Differentiation of the genus Thermochromatium from

other genera

According to the results of 16S rDNA sequence analyses, the genus Thermochromatium belongs to the branch of the family Chro-

matiaceae containing motile freshwater bacteria, characterized by high temperature optima, above 40°C (T tepidum at 48-50°C) and lack of capability for chemotrophic growth. The type strain of T. tepidum has an unusual and quite characteristic absorption spectrum. Intact cells and photosynthetic membrane preparations show a near-infrared absorption peak at 920 nm (Garcia et al., 1986). This absorption is due to a novel light-harvesting complex, and is the longest wavelength-absorbing band observed for a photosynthetic light-harvesting complex containing bacterio-chlorophyll a. Whether this unusual complex is related to the thermophilic lifestyle of T. tepidum is unknown.

Morphological and physiological characteristics for the differentiation of Thermochromatium species from other photo-trophic members of the family Chromatiaceae are shown in Table BXII.y.4 of the chapter describing the family Chromatiaceae. The phylogenetic relationship of Thermochromatium tepidum to other members of the family Chromatiaceae, based on 16S rDNA sequence comparison, is shown in Fig. BXII.y.3 of the chapter describing the family Chromatiaceae.

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