三峡库区水华蓝藻营养物吸收的胶被作用机理研究

The frequent outbreaks of cyanobacterial bloom with large-scale and high biomass in the Three Gorges Reservoir have become a typical ecological disaster, resulting in a continuous threat to the sustainability of freshwater ecosystem and human health. The bloom-forming cyanobacteria always occurs in a colony under natural conditions, but exists mainly as single cells and a few paired cells in axenic cultures in the laboratory. Although cyanobacterial sheath is considered to be the main factor between the differences, little is known on the role and mechanisms of sheath for the communication between algal cells and the external environment. This project will focus on the role of sheath in the cyanobacteria nutrient absorptive strategy from its natural physiological functions. Based on the comparison study of the structure, distribution and composition, the biomechanical properties, and the nutrient uptake rate and flux in the different cyanobacterial sheath under varying nutrient conditions, and coincidently with the analyzation of thermal effect in the nutrient absorption process, the present study will illustrate how the cyanobacterial sheath to regulate the nutrient transport from the view of physiology, biochemistry and thermodynamics. Then it will reveal the synergistic mechanism of the energy transfer direction and flux change in the cyanobacteria nutrient absorptive strategy in terms of stoichiometric relationship in the sheath. The results of this research project will provide theoretical and practical significance in further understand of cyanobacteria as a strong competitor in natural water bodies, and the mechanism of the dominancy and persistence of cyanobacteria bloom in freshwater ecosystem.

三峡库区蓝藻水华大范围、高频次、高生物量的暴发已成为典型的生态灾害。发生蓝藻水华的典型藻种在野外自然条件下主要以群体形态存在，而蓝藻的群体和单细胞形态差异在于细胞外的胶被，但其作为藻类细胞与外界环境沟通“桥梁”的作用机理尚未完全明晰。本项目拟从三峡库区多种典型水华蓝藻胶被的自然生理功能入手，探讨其在蓝藻营养物吸收中的作用。通过对不同生源要素梯度，不同蓝藻胶被的结构、分布特征和组分变化、生物力学特性、胶被中营养盐吸收速率及通量等的研究，同时结合营养物质在胶被中吸收传递的热效应解析，从生理生化及热力学角度探寻胶被是如何调节细胞内外物质传递和处理，继而以化学计量关系研究胶被在蓝藻营养物吸收传输中的物质通量与能量驱动。该研究对进一步了解蓝藻在自然水体中成为营养竞争者、以及三峡库区蓝藻水华暴发和优势维持机制提供重要理论和实验依据。

发生蓝藻水华的典型藻种在野外自然条件下多以群体形态存在，而蓝藻的群体和单细胞形态差异在于细胞外的胶被，但其作为藻类细胞与外界环境沟通“桥梁”的作用机理尚未完全明晰。本项目在文献知识梳理与综述基础上，开发了蓝藻胶被提取的新方法，从细胞内大分子物质组成、营养物代吸收速率及通量、生物力学特性等方面构建了针对蓝藻单细胞或小群体蓝藻胶被特性与营养通量的吸收的方法。结合蓝藻胶被热通量方程一般形式的推导和藻类磷吸收的“势-通量”热力学模型的构建，逐步形成了探索蓝藻适应性策略的工具箱，并对蓝藻胶被调节细胞内外物质能量传递进行了分析研究。项目取得了以下主要的发现：1）不同类型胶被（鞘、荚膜、粘液层）的生物化学组成各不相同，单糖和氨基酸种类组成呈现出显著差异。这诱导不同类型胶被粘附性、亲疏水性具有显著差别。2）胶被对营养物质有一定程度的富集作用，荚膜比鞘对营养物质的富集效率更高，而粘液与藻细胞结合松散，且易发送水化，导致对营养物质的富集作用弱。胶被促进了蓝藻细胞对NH4+的吸收和O2的释放。不同形态的胶被表面特性不同，影响物质的扩散速率，并在一定程度上调节细胞内外营养元素的传输条件，加快了物质的相互传递。3）高温会使胶被弹性变小而厚度及粘附性能增大，低温则相反，变化幅度粘液>荚膜>鞘，且对于胶被粘附性能和厚度，高温比低温带来更大的影响。胶被传导过程仅受胶被厚度及胶被材料性质影响，胶被越厚，其热阻越大，胶被弹性模量越大，其导热系数越小。高温胁迫对群体的大小有促进作用，低温下表现为抑制。4）藻类通过改变细胞力学特性和不同胶被的厚度粘性等，来稳定细胞的热通量，促进对胞外养分NP等的吸收。藻类吸收的营养物质又可以进一步反馈调节形态变化中胶被的合成，并且在低温和高温下胶被合成具有不同的碳代谢分子途径。