Molecular characterization of the aggregate formation-inducing interaction between Marinobacter adhaerens HP15 and the diatom, Thalassiosira weissflogii

在暴露在阳光下的海洋环境中，浮游植物细胞，如硅藻，通过光合作用固定大气中的二氧化碳，并将其导入生物泵。因此，大多数固定碳通过消费者重新进入大气，而1-10%的浮游植物固定二氧化碳通过快速下沉的海洋雪团运往深海。细菌和连接硅藻细胞的含多糖涂层在很大程度上促进了海洋雪层的形成。剖析硅藻和细菌之间的细胞相互作用是最初资助期的主题。为此，我们使用了一个双边硅藻-细菌模型系统，结合假说驱动的突变、体内表达技术(IVET)筛选和蛋白质组学方法来系统地研究这种相互作用，并鉴定这种相互作用所需的细菌基因和基因产物。该模型系统由硅藻(Thalassiosira Weissflgii)和伽马变形杆菌(Marinbacter Adhaerens HP15)组成，它对硅藻细胞表现出可重复性和特异性的附着，诱导硅藻携带的胞外聚合物分泌，并引发海洋雪层的形成。利用基因特异性突变，可以证明，鞭毛和菌毛介导的细菌趋化作用是初始附着到硅藻细胞和形成聚集体所必需的。可以进一步证明，由硅藻产生的透明的胞外聚合物颗粒的化学组成受到细菌存在的影响。令人惊讶的是，依维特筛选和蛋白质组学方法显示，阿达氏支原体似乎受益于硅藻释放的氨基酸，而不是碳水化合物，这可能是一种磷酸盐限制的依赖方式。IVET方法进一步表明，细菌可能通过质粒携带的紧贴附着点编码的IVb型菌毛附着在硅藻表面，并可能受益于其承受强烈升高和有毒浓度锌的能力。这张复杂但尚未得到很好解决的图片揭示了令人兴奋的新研究问题，即上述遗传性状在相互作用过程中的确切功能和相互作用，以及硅藻的特定反应(S)，这些问题应该在研究项目的扩展中解决。

In light-exposed marine environments, phytoplankton cells such as diatoms photosynthetically fix atmospheric carbon dioxide and channel it into the biological pump. Most of thus-by fixed carbon re-enters the atmosphere via consumers while 1-10% of phytoplankton-fixed CO2 is transported to the deep sea via fast-sinking marine snow aggregates. Marine snow formation is significantly fostered by bacteria and by polysaccharide-containing coatings, which connect diatom cells. Dissecting the cellular interplay between diatoms and bacteria was the topic of the initial funding period. For this, a bilateral diatom-bacteria model system in combination with hypotheses-driven mutagenesis, an in vivo expression technology (IVET) screen, and a proteomics approach were used to systemically investigate this interaction and to identify bacterial genes and gene products required for it. The model system consisted of the diatom, Thalassiosira weissflogii, and the gamma-proteobacterium, Marinobacter adhaerens HP15, which showed reproducible and specific attachment to diatom cells, induced diatom-borne exopolymer secretion, and gave rise to marine snow formation. Using gene-specific mutagenesis, it could be demonstrated that both, flagellum- and pilus-mediated bacterial chemotaxis were required for initial attachment to diatom cells and aggregate formation. It could be further demonstrated that the chemical composition of transparent exopolymeric particles produced by diatoms is impacted by the presence of the bacterium. Surprisingly, IVET screen and proteomics approach revealed that M. adhaerens seems to benefit from diatom-released amino acids but not carbohydrates in a potentially phosphate limitation-dependent manner. The IVET approach furthermore showed that the bacterium might adhere to diatom surfaces via a plasmid-borne, tight adhesion locus-encoded Type IVb pilus, and might profit from its ability to withstand strongly elevated and toxic concentrations of zinc. This complex but not-yet fine-resolved picture revealed exciting new research questions on the precise functions and interplay of the above genetic traits during the interaction and the specific response(s) of the diatom which should be addressed in the extension of the research project.