DNA-DNA Hybridization of Methylotrophic Halophilic Methanogenic Bacteria and Transfer of Methanococcus halophilusvp to the Genus Methanohalophilus as Methanohalophilus halophilus comb. nov.

Abstract

Twelve strains of halophilic, methylotrophic, methanogenic bacteria, including the type strains of the validly published species Methanohalophilus mahii, Methanohalophilus zhilinae, and Methanohalobium evestigatum, as well as the type strain of Methanococcus halophilus, were compared on the basis of DNA-DNA hybridization and phenotypic characteristics. Two groups (moderately halophilic Methanohalophilus strains and extremely halophilic Methanohalobium strains) were formed on the basis of G+C contents and DNA-DNA hybridization data. Type strain SLP of Methanohalophilus mahii and type strain Z-7982 of Methanococcus halophilus exhibited 87% sequence similarity and belong to the same genus (Methanohalophilus), if not to the same species. A regulation of the International Code of Nomenclature of Bacteria implies that the specific epithet halophilus has priority by validation and should be retained. Therefore, we propose that Methanococcus halophilus should be transferred to the genus Methanohalophilus as Methanohalophilus halophilus comb. nov. (type strain, strain Z-7982 [= DSM 3094 = OCM 160]); an emended description is given. Moderately halophilic strains TR-7, Z-7404, and Z-7405, which were isolated from geographically separated sites, exhibited high levels of DNA-DNA hybridization with Methanohalophilus mahii and might be regarded as strains of the same Methanohalophilus species. The alkaliphilic organism Methanohalophilus zhilinae is a valid species in the genus Methanohalophilus. The extremely halophilic organism Methanohalobium evestigatum exhibited no significant DNA-DNA hybridization with moderately halophilic species and should be regarded as a member of an independent genus. Other extremely halophilic strains (strains Z-7408 and Z-7403) exhibited low levels of DNA-DNA hybridization with Methanohalobium evestigatum Z-7303T (= DSM 3721) (T = type strain).