ISOLATION AND CHARACTERIZATION OF A NOVEL FACULTATIVELY ALKALIPHILIC NITROBACTER SPECIES, N-ALKALICUS SP. NOV

Five strains of lithotrophic, nitrite-oxidizing bacteria (AN1-AN5) wer e isolated from sediments of three soda lakes (Kunkur Steppe, Siberia; Crater Lake and Lake Nakuru, Kenya) and from a soda soil (Kunkur Step pe, Siberia) after enrichment at pH 10 with nitrite as sole electron s ource. Morphologically, the isolates resembled representatives of the genus Nitrobacter. However, they differed from recognized species of t his genus by the presence of an additional S-layer in their cell wall and by their unique capacity to grow and oxidize nitrite under highly alkaline conditions. The influence of pH on growth of one of the strai ns (AN1) was investigated in detail by using nitrite-limited continuou s cultivation. Under such conditions, strain AN1 was able to grow at a broad pH range from 6.5 to 10.2, with an optimum at 9.5. Cells grown at pH higher than 9 exhibited a clear shift in the optimal operation o f the nitrite-oxidizing system towards the alkaline pH region with res pect to both reaction rates and the affinity. Cells grown at neutral p H values behaved more like neutrophilic Nitrobacter species. These dat a demonstrated the remarkable potential of the new nitrite-oxidizing b acteria for adaptation to varying alkaline conditions. The 16S rRNA ge ne sequences of isolates AN1, AN2, and AN4 showed high similarity (gre ater than or equal to 99.8%) to each other, and to sequences of Nitrob acter strain R6 and of Nitrobacter winogradskyi. However, the DNA-DNA homology in hy hybridization studies was too low to consider these iso lates as new strains. Therefore, the new isolates from the alkaline ha bitats are described as a new species of the genus Nitrobacter, N. alk alicus, on the basis of their substantial morphological, physiological , and genetic differences from the recognized neutrophilic representat ives of this genus.