Chloroflexus aggregans sp. nov., a Filamentous Phototrophic Bacterium Which Forms Dense Cell Aggregates by Active Gliding Movement

Abstract

Two strains of thermophilic photosynthetic bacteria, designated MD-66T (T = type strain) and YI-9, were isolated from bacterial mats in two separate hot springs in Japan. These new isolates were phenotypically similar to ChloroJlexus uurantiacus in some respects. They were thermophilic filamentous photosynthetic bacteria that grew well at 55°C either anaerobically as photoheterotrophs or aerobically as chemoheterotrophs. They exhibited gliding motility, produced bacteriochlorophylls a and c, contained chlorosomes, and required thiamine and folic acid as growth factors. However, isolates MD-66T and YI-9 had the ability to rapidly form mat-like dense aggregates of filaments, an ability which has not been observed in any C. uurantiacus strain. Carbon source utilization tests revealed that unlike C. uurantiucus, the new isolates did not utilize acetate, citrate, ethanol, or glycylglycine. An analysis of the carotenoid components revealed that isolates MD-66T and YI-9 contained mainly y-carotene and OH-y-carotene glucoside fatty acid esters. These isolates also contained only trace amounts of p-carotene, which is a major carotenoid component (28.4% of the total carotenoids) in C. auruntiacus. The results of DNA hybridization studies suggested that the new strains were genetically distinct from C. uurantiacus (levels of similarity, 9 to 18%), and 16s rRNA sequence comparisons showed that strain MI)-66T was related to C. auruntiacus at a similarity level of 92.8%. On the basis of our data, we propose that a new ChloroJlexus species should be created for our new isolates; the name of this new species is ChZoroJlexus aggregans, and the type strain is strain MD-66 (= DSM 9485).The first filamentous anoxygenic phototrophic bacteria were discovered by Pierson and Castenholz in thermophilic environments (25). These organisms were described as members of a new genus and species, Chlorojlexus aurantiacus, and strain J-10-fl, which was isolated from a Japanese hot spring, was designated the type strain of this species (26). Following this description, Gorlenko and coworkers found members of two apparently related mesophilic filamentous phototrophic genera in freshwater habitats, the genera Chloronerna (4) and "Oscillochloris" (9). These organisms exhibited gliding motility and contained pigment-bearing vesicles called "chlorosomes" (32). The group containing the phototrophic bacteria includes representatives of another genus, the genus Heliothrix, as described by Pierson et al. (29). Members of the genus Heliothrix have not been isola.ted in pure culture yet, but they are known to lack chlorosomes. The results of a 5s rRNA sequence analysis showed that H'eliothrix oregonensis is phylogenetically related to Chloroflexus aurantiacus.A phylogenetic analysis based on 16s rRNA sequence data revealed that Chloroflexus aurantiacus is not closely related to any other phototrophic bacteria and forms a remarkably deep branch on the common line of eubacterial descent (23). Consequently, the filamentous phototrophs are now placed in a grou...

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