Superior antagonistic activity against axenic Microcystis aeruginosa PCC7806 was observed with Paucibacter sp. B51 isolated from cyanobacterial bloom samples among 43 tested freshwater bacterial species. Complete genome sequencing, analyzing average nucleotide identity and digital DNA–DNA hybridization, designated the B51 strain as Paucibacter aquatile. Electron and fluorescence microscopic image analyses revealed the presence of the B51 strain in the vicinity of M. aeruginosa cells, which might provoke direct inhibition of the photosynthetic activity of the PCC7806 cells, leading to perturbation of cellular metabolisms and consequent cell death. Our speculation was supported by the findings that growth failure of the PCC7806 cells led to low pH conditions with fewer chlorophylls and down-regulation of photosystem genes (e.g., psbD and psaB) during their 48-h co-culture condition. Interestingly, the concentrated ethyl acetate extracts obtained from B51-grown supernatant exhibited a growth-inhibitory effect on PCC7806. The physical separation of both strains by a filter system led to no inhibitory activity of the B51 cells, suggesting that contact-mediated anti-cyanobacterial compounds might also be responsible for hampering the growth of the PCC7806 cells. Bioinformatic tools identified 12 gene clusters that possibly produce secondary metabolites, including a class II lasso peptide in the B51 genome. Further chemical analysis demonstrated anti-cyanobacterial activity from fractionated samples having a rubrivinodin-like lasso peptide, named paucinodin. Taken together, both contact-mediated inhibition of photosynthesis and the lasso peptide secretion of the B51 strain are responsible for the anti-cyanobacterial activity of P. aquatile B51.

中文翻译：

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水生少杆菌B51及其套索肽寡菌素对蓝藻水华形成铜绿微囊藻PCC7806的拮抗作用

观察到Paucibacter sp. 对无菌铜绿微囊藻PCC7806具有优异的拮抗活性。B51 从 43 个测试的淡水细菌物种中的蓝藻水华样本中分离出来。完整的基因组测序、分析平均核苷酸同一性和数字 DNA-DNA 杂交，将 B51 菌株命名为水生寡杆菌。电子和荧光显微镜图像分析显示，铜绿微囊藻细胞附近存在 B51 菌株，这可能会引起 PCC7806 细胞光合活性的直接抑制，导致细胞代谢紊乱并随后导致细胞死亡。我们的推测得到了以下发现的支持：PCC7806 细胞的生长失败导致低 pH 条件，叶绿素较少，并且在 48 小时共培养条件下光系统基因（例如psb D 和psa B）下调。有趣的是，从 B51 生长的上清液中获得的浓缩乙酸乙酯提取物对 PCC7806 表现出生长抑制作用。通过过滤系统对两种菌株进行物理分离，导致 B51 细胞没有抑制活性，这表明接触介导的抗蓝藻化合物也可能阻碍 PCC7806 细胞的生长。生物信息工具鉴定出 12 个可能产生次级代谢产物的基因簇，其中包括 B51 基因组中的 II 类套索肽。进一步的化学分析表明，分级样品具有抗蓝藻活性，该样品含有红布维诺丁 (rubrivinodin) 样套索肽 (paucinodin)。总而言之，接触介导的光合作用抑制和 B51 菌株的套索肽分泌是P. aquatile B51 的抗蓝藻活性的原因。