Methane-oxidizing microorganisms perform an important role in reducing emissions of the greenhouse gas methane to the atmosphere. To date, known bacterial methanotrophs belong to the Proteobacteria, Verrucomicrobia, and NC10 phyla. Within the Proteobacteria phylum, they can be divided into type Ia, type Ib, and type II methanotrophs. Type Ia and type II are well represented by isolates. Contrastingly, the vast majority of type Ib methanotrophs have not been able to be cultivated so far. Here, we compared the distributions of type Ib lineages in different environments. Whereas the cultivated type Ib methanotrophs (Methylococcus and Methylocaldum) are found in landfill and upland soils, lineages that are not represented by isolates are mostly dominant in freshwater environments, such as paddy fields and lake sediments. Thus, we observed a clear niche differentiation within type Ib methanotrophs. Our subsequent isolation attempts resulted in obtaining a pure culture of a novel type Ib methanotroph, tentatively named "Methylotetracoccus oryzae" C50C1. Strain C50C1 was further characterized to be an obligate methanotroph, containing C16:1ω9c as the major membrane phospholipid fatty acid, which has not been found in other methanotrophs. Genome analysis of strain C50C1 showed the presence of two pmoCAB operon copies and XoxF5-type methanol dehydrogenase in addition to MxaFI. The genome also contained genes involved in nitrogen and sulfur cycling, but it remains to be demonstrated if and how these help this type Ib methanotroph to adapt to fluctuating environmental conditions in freshwater ecosystems.@en