Two strains of thermophilic photosynthetic bacteria, designated MD-66 (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 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 and , contained chlorosomes, and required thiamine and folic acid as growth factors. However, isolates MD-66 and YI-9 had the ability to rapidly form mat-like dense aggregates of filaments, an ability which has not been observed in any strain. Carbon source utilization tests revealed that unlike , the new isolates did not utilize acetate, citrate, ethanol, or glycylglycine. An analysis of the carotenoid components revealed that isolates MD-66 and YI-9 contained mainly γ-carotene and OH-γ-carotene glucoside fatty acid esters. These isolates also contained only trace amounts of β-carotene, which is a major carotenoid component (28.4% of the total carotenoids) in . The results of DNA hybridization studies suggested that the new strains were genetically distinct from (levels of similarity, 9 to 18%), and 16S rRNA sequence comparisons showed that strain MD-66 was related to at a similarity level of 92.8%. On the basis of our data, we propose that a new species should be created for our new isolates; the name of this new species is , and the type strain is strain MD-66 (= DSM 9485).


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