Rhodobium pfennigii sp. nov., a phototrophic purple non-sulfur bacterium with unusual bacteriochlorophyll a antennae, isolated from a brackish microbial mat on Rangiroa atoll, French Polynesia

Abstract

A novel budding purple non-sulfur bacterium (strain AR2102T) was isolated in pure culture from a microbial mat that had developed in brackish-water ponds on the coral rim of the atoll of Rangiroa (Tuamotu Islands, French Polynesia). Single cells of this strain were rod-shaped and motile by means of polar flagella and divided by budding. Their intracellular photosynthetic membranes were of the lamellar type. Bacteriochlorophyll a and carotenoids of the normal spirilloxanthin series, with spirilloxanthin as the main carotenoid, were present as photosynthetic pigments. Bacteriochlorophyll a absorption in the infrared portion of the light spectrum exhibited an unusual in vivo absorption peak at 909 nm. The strain grew optimally under photoheterotrophic conditions, but could grow photolithotrophically on thiosulfate or chemo-organotrophically under micro-oxic conditions. Optimal growth occurred in the presence of 1–2 % NaCl. Comparative sequence analysis of the 16S rRNA gene placed strain AR2102T within the class Alphaproteobacteria, in a cluster with Rhodobium species. Representatives of this cluster form a closely related group of slightly to moderately halotolerant to halophilic, rod-shaped, purple non-sulfur bacteria that divide by budding. The new isolate exhibited some differences in physiology (no utilization of alcohols or carbohydrates) and genetic characteristics (low relatedness in DNA–DNA hybridization) as well as in its relation to light (differences in absorption wavelengths) from previously described Rhodobium species. Consequently, we propose that strain AR2102T (=DSM 17143T=ATCC BAA-1145T) should be considered as the type strain of a novel species within the genus Rhodobium, Rhodobium pfennigii sp. nov.