Further descriptive information

Nitrococcus mobilis belongs to the family Ectothiorhodospiraceae in the class Gammaproteobacteria (Woese et al., 1985). It is the only example of a nitrite-oxidizing bacterium that is phylogenetically related to ammonia-oxidizing bacteria (Nitrosococcus oceani and "Nitrosococcus halophilus") in the same class. All these organisms are from marine habitats. No growth occurs in freshwater media even if NaCl is included. The temperature range for growth is 15-30°C and the pH range is 6.8-8.0. No terrestrial strains are known so far. Ectothiorhodospira, Arhodomonas, Halorhodospira, and Thiorhodospira are the closest relatives of Nitrococcus.

Additional details and a comparison of the biochemical properties of Nitrococcus to those of other nitrite-oxidizing genera can be found in the introductory chapter "Lithoautotrophic Nitrite-Oxidizing Bacteria." Detailed treatments of the ecology of nitrite-

oxidizing bacteria and of the phylogeny of these organisms can be found in the introductory chapter "Nitrifying Bacteria."

Because of the presence of extensive intracytoplasmic membrane systems in both genera, Teske et al. (1994) hypothesized that Nitrobacter and Nitrococcus derived from an immediate photosynthetic ancestry. Additional features that the two genera have in common include a-type cytochromes, flagella, carboxysomes, and storage materials. The tubular membranes of Nitrococcus are quite similar to the internal photosynthetic membrane system of Thiocapsa pfennigii (Thiococcus pfennigii).

A spherical cell of Nitrococcus with two flagella is shown in Fig. BXII.y.23. The tubular intracytoplasmic membranes are visible in Fig. BXII.y.24. Cells are elongated before division (1.8 X 3.5 | m). They may occur as small clumps of cells embedded in a slime matrix. The cell wall is similar to that seen in most other Gram-negative bacteria. The tubular membrane system arises by invaginations of the cytoplasmic membrane and has a branching nature. As in Nitrobacter, the cytoplasmic and intracytoplasmic membranes are asymmetric, with an electron-dense layer on the inner side; the structure of the membranes becomes apparent when cells are ruptured and negatively stained. The surface of the membrane is covered with particles, which are arranged in rows (Fig. BXII.y.24 and BXII.y.25). Watson and Waterbury (1971) described doughnut-shaped particles 6-8 nm in diameter, which occasionally form rectilinear arrays. The particulate intra-cytoplasmic membranes of Nitrococcus were labeled with gold particles using monoclonal antibodies (MAbs) that recognize the b-

FIGURE BXII.y.25. Isolated intracytoplasmic membranes of Nitrococcus mobilis in negative contrast. The membrane surface is densely packed with particles of about 6 nm in diameter (A) which can be regularly arranged in rows (B). Bars = 50 nm.

0 0

Post a comment