Further descriptive information

L. modestohalophilus exhibits considerable polymorphism under various growth conditions (Gorlenko et al., 1979). On the mineral medium of Pfennig (1965) with sulfide or thiosulfate, rod-shaped or ovoid cells of different strains may be 0.5-1.5 X 2.55 | m. These cells display dimorphism during normal development of cultures on bicarbonate-containing media with acetate, glycerol, or other organic compounds. In freshly transferred cultures, cells are rod shaped, motile by means of flagella, generally devoid of gas vesicles, and contain globules of S0. Late in the exponential growth phase, cells lose their motility, gas vesicles appear, and slime capsules are formed. In stationary phase, slime capsules consolidate and cells are transformed into cysts. Cells with gas vesicles tend to concentrate in the upper part of the bottle at room temperature. Gas vesicles, short point-ended cylinders of 60 X 80 nm to 120 X 140 nm, are grouped near the cell periphery, while sulfur is deposited in the central vesicle-free part of the cells. Multiple passages on mineral salts medium result in a temporary loss of the ability to produce motile cells, and bacteria that are surrounded by loose or dense slime capsules and contain gas vesicles during all stages of growth. Cells are large and irregularly shaped at high NaCl concentrations and an acid pH of 5.9-6.6, but are thin, elongated, and slightly bent at a pH of 8.0 or higher.

The cell wall is of the Gram-negative type and, characteristically, the outer membrane is overlaid by an external layer (Slayer) consisting of hexagonal subunits which are 20 nm long. Vegetative cells possess a loose fibrous slime capsule. Slime capsules of the cyst-like cells are dense, with a clear margin. The photosynthetic membrane system occupies the major part of the cytoplasm in bacteria growing anaerobically in the light. Storage materials are abundantly produced during growth with glycerol, both under chemotrophic (aerobically in the dark) as well as under phototrophic (anaerobically in the light) growth conditions.

Suspensions of bacteria grown anaerobically in the light appear purple-pink. In vivo absorption maxima at 370, 655, 804, and 827 nm and a shoulder at 880 nm indicate the presence of bacteriochlorophyll a, while a major peak at 518 nm and shoulders at 486 and 549 nm reveal the presence of carotenoids of the okenone group. By chemical analysis, the major components were identified as okenone (64.2%), the compound thiothece-OH-484 (18.5%), also present in Thiocystis gelatinosa (Schmidt, 1978), and spirilloxanthin (10.5%) (Sidorova et al., 1998).

Among the organic compounds that are photoassimilated, glycerol, acetate, pyruvate, lactate, maltose, and lactose strongly increase the yield. Glycerol metabolism involves the functioning of glycerol kinase and a-glycerophosphate dehydrogenase independent of pyridine nucleotides (Krasil'nikova et al., 1979). Organic acids are assimilated through the incomplete tricarbox-ylic acid cycle. The glyoxylate cycle is absent (Krasil' nikova, 1985; Krasil'nikova and Kondrat'eva, 1979). Sugars are utilized through the Embden-Meyerhof pathway.

Optimum conditions for chemotrophic growth in the dark are ~10% O2 in the gas phase. Phototrophic growth in the light is suppressed at O2 concentrations above 2%. In a stab culture in 0.8% agar medium in the dark, growth occurs 1 cm below the surface. Thiosulfate or S0 are required as electron donor and sulfur source for assimilatory purposes (Krasil' nikova, 1981). Optimum concentration of thiosulfate is 0.2%. At a concentration of 0.5% Na2S2O3-5H20, growth of aerobic cultures is strongly inhibited. In the dark, thiosulfate is oxidized to S0 and sulfate. S0 is deposited inside the cells as an intermediate product, accumulation of S0 and generation of sulfate occur in parallel. Vitamin B12 is required for chemotrophic aerobic growth (Kon-drat' eva et al., 1981). Best growth in the dark occurs with glycerol as carbon source, while lactate, pyruvate, and propionate also stimulate growth.

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