Paludisphaera borealis gen. nov., sp. nov., a hydrolytic planctomycete from northern wetlands, and the proposal of Isosphaeraceae fam. nov

Two isolates of aerobic, budding, pink-pigmented bacteria, designated strains PX4T and PT1, were isolated from a boreal Sphagnum peat bog and a forested tundra wetland. Cells of these strains were non-motile spheres that occurred singly or in short chains. Novel isolates were capable of growth at pH values between 3.5 and 6.5 (optimum at pH 5.0-5.5) and at temperatures between 6 and 30C (optimum at 15-25C). Most sugars and a number of polysaccharides including pectin, xylan, lichenan and Phytagel were used as growth substrates. The major fatty acids were C16:0, C18:1w9 and C18:0; the major polar lipids were phosphocholine and trimethylornithine. The quinone was MK-6, and the G+C content of the DNA was 66 mol%. Strains PX4T and PT1 were members of the order Planctomycetales and displayed 93-94% 16S rRNA gene sequence similarity to Aquisphaera giovannonii, 91-92% to Singulisphaera species and 90-91% to Isosphaera pallida. Two novel strains, however, differed from members of these genera by cell morphology, substrate utilization pattern and a number of physiological characteristics. Based on these data, the novel isolates should be considered as representing a novel genus and species of planctomycetes, for which the name Paludisphaera borealis gen. nov., sp. nov, is proposed. The type strain is PX4T (= DSM 28747T = VKM B-2904T). We also suggest to establish a novel family, the Isosphaeraceae fam. nov., to accommodate stalk-free planctomycetes with spherical cells, which can be assembled in short chains, long filaments or shapeless aggregates. This family includes the genera Isosphaera, Aquisphaera, Singulisphaera, and Paludisphaera.