An aerobic, budding, non-pigmented and rosette-forming bacterium was isolated from a littoral wetland of a boreal lake located in Valaam Island, northern Russia, and designated strain P3(T). Ellipsoidal to pear-shaped cells of this bacterium were covered with crateriform pits and possessed stalks suggesting a planctomycete morphotype. 16S rRNA gene sequence analysis confirmed that strain P3(T) was a member of the order Planctomycetales and belonged to a phylogenetic lineage defined by the genus Planctomyces , with 89 and 86% sequence similarity to Planctomyces brasiliensis and Planctomyces maris , respectively. Strain P3(T) was a mildly acidophilic, mesophilic organism capable of growth at pH values between pH 4.2 and 7.1 (with an optimum at pH 6.0-6.5) and at temperatures between 10 and 30 °C (optimum at 20-28 °C). Most sugars, a number of polysaccharides and several organic acids were the preferred growth substrates. Compared with Planctomyces brasiliensis and Planctomyces maris , which require NaCl for growth, strain P3(T) was salt-sensitive and did not develop at NaCl concentrations above 0.5% (w/v). The major fatty acids were C16 : 0 and C16 : 1ω7c; the cells also contained significant amounts of C18 : 1ω7c and C18 : 0. The major intact polar lipids were diacylglycerol-O-(N,N,N-trimethyl)homoserine (DGTS) lipids; the major neutral lipids were long-chain 1,(ω-1)-diols and C31 : 9 hydrocarbon. The quinone was MK-6, and the G+C content of the DNA was 59.0 mol%. Strain P3(T) differed from Planctomyces brasiliensis and Planctomyces maris by cell morphology, substrate utilization pattern and a number of physiological characteristics. Based on these data, the novel isolate should be considered as representing a novel genus and species of planctomycetes, for which the name Planctomicrobium piriforme gen. nov., sp. nov., is proposed. The type strain is P3(T) ( =DSM 26348(T) =VKM B-2887(T)).
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