Thiorhodovibrio frisius and Trv. litoralis spp. nov., Two Novel Members from a Clade of Fastidious Purple Sulfur Bacteria That Exhibit Unique Red-Shifted Light-Harvesting Capabilities.

In the pursuit of cultivating anaerobic anoxygenic phototrophs with unusual absorbance spectra, a purple sulfur bacterium was isolated from the shoreline of Baltrum, a North Sea island of Germany. It was designated strain 970, due to a predominant light harvesting complex (LH) absorption maximum at 963-966 nm, which represents the furthest infrared-shift documented for such complexes containing bacteriochlorophyll a. A polyphasic approach to bacterial systematics was performed, comparing genomic, biochemical, and physiological properties. Strain 970 is related to Thiorhodovibrio winogradskyi DSM 6702T by 26.5, 81.9, and 98.0% similarity via dDDH, ANI, and 16S rRNA gene comparisons, respectively. The photosynthetic properties of strain 970 were unlike other Thiorhodovibrio spp., which contained typical LH absorbing characteristics of 800-870 nm, as well as a newly discovered absorption band at 908 nm. Strain 970 also had a different photosynthetic operon composition. Upon genomic comparisons with the original Thiorhodovibrio strains DSM 6702T and strain 06511, the latter was found to be divergent, with 25.3, 79.1, and 97.5% similarity via dDDH, ANI, and 16S rRNA gene homology to Trv. winogradskyi, respectively. Strain 06511 (=DSM 116345T) is thereby described as Thiorhodovibrio litoralis sp. nov., and the unique strain 970 (=DSM 111777T) as Thiorhodovibrio frisius sp. nov.

Puf operon sequences and inferred structures of light-harvesting complexes of three closely related Chromatiaceae exhibiting different absorption characteristics.

Whole cells of the purple sulfur bacterium strain 970 exhibit an unusual absorption peak at 963 nm. Its closest relatives, Thiorhodovibrio (Trv.) winogradskyi DSM6702(T) and strain 06511 display a bacteriochlorophyll (BChl) a absorption peak at 867 nm that is characteristic for most light-harvesting complexes 1 (LHC1) of proteobacteria. The puf operons encoding the LHC1 and reaction center proteins were amplified, cloned, and sequenced, and for the Trv. winogradskyi, strains show the common pufBALMC gene arrangement, whereas strain 970 contains a second pufBA copy downstream of pufC. Only pufB(1)A(1) is transcribed, and the corresponding mRNA fragment had an increased stability. Alignments of the deduced protein sequences showed that the LHC1 polypeptides are closely related to those of Thermochromatium (Tch.) tepidum. A deletion between αHis(0) and αTrp(+11), thought to be responsible for the redshifted Q(y) absorption in Tch. tepidum, was also detected in strain 970 and Trv. winogradskyi, whereas αLys(+12) is replaced by histidine only in strain 970. Based on our structural modeling, the side chain of this αHis is predicted to be in close proximity to the BChl a, suggesting that it exerts a modulating effect on the spectral properties of the highly unusual LHC1 complex of strain 970.