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

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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.