ULTRASTRUCTURE OF THE HYPERTHERMOPHILIC ARCHAEON PYRODICTIUM-ABYSSI

The ultrastructure of a new isolate of Pyrodictium abyssi, obtained fr om the Midatlantic Ridge at a depth of 3600 m, was studied by scanning and transmission electron microscopy using cryopreparation techniques . Scanning electron microscopy revealed an extensive three-dimensional network, consisting of cells and extracellular tubules. Cells are int ertwined with the tubules. The presence of 10-nm thin flagella was ver ified by low-angle shadowing, freeze etching, and negative staining. I n scanning electron microscopy and in ultrathin sections in transmissi on electron microscopy, cells are highly irregular with ultraflat area s. Transmission electron microscopy studies showed that the tubules ar e hollow cylinders, made of helically arrayed protein subunits, with a n outer diameter of approximately 25 nm. The tubules often conglomerat e into large bundles. Sections of high-pressure-frozen, freeze-substit uted cells revealed many new details: the cytoplasm is homogenous and densely packed, occasionally containing regular structures of unknown nature in ribosome-free areas; the cell envelope consists of a cytopla smic membrane, a periplasmic space, containing lightly stained materia l, and a zigzag-shaped surface-layer protein. The subunits of the surf ace layer are arranged on a p6 lattice with a center-to-center spacing of 21 nm. (C) 1995 Academic Press, Inc.