The first representative of the phylum Planctomycetes, Planctomyces bekefii, was described nearly one century ago. This morphologically conspicuous freshwater bacterium is a rare example of as-yet-uncultivated prokaryotes with validly published names and unknown identity. We report the results of molecular identification of this elusive bacterium, which was detected in a eutrophic boreal lake in Northern Russia. By using high-performance cell sorting, P. bekefii-like cell rosettes were selectively enriched from lake water. The retrieved 16S rRNA gene sequence was nearly identical to those in dozens of metagenomes assembled from freshwater lakes during cyanobacterial blooms and was phylogenetically placed within a large group of environmental sequences originating from various freshwater habitats worldwide. In contrast, 16S rRNA gene sequence similarity to all currently described members of the order Planctomycetales was only 83%-92%. The metagenome assembled for P. bekefii reached 43% genome coverage and showed the potential for degradation of peptides, pectins, and sulfated polysaccharides. Tracing the seasonal dynamics of P. bekefii by Illumina paired-end sequencing of 16S rRNA gene fragments and by fluorescence in situ hybridization revealed that these bacteria only transiently surpass the detection limit, with a characteristic population peak of up to 104 cells ml-1 following cyanobacterial blooms.

中文翻译：

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解决了100年的谜团：尚未培养的拟南芥（Planctomyces bekefii）的鉴定，基因组表征和生物地理学。

大约在一个世纪前就描述了Planctomycetes门的第一个代表，即Planctomyces bekefii。这种形态学上引人注目的淡水细菌是尚未培养的原核生物的罕见例子，原核生物具有有效的名称和未知身份。我们报告了这种难以捉摸的细菌的分子鉴定的结果，这是在俄罗斯北部富营养化的北方湖泊中发现的。通过使用高效细胞分选法，从湖水中选择性地富集了苯丙酸杆菌样细胞玫瑰花结。检索到的16S rRNA基因序列与蓝藻水华期间在淡水湖中组装的数十个元基因组中的序列几乎相同，并且在系统发育上位于源自世界各地各种淡水生境的一大批环境序列中。相比之下，16S rRNA基因序列与所有当前描述的浮游动物纲成员的相似性仅为83％-92％。为苯丙酸杆菌组装的元基因组达到43％的基因组覆盖率，显示出降解肽，果胶和硫酸化多糖的潜力。通过Illumina的16S rRNA基因片段的双末端测序和荧光原位杂交追踪B. fifi的季节动态，发现这些细菌仅短暂地超过了检测极限，其特征性种群峰值高达104个细胞ml-1，随后蓝藻水华。