Dissertation Research: Phylogeny and Comparative Analysis of Salinity and Silicic Acid Uptake in the Thalassiosirales (Bacillariophyceae)

非技术摘要硅藻是构成河流、湖泊和海洋食物网的基础的微生物。虽然在地球上几乎每一个永久性和临时性水体中都发现了硅藻，但它们如何进入新的栖息地并在新的栖息地繁衍生息尚不清楚。最困难的入侵似乎是从咸水到淡水。所研究的硅藻类群--海链藻类--为了解海洋和淡水生境中硅藻的起源和演化提供了一个极好的机会。来自海洋和淡水栖息地的海链藻类物种在对溶解硅的利用方面存在根本差异，溶解硅是硅藻细胞壁的主要成分，也是地壳中第二丰富的元素。这项研究的目的是：1)根据核、叶绿体和线粒体基因组的DNA序列重建中心硅藻目海链藻目的系统发育；2)通过确定这两种栖息地类型之间的转换次数，利用系统发育图检验关于海洋和淡水物种之间关系的相互竞争的假说；以及3)测试盐度最适和硅酸吸收效率是否与海链藻目物种的硅酸吸收效率相关。系统发育将表明海洋和淡水栖息地之间的过渡次数，实验数据将揭示可能促进这些过渡的生理变化。硅藻对维持地球上的生命至关重要。仅海洋硅藻的初级生产量(通过光合作用产生的有机物和游离氧分子)就占地球氧气的大约25%，比世界上所有雨林的总和还要多，并维持着世界上大部分海洋和淡水渔业。除了对氧气循环和食物网的重要性外，硅藻对硅的加工也很重要，硅是地壳中第二丰富的元素。它们使用硅(化学形式称为二氧化硅)来制造细胞壁。许多硅藻细胞壁下落到湖泊和海洋的底部，形成了大量的硅藻土沉积物。它被开采用于许多工业目的，可以形成石油储藏库，并记录了地球历史上的变化。这种沉积物的形成以及硅藻循环的碳、氧和二氧化硅的数量取决于影响它们生长的条件。硅藻的生产往往是营养有限的，因此了解硅藻如何利用溶解的二氧化硅等营养物质是了解在世界海洋中观察到的初级生产模式的必要的第一步。硅藻也是少数几个成功占据海洋和淡水生境的群体之一，有证据表明，盐度极大地影响了硅藻对二氧化硅的新陈代谢，也影响了硅藻的生产。理解生物体如何以及为什么通过进化过程克服障碍具有基本的科学价值。从实践的角度来看，了解硅藻如何完成从咸水到淡水的艰难过渡，可以帮助我们了解硅藻适应这些不同环境的机制，从而帮助我们理解并更好地预测人类和自然造成的环境变化可能如何影响硅藻的分布和生产，从而影响碳、二氧化硅和氧气的循环。

Non-technical abstractDiatoms are microscopic organisms that form the basis of the food web in rivers, lakes and oceans. Although diatoms are found in nearly every permanent and temporary water body on earth, how they enter and flourish in new habitats is unknown. The most difficult invasion seems to be from saltwater to freshwater. The diatom group studied, Thalassiosirales, provides an excellent opportunity for understanding the origin and evolution of diatoms in marine and freshwater habitats. Thalassiosirales species from marine and freshwater habitats differ fundamentally in utilization of dissolved silicathe primary component of diatom cell walls, and the second most abundant element in the earth's crust. The objectives of this research are to 1) reconstruct the phylogeny of the centric diatom order Thalassiosirales based on DNA sequences from the nuclear, chloroplast, and mitochondrial genomes, 2) use the phylogeny to test competing hypotheses about the relationship between marine and freshwater species by determining the number of transitions between these 2 habitat types, and 3) test whether the salinity optima and efficiency of silicic-acid uptake are correlated for species of Thalassiosirales. The phylogeny will indicate the number of transitions between marine and freshwater habitats, and the experimental data will reveal physiological changes that potentially facilitated these transitions. Diatoms are critical to sustaining life on earth. Primary production (creation of organic matter and free oxygen molecules through photosynthesis) by marine diatoms alone accounts for approximately 25% of the earth's oxygen more than all of the world's rainforests combined and sustains much of the world's ocean and freshwater fisheries. Besides their importance to the oxygen cycle and food webs, diatoms are also important to processing of silicon, the second most abundant element in the earth's crust. They use silicon (in the chemical form called silica) to make their cell walls. Many diatom cell walls fall to the bottoms of lakes and oceans, forming vast deposits of diatomaceous earth. This is mined for many industrial purposes, can form a reservoir for petroleum, and records changes in earth history. The creation of such deposits, and the amount of carbon, oxygen and silica cycled by diatoms depends on conditions affecting their growth. Diatom production is often nutrient limited, so understanding how diatoms utilize nutrients such as dissolved silica is a necessary first-step towards understanding the patterns of primary production observed in the world's oceans. Diatoms also are one of the few groups that successfully occupy marine and freshwater habitats, and evidence suggests that salinity greatly affects the metabolism of silica by diatoms and also diatom production. There is basic scientific value in understanding how and why organisms overcome barriers through evolutionary processes. From a practical standpoint, understanding how diatoms made the difficult transition from saltwater to freshwater can help us understand the mechanisms by which diatoms adapt to these different environments, and so help us understand and perhaps better predict how environmental change caused by humans and nature might affect the distribution and production of diatoms, and so cycling of carbon, silica and oxygen.