in other organisms. In fr eeze fractures, the protoplasmic fracture faces of thylakoid membranes were densely covered with particles of inhomogenous size. The particle size his togram peaked at 10-11, 13 and 18 nm. The 18-nm particles are assumed to re present photosystem I trimers. The particles on exoplasmic fracture faces, proposed to represent photosystem II complexes, were significantly larger t han the corresponding particles of cyanobacteria and clustered to form larg e aggregates. This kind of arrangement is unique among photosynthetic organ isms.