Methylovulum psychrotolerans sp nov:, a cold-adapted methanotroph from low-temperature terrestrial environments, and emended description of the genus Methylovulum

Two isolates of aerobic methanotrophic bacteria, strains Sph1(T) and Sph2, were obtained from cold methane seeps in a floodplain of the river Mukhrinskaya, Irtysh basin, West Siberia. Another morphologically and phenotypically similar methanotroph, strain OZ2, was isolated from a sediment of a subarctic freshwater lake, Archangelsk region, northern Russia. Cells of these three strains were Gram-stain-negative, light-pink-pigmented, non-motile, encapsulated, large cocci that contained an intracytoplasmic membrane system typical of type I methanotrophs. They possessed a particulate methane monooxygenase enzyme and utilized only methane and methanol. Strains Sph1(T), Sph2 and OZ2 were able to grow at a pH range of 4.0-8.9 (optimum at pH 6.0-7.0) and at temperatures between 2 and 36 degrees C. Although their temperature optimum was at 20-25 degrees C, these methanotrophs grew well at lower temperatures, down to 4 degrees C. The major cellular fatty acids were C-16:1 omega 5c, C-16:1 omega 6c, C-16:1 omega 7c, C-16:1 omega 8c, C-16:0 and C-14:0; the DNA G+C content was 51.4-51.9 mol%. Strains Sph1(T), Sph2 and OZ2 displayed nearly identical (99.1-99.7 % similarity) 16S rRNA gene sequences and belonged to the family Methylococcaceae of the class Gammaproteobacteria. The most closely related organism was Methylovulum miyakonense HT12(T) (96.0-96.5 % 16S rRNA gene sequence similarity and 90 % pmoA sequence similarity). The novel isolates, however, differed from Methylovulum miyakonense HT12(T) by cell morphology, pigmentation, absence of soluble methane monooxygenase, more active growth at low temperatures, growth over a broader pH range and higher DNA G+C content. On the basis of these differences, we propose a novel species, Methylovulum psychrotolerans sp. nov., to accommodate these methanotrophs. Strain Sph1(T) (=LMG 29227(T)=VKM B-3018(T)) is the type strain.