Recognition, signalling and response of the diatom Fragilariopsis to epibiotic bacterial colonization.

硅藻Fragilariopsis在南大洋的浮游植物中占主导地位，在南极地区重要的(占全球15%的)初级生产力中发挥着核心作用。海洋初级生产力是由微藻和细菌之间在空间上的密切相互作用以及它们对基本微量元素、微营养素和宏观营养素的相互需求推动的。最近对不同浮游植物物种的几项微生物组研究表明，它们拥有独特的微生物群落，这些微生物群落在浮游植物物种内和跨时间尺度上是一致的和特定的。然而，在不同的专门化水平上导致这些联系的机制过程被很少地理解。这一建议中涉及的主要假说假设Fragilariopsis依赖或使用类似的识别和转导策略来响应环境细菌作为动物和植物，尽管它们与硅藻的系统发育距离。因此，在理论上，这种类比允许将方法论从动植物中经过充分研究的免疫学和微生物群科学转移到浮游植物物种。主动识别、选择和过滤过程的概念被认为引导与Fragilariopsis相关的微生物群的组装，在多细胞生物的免疫环境中被很好地理解，甚至在多细胞海洋大型藻类中也是如此，但在硅藻中则不然。这些反应是通过受体介导的细菌激发子的识别而触发的，并由经过充分研究的信号级联反应(如通过一氧化氮(NO))来介导。因此，这会导致各种可量化的应激标志物上调，如细胞内的活性氧物种或谷胱甘肽与谷胱甘肽-二硫化物比率的变化。在一定程度上，这些胁迫标记已经在微藻中得到验证，并首次出现了海洋硅藻中没有信号和没有合酶样基因序列的证据。因此，该提案解决了DFG SPP的相关问题，例如需要“更好地了解极地过程和机制”，以及“[SO生物体]对环境变化的反应”。具体地说，我们将从Fragilariopsis微生物群中分离、培养和鉴定各种细菌。同时，不同的Fragilariopsis生态型被轴化。在共培养实验中，我们将使用已知和成熟的化学和分子工具来表征和量化Fragilariopsis对细菌和细菌分子激发子(如细菌脂多糖)的胞内和胞外响应。这项建议的短期目标是证实Fragilariopsis对细菌或细菌信号分子的主动响应。这项工作的长期目标是分析Holobion组装，并通过实验调整未来的气候制度来挑战识别过程，以了解Fragilariopsis holobion对预测的气候变化情景的敏感性。

The diatom Fragilariopsis dominates phytoplankton in the Southern Ocean and plays a central role in the significant (15% of global) primary productivity of Antarctic regions. Marine primary production is fuelled by spatially close interactions between microalgae and bacteria and driven by their reciprocal needs for essential trace elements, micro- and macro-nutrients. Several recent microbiome studies of diverse phytoplankton species suggest that they harbour unique microbiomes, which are consistent and specific within phytoplankton species and across temporal scales. Yet, the mechanistic processes causing these associations at different levels of specialization are poorly understood.The main hypothesis addressed in this proposal postulates that Fragilariopsis relies on, or employs comparable recognition and transduction strategies in response to environmental bacteria as animals and plants, despite their phylogenetic distance to the diatom. Thus, in theory this analogy allows a cross-disciplinary transfer of methodology from well investigated immunity and microbiome science in animals and plants to a phytoplankton species.The concept of active recognition, selection and filtering processes, which supposedly steer the assembly of Fragilariopsis-associated microbiomes, is well understood in the context of immunity in multicellular organisms, even in multicellular marine macroalgae, but not in diatoms. Such responses are triggered via receptor-mediated recognition of bacterial elicitors and mediated by well-investigated signaling cascades (e.g. via nitric oxide (NO)). As a consequence, this leads to the upregulation of a variety of quantifiable stress markers, such as intracellular reactive oxygen species or a shift of the glutathione to glutathione-disulfide ratio. Partially, these stress markers have been verified in microalgae and first evidence of NO signaling and NO synthase-like gene sequences in marine diatoms is emerging. This proposal thus addresses pertinent DFG SPP questions, such as the need of “improved understanding of polar processes and mechanisms'' and the “response [of SO organisms] to environmental change”. Specifically, we will isolate, culture and identify various bacteria from Fragilariopsis microbiomes. In parallel, different Fragilariopsis ecotypes are axenified. In co-culture assays, we will employ known and well established chemical and molecular tools to characterize and quantify intra- and extracellular responses of Fragilariopsis to bacteria and bacterial molecular elicitors, such as bacterial lipopolysaccharides.The short term goal of this proposal is to corroborate an active response of Fragilariopsis to bacteria or bacterial signaling molecules. The long-term goal of this work is to analyse the holobiont assembly and challenge the recognition process by experimentally adjusting future SO climate regimes to understand the susceptibility of the Fragilariopsis holobiont to predicted climate change scenarios.