Nitrogen metabolism in phytoplankton under natural conditions and the presence of inorganic fertiliser and organic waste

氮是淡水生态系统中控制浮游植物生产力的重要营养物质。在水生环境中，一些细菌可以将大气中的氮气（N2）转化为生物可利用的形式（铵、硝酸盐）。溶解的N库以溶解的有机N化合物为主（80-90%），约10-20%为无机N化合物，有机和无机N化合物都很容易被生物系统吸收。在世界范围内，由于使用人工肥料（Haber-Bosch）、畜牧业生产和废水处理厂排放富氮废水以及大气氮沉降，向水体输出的氮不断增加，导致淡水生态系统的营养污染，对生态系统健康产生不利影响。这些可能影响了浮游植物群落的物种组合、分布和生产力，尽管我们对这些影响的了解还不完整，特别是在淡水生态系统中。浮游植物在全球初级生产力中发挥着关键作用，特别是在天然淡水中。生态学研究表明，在低营养生境中，海洋和淡水picocyanobacteria都是浮游植物的主要组成部分，超过了真核生物组成部分。尽管淡水壁青杆菌在食物链动态中发挥着核心作用（例如作为鞭毛虫和大型食草动物的食物来源），但它们的研究仍然很少，特别是在基因组水平上，并与它们在海洋栖息地的近亲进行比较。此外，传统的细胞计数方法和分子生态学研究已经确定了淡水湖中picocyanobacteria丰度的季节性变化，但很少有研究研究浮游植物群落的季节性变化，以及/或溶解N池的化学变化，因为这种化学变化沿着营养富集的梯度变化。一些淡水picocyanobacteria基因组研究已经确定了参与硝酸盐整合和几丁质降解（一种由真菌和节肢动物产生的聚合物）的基因。后者表明复杂的有机分子可能是生物可利用的，支持了最近由导师领导的实验室生物测定研究中出现的证据。然而，淡水浮游植物吸收环境氮源的代谢途径、遗传信息和生化机制尚不清楚。采用跨学科的方法，本项目旨在研究花青菌群落如何在田间和受控实验室实验中对不同梯度和化学形式的氮有效性做出反应。该学生将设计并进行新颖的实验，从英国湖区分离、培养和测序picocyanobacteria，在那里可以获得长期的背景数据和附近的Cotswold Water Park在South Cerney。该奖学金将解决以下问题：picocyanobacteria群落如何在不同的营养/营养梯度中变化，以及与其他微生物的关系？2. 什么样的氮同化机制存在于花青菌？这些基因在不同的实验条件下是如何表达的？3. 在受控的实验室条件下，人工肥料和有机废物的存在如何影响花青菌对氮的同化？

Nitrogen is an essential nutrient controlling phytoplankton productivity in freshwater ecosystems. In aquatic environments, some bacteria can transform atmospheric nitrogen gas (N2) into bioavailable forms (ammonium, nitrates). The dissolved N pool is dominated by dissolved organic N compounds (80-90%), with around 10-20% in the form of inorganic N compounds, both organic and inorganic N compounds can be easily assimilated into living systems. Worldwide, the increasing export of N to waters from the use of artificial fertilisers (Haber-Bosch), livestock production and the discharge of N rich effluent from wastewater treatment works together with atmospheric N deposition, have contributed to nutrient pollution of freshwater ecosystems with adverse consequences for ecosystem health. These have likely affected the species assemblages, distribution and productivity of the phytoplankton community, though our understanding of these impacts is incomplete, particularly in freshwater ecosystems. Picophytoplankton play a key role in global primary productivity, particularly in natural freshwaters. Ecological studies have previously suggested that both marine and freshwater picocyanobacteria represent the main component of picophytoplankton in low nutrient habitats exceeding the eukaryotic component. Despite their central role in the food chain dynamics (e.g. as a food source for flagellates and larger grazers), freshwater picocyanobacteria remain poorly studied, especially at the genomic level and in comparison, to their closest relatives in marine habitats. Furthermore, traditional cell count methodologies and molecular ecology studies have identified picocyanobacteria seasonal changes in terms of abundance in freshwater lakes, yet few studies have studied seasonal changes in phytoplankton communities, and/or in relation to the changing chemistry of the dissolved N pool as this varies along gradients of nutrient enrichment. A handful of freshwater picocyanobacteria genomes studies have identified genes involved in nitrate incorporation as well as chitin degradation (a polymer produced by fungi and arthropods). The latter suggests that complex organic molecules might be bioavailable, supporting evidence emerging from recent laboratory bioassay research led by the supervisors. However, the metabolic pathway, genetic information and biochemical mechanisms used by the freshwater picophytoplankton in assimilating environmental sources of nitrogen are not well understood. Using an interdisciplinary approach this project aims to study how picocyanobacteria communities respond to different gradients and chemical forms of nitrogen availability both in the field and under controlled lab experiments. The student will design and carry out novel experiments to isolate, culture, and sequence picocyanobacteria from the English Lake District, where long-term contextual data are available and the nearby Cotswold Water Park at South Cerney. The questions this studentship will address are as follows: 1. How do picocyanobacteria communities change across different trophic/nutrient gradients, and in relation to other microorganisms? 2. What is the nitrogen assimilation machinery present in picocyanobacteria? How are these genes expressed under different experimental conditions? 3. How does the presence of artificial fertilisers and organic waste affect N assimilation by picocyanobacteria under controlled lab conditions?