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

在光照的海洋环境中，浮游植物细胞（例如硅藻）通过光合作用固定大气中的二氧化碳，并将其引导到生物泵中。其中大部分固定碳通过消费者重新进入大气，而1-10%的植物生长素固定的CO2通过快速下沉的海洋雪聚集体被输送到深海。海洋雪的形成是由细菌和含有多糖的涂层，连接硅藻细胞显着促进。研究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，我们采用了一个双向的细菌-硅藻模型系统，结合假设驱动的诱变、体内表达技术（IVET）筛选和蛋白质组学方法，系统地研究了这种相互作用，并鉴定了所需的细菌基因和基因产物。其显示出可再现的和特异性的附着于硅藻细胞，诱导了硅藻携带的外聚合物分泌，并引起了海洋雪的形成。使用基因特异性诱变，它可以证明，鞭毛和菌毛介导的细菌趋化性所需的初始附着硅藻细胞和聚集体的形成。可以进一步证明，由硅藻产生的透明外聚物颗粒的化学组成受到细菌存在的影响。令人惊讶的是，IVET筛查和蛋白质组学方法显示M.粘附蛋白似乎以潜在的磷酸盐限制依赖性方式受益于β-释放的氨基酸而不是碳水化合物。IVET方法进一步表明，细菌可能通过质粒携带的紧密粘附位点编码的IVb型菌毛粘附在硅藻表面，并可能受益于其承受强烈升高和有毒浓度的锌的能力。这一复杂但尚未精细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和硅藻的特定反应期间的精确功能和相互作用，这些问题应在研究项目的扩展中得到解决。

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.