蓝藻以非磷脂类替代磷脂响应磷胁迫的生物化学机制研究

Phosphorus potentially limits primary productivity in lakes. Adaptation to low bioavailable phosphorus supply of bloom-forming cyanobacteria is the most important mechanism to support their dominance. Algal and plant production of nonphosphorus lipids in place of phospholipids is a physiological response to low phosphorus availability. but examples from freshwater bloom forming cyanobacteria remain minimal. Lakes Taihu, Chaohu and some urban fishponds in City Wuhan with gradient of phosphorus concentration suffered from serious canobacterial bloom are selected as the studied water bodies. Together with traditional and new techniques (e.g. high-performance liquid chromatography/electrospray ionization multistage mass spectrometry, third-generation sequencing, also known as long-read sequencing), metagenomic and metatranscriptomic analysis), we will investigate the contents of non-phosphorus lipids and phosphorus lipids and their relationship between ambient inorganic phosphorus concentrations, find out the significant role of phosphorus stress in reshaping the proportions of phospholipids, sulfolipids, betaine lipids and galactolipids. This physiological response of N2-fixing and non-N2-fixing cyanobacteria might be different and identify expressions of genes encoding above lipids, testing the hypothesis that cyanobacterium may rely on extensively replace phospholipids with non-phospholipids during growth to overcome phosphorus stress and dominate the phytoplankton community, which may illustrate the process of cyanobacterial bloom and the succession of dominant species from biochemical and molecular point of view. This study centres on the key research program focusing on the hydrosphere microorganisms involved in phosphorus, sulfur, carbon and nitrogen biogeochemical cycle and its ecological effect, and a new pathway of nutrition metabolism is expected.

磷是湖泊初级生产力的关键限制性营养元素，对低磷的高适应性是藻类在无机磷浓度较低的诸多水域形成水华的重要机制。水华蓝藻是否能合成非磷脂类代替磷脂尚不明晰。本项研究拟以蓝藻水华频发的且具明显营养梯度的太湖、巢湖和武汉市池塘等水体为实验对象，以经典与新兴的技术（如高效液相色谱、宏基因和宏转录分析和第三代测序等）为实验手段，分析固氮和非固氮蓝藻细胞磷脂、硫脂、含氮脂质和糖脂的含量与环境中生物可利用性磷浓度的关系，判别磷的胁迫对蓝藻胞内非磷脂类和磷脂比例重塑的驱动作用以及这一响应的种类特异性，探明其中关键的调节基因，验证水华蓝藻能以非磷脂类替代磷脂降低对磷的需求从而克服磷的胁迫进而取得竞争优势这一科学假设，最终从营养利用策略的角度阐明蓝藻水华的发生和演替的分子生物学机理。该研究紧紧围绕“水圈微生物参与碳氮硫等元素生物地球化学循环的宏观机制与生态效应”这一核心问题，且有望发现蓝藻营养代谢的新途径。

在营养盐浓度得以控制的情况下湖泊仍然可持续暴发蓝藻水华。蓝藻可通过改变细胞内脂质组成，以非磷脂质替代磷脂来降低对磷的需求，这一机制在淡水蓝藻中尚未见报到。本项研究以蓝藻水华频发的武汉市湖泊和池塘为研究对象，结合生态学、生物化学和分子生物学方法，分析了不同磷浓度梯度且发生蓝藻水华的水体中水华优势种胞内磷脂和非磷脂类含量和比例的变化，同时比较了磷脂和非磷脂代谢相关基因的表达水平。在不同水华优势种之间，相比丝状蓝藻束丝藻，水华分布更为广泛的微囊藻胞内氮脂、磷脂、氮磷脂和硫脂含量明显低于束丝藻的相应值，暗示微囊藻对氮和磷的需求更低；就微囊藻而言，在水中溶解反应性磷（SRP）浓度低于0.02mg/L时，随着SRP浓度的降低，胞内磷脂（PG）含量逐渐减少，而双半乳糖甘油二脂（DGDG）、单半乳糖甘油二脂（MGDG）和硫代异鼠李糖甘油二酯(SQDG)的含量逐渐增加；同时，SRP与PG/ SQDG、PG/ MGDG 、PG/ DGDG呈显著正相关关系（P<0.05），表明在磷胁迫条件下，微囊藻细胞中PG占比减少，MGDG 、DGDG和SQDG的占比增加，暗示糖脂和磷脂可能替代了磷脂，降低了藻细胞对磷的需求量，保障了低磷条件下的生物量。实时荧光定量 PCR（qPCR）结果表明，在磷浓度较低时(磷浓度小于0.05mg/ L)，MGDG合成酶基因mgdE和SQDG合成酶基因sqdB的表达量均升高，当高于这一浓度时，非磷脂合成酶基因随SRP的增加而升高。上述结果表明，磷的富集并非蓝藻水华发生的先决条件，而导致表观生物可利用性组分相对缺乏的磷的低剂量持续供给方式则为其关键诱因。这是对传统的富营养化观念的重要修正，可为富营养化水体生态修复方案的确定与蓝藻水华防控技术的研发提供理论指引。