Methylovulum psychrotolerans sp. nov., a coldadapted methanotroph from low-temperature terrestrial environments, and emended description of the genus Methylovulum

Two isolates of aerobic methanotrophic bacteria, strains Sph1T 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 Sph1T, 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°C. Although their temperature optimum was at 20–25°C, these methanotrophs grew well at lower temperatures, down to 4°C. The major cellular fatty acids were C16: 1ω5c, C16: 1ω6c, C16: 1ω7c, C16: 1ω8c, C16: 0 and C14: 0; the DNA G+C content was 51.4–51.9 mol%. Strains Sph1T, 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 HT12T (96.0–96.5 % 16S rRNA gene sequence similarity and 90 % pmoA sequence similarity). The novel isolates, however, differed from Methylovulum miyakonense HT12T 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 Sph1T (=LMG 29227T=VKM B-3018T) is the type strain.