Description of New Strains of a Purple Sulfur Bacterium Thiorhodospira sibirica and Characterization of Their Ultrastructural Features

The present article describes the characteristics of four strains of the alkaliphilic halotolerant purple sulfur bacterium Thiorhodospira sibirica isolated from steppe soda lakes with a mineralization level of 7−35 g/L (рН 9.0−9.8) in Zabaykalsky krai, the Republic of Buryatia (Russia), and northeastern Mongolia. All studied strains exhibited the morphological and physiological features characteristic of Trs. sibirica, such as a unique pigment absorption spectrum in vivo with four bacteriochlorophyll a absorption maxima in the near infrared region and extraordinarily arranged lamellar-type photosynthetic membranes. Similar to the type strain A12T (=ATCC 700588T), the bacteria of all tested strains formed elemental sulfur as an intermediate sulfide oxidation product; its globules were located intracellularly and not extracellularly, as in other Ectothiorhodospiraceae. Using the type strain, the intracellular arrangement of elemental sulfur globules was demonstrated in ultrathin sections by means of the specific silver nitrate reaction. All studied strains were characterized by 93−95% similarity based on DNA–DNA hybridization results or 98.55−98.61% similarity to Trs. sibirica A12T (=ATCC 700588T) in terms of 16S rRNA gene sequences. This confirms that they belong to the Trs. sibirica species.