Chlamydiae are ubiquitous intracellular bacteria and infect a wide diversity of eukaryotes, including mammals. However, chlamydiae have never been reported to infect photosynthetic organisms. Here, we describe a novel chlamydial genus and species, Candidatus Algichlamydia australiensis, capable of infecting the photosynthetic dinoflagellate Cladocopium sp. (originally isolated from a scleractinian coral). A. australiensis was confirmed to be intracellular by fluorescence in situ hybridization and confocal laser scanning microscopy, and temporally stable at the population level by monitoring its relative abundance across four weeks of host growth. Using a combination of short- and long-read sequencing, we recovered a high-quality (completeness 91.73% and contamination 0.27%) metagenome-assembled genome of A. australiensis. Phylogenetic analyses show that this chlamydial taxon represents a new genus and species within the Simkaniaceae family. A. australiensis possesses all the hallmark genes for chlamydiae-host interactions, including a complete type III secretion system. In addition, a type IV secretion system is encoded on a plasmid and has previously been observed for only three other chlamydial species. Twenty orthologous groups of genes are unique to A. australiensis, one of which is structurally similar to a protein known from Cyanobacteria and Archaeplastida involved in thylakoid biogenesis and maintenance, hinting at potential chlamydiae interactions with the chloroplasts of Cladocopium cells. Our study shows that chlamydiae infect dinoflagellate symbionts of cnidarians, the first photosynthetic organism reported to harbor chlamydiae, thereby expanding the breadth of chlamydial hosts and providing a new contribution to the discussion around the role of chlamydiae in the establishment of the primary plastid.

中文翻译：

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衣原体作为光合甲藻的共生体


衣原体是普遍存在的细胞内细菌，可感染多种真核生物，包括哺乳动物。然而，从未有衣原体感染光合生物的报道。在这里，我们描述了一种新的衣原体属和种，即 Candidatus Algichlamydia australiensis，它能够感染光合甲藻 Cladocopium sp。 （最初是从石珊瑚中分离出来的）。通过荧光原位杂交和共焦激光扫描显微镜证实澳大利亚澳大利亚拟南芥存在于细胞内，并且通过监测其在宿主生长四个星期内的相对丰度，在群体水平上暂时稳定。通过结合短读长和长读长测序，我们恢复了高质量（完整性为 91.73%，污染率为 0.27%）的澳大利亚拟南芥宏基因组组装基因组。系统发育分析表明，这种衣原体分类单元代表了 Simkaniaceae 科中的一个新属和新种。澳大利亚衣原体拥有衣原体与宿主相互作用的所有标志基因，包括完整的 III 型分泌系统。此外，IV 型分泌系统是在质粒上编码的，并且之前仅在其他三种衣原体物种中观察到。二十个直系同源基因组是澳大利亚 A. a. australiensis 所特有的，其中一组在结构上与蓝细菌和古质体中已知的参与类囊体生物发生和维持的蛋白质相似，暗示衣原体与枝萼细胞的叶绿体存在潜在的相互作用。 我们的研究表明，衣原体感染刺胞动物的甲藻共生体，这是第一个报道的含有衣原体的光合生物，从而扩大了衣原体宿主的范围，并为围绕衣原体在初级质体建立中的作用的讨论提供了新的贡献。