Elucidating patterns and processes structuring protist biodiversity across salinity gradients

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Most organismal diversity is microbial, including the numerous lineages of protists that are related to animals and plants. Protists are incredibly diverse, abundant, and have huge impacts on aquatic ecosystems because they influence the flow of nutrients and energy. Powerful sequencing approaches now enable us to rapidly characterize microbial communities and assess biodiversity on an unprecedented scale. We will use these tools to ask how protist diversity is distributed across environments and how different ecological and evolutionary forces, including selection and dispersal, combine to produce these biodiversity patterns. The transition from freshwater to marine ecosystems is of the strongest divisions in microbial diversity, with very different types of organisms inhabiting these environments. Environmental factors such as salinity and pH are thought to be largely responsible for this difference because together they constitute a large physiological barrier and crossing it usually requires evolutionary innovation. However, it is also possible that interactions among organisms are important, and they may reinforce the environmental boundary by out-competing or preferentially preying on newly arrived microbes. We will collect data on environmental parameters and the whole microbial community (protists and bacteria) to address this question from the perspective of the whole community for the first time. This research will use correlative studies along replicate salinity gradients combined with manipulative experiments that directly test what happens to community composition when salinity changes or a large predator is added, for example. We will also transplant entire communities to new environments under different conditions to see which microbes survive. Together these results will help uncover the mechanisms that determine which species of protists live in different environments, and how these processes differ within and between habitats, across local and regional scales, and over time. Along the way, we will also assess the role of dispersal plays in structuring microbial communities and test general ecological theories of biogeography and the maintenance of biodiversity. This research takes place in coastal environments and estuaries that are characterized by salinity gradients. Coastal ecosystems and estuaries deliver numerous ecosystem services that benefit humans, but these important ecosystems are threatened by human activities including agricultural runoff and sewage effluent. Coastal ecosystems are also on the frontlines of impending effects of climate change such as ocean acidification and increased variability in weather. Thus, understanding the processes that govern microbial eukaryotic community assembly in coastal ecosystems will enable better predictions of the response of microbial organisms to environmental changes that are among the most pressing concerns of our time and how these systems work in general.