Ultrastructural characterization of the prokaryotic symbiosis in Chlorochromatium aggregatum

The phototrophic consortium Chlorochromatium aggregatum currently represents the most highly developed interspecific association of bacteria and consists of green sulfur bacteria, so-called epibionts, surrounding a central, motile, chemotrophic bacterium. In order to identify subcellular structures characteristic of this symbiosis, consortia were studied by a combination of high-resolution analytical scanning electron microscopy, transmission electron microscopy, and three-dimensional reconstruction and image analyses. Epibionts are interconnected and to a lesser extent are also connected with the central bacterium, by electron-dense, hair-like filaments. In addition, numerous periplasmic tubules extend from the outer membrane of the central bacterium and are in direct contact with the outer membrane of the epibionts. In each epibiont cell, the attachment site to the central bacterium is characterized by the absence of chlorosomes and an additional 17-nm-thick layer (epibiont contact layer [ECL]) attached to the inner side of the cytoplasmic membrane. The ECL is only occasionally observed in pure cultures of the epibiont, where it occurs in about 10 to 20% of the free-living cells. A striking feature of the central bacterium is the presence of one or two hexagonally packed flat crystals (central bacterium crystal [CBC]) per cell. The CBC reaches 1 mu m in length, is 35 nm thick, and consists of bilayers of subunits with a spacing of 9 nm. A detailed model for consortia is presented, summarizing our conclusions regarding (i) cohesion of the cells, (ii) common periplasmic space between the central bacterium and the epibiont, (iii) ECL as a symbiosis-specific structure, and (iv) formation of the interior paracrystalline structures, central bacterium membrane layer, and CBC.