Diversity, phylogeny and ecological role of heterotrophic protists in the pelagic redoxcline of the central Baltic Sea

波罗的海以及其他几个边缘海和沿海海的水柱在较深的盆地中具有明显的氧气和氧化还原条件的垂直梯度。我们的知识的多样性和功能的微生物在远洋氧化还原跃层的基础上，主要是从原核生物群落（细菌，古生菌），这是介导重要的生物地球化学过程中的过渡带从亚氧到缺氧沃茨（氧化还原跃层）的数据。微生物真核生物（原生生物）的比较研究仍然在很大程度上缺乏，尽管一般的假设，他们施加了强烈的影响，原核生物组合。本研究的目的是探讨沿着氧化还原梯度上的原生生物（主要是鞭毛虫和纤毛虫）的组成和功能。主要假设是：（1）微生物食物网的结构发生根本性转变，从好氧水转变为缺氧/含硫沃茨，从异养纳米鞭毛虫（HNF）转变为微需氧纤毛虫和噬菌体，作为细菌损失的主要媒介;（2）原生生物和原核生物之间的相互作用，包括噬菌和共生协会，影响原核生物群落的结构和活性并调节由原核生物功能性团介导的生物地球化学过程（例如，脱硫剂、氨氧化剂、硫氧化剂等）。为此目的，定量的种类组成的噬食性原生生物将检查结合分子技术（18 S DNA/RNA指纹和克隆库，设计和应用特定的基因探针原位杂交和定量PCR），经典的显微镜方法，与自然组合和分离的生物体的孵育实验。为了了解氧气梯度对浮游原核生物主要死亡因素的影响，我们将量化死亡率，由于原生动物放牧和病毒裂解沿着梯度。不同的工艺（暗CO2固定、硝化、反硝化、硫氧化等）将在实验孵育中进行评估，以检查这些速率是否受到原生生物-原核生物相互作用的调节。波罗的海的氧化还原跃层将作为一个模型系统，揭示了在缺氧-缺氧界面的细菌-原生生物相互作用的一般理解。来自其他海洋氧化还原跃层的样品和实验（例如，黑海、挪威峡湾、卡里亚科盆地）的研究将有助于检验在波罗的海观察到的模式是否可以推广。

The water column of the Baltic Sea, but also of several other marginal and coastal seas, is characterized in the deeper basins by pronounced vertical gradients in oxygen and redox conditions. Our knowledge regarding the diversity and function of microorganisms in pelagic redoxclines is based mainly on data from prokaryotic communities (bacteria, archaea) which are mediating important biogeochemical processes in the transition zone from suboxic to anoxic waters (redoxcline). Comparable studies on microbial eukaryotes (protists) are still largely lacking despite the general assumption that they exert a strong impact on the prokaryotic assemblages. The aim of this project is to investigate the composition and functional role of phagotrophic protists (mainly flagellates and ciliates) along the redox gradients. The major hypotheses are: (1) that there occurs a fundamental shift in the structure of the microbial food web from oxic to anoxic/sulfidic waters, with a shift from heterotrophic nanoflagellates (HNF) to microaerophilic ciliates and bacteriophages as major agents of bacterial losses; (2) that interactions between protists and prokaryotes, which comprise both bacterivory and symbiotic associations, impact the structure and activity of the prokaryotic communities and modulate biogeochemical processes mediated by functional guilds of prokaryotes (e.g., denitrifiers, ammonia oxidizers, sulfur oxidizers etc.). To this aim, the quantitative species composition of phagotrophic protists will be examined by a combination of molecular techniques (18S DNA/RNA fingerprints and clone libraries, design and application of specific gene probes for in situ hybridization and quantitative PCR), classical microscopical methods, and incubation experiments with natural assemblages and isolated organisms. In order to understand the impact of the oxygen gradients on the major mortality factors for planktonic prokaryotes, we will quantify mortality rates due to protist grazing and viral lysis along the gradients. Different processes (dark CO2 fixation, nitrification, denitrification, sulfur oxidation etc.) will be assessed in experimental incubations to examine whether these rates are modulated by protist-prokaryote interactions. The Baltic Sea redoxcline will serve as a model system to reveal a general understanding of bacteria-protist interactions at oxic-anoxic interfaces. Samples and experiments from other marine redoxclines (e.g., Black Sea, Norwegian fjords, Cariaco Basin) will serve to test whether the patterns observed in the Baltic Sea can be generalized.