| Summary: | Thesis (Ph.D.)--University of Washington, 2022 Polar oceans and sea ice are among Earth’s major biomes, but are experiencing rapid environmental changes associated with climate change that may shift polar marine ecosystems into new, potentially unstable, states. Warming temperatures, alterations in sea-ice dynamics, and enhanced glacial freshwater input into coastal regions at the poles all stand to alter the temperature and salinity seascapes of an environment already marked by pronounced seasonal fluctuations. This dissertation examines the role of temperature and salinity change associated with sea-ice formation and melt in structuring the chemical inventory of organic matter in microbially dominated polar systems, with a focus on sea-ice algae. Much of this work utilizes liquid chromatography-mass spectrometry (LC-MS) to observe pools of small biomolecules (metabolites) that can serve as currencies of microbial metabolism and provide a snapshot of cellular activity. Specifically, I focus on gaps in our knowledge regarding protective compounds that are temperature- and salinity-sensitive (compatible solutes), many of which are highly labile metabolites with the capacity to fuel the microbial loop upon their release from cells. Chapter 1 introduces sea-ice microbial communities and the cellular strategies they use to grapple with the temperature and salinity fluctuations that characterize polar marine habitats. In Chapters 2 and 3, I examine the covarying impact of temperature and salinity on compatible solutes in the sea-ice diatom Nitzschia lecointei (Chapter 2) and analyze organic metabolite pools across cultured diatom species (Chapter 3). These chapters reveal that sea-ice algae can contain diverse and species-specific suites of labile compatible solutes at high concentrations (up to ~1 M; Chapter 3) with complex and varying cellular sensitivities to temperature and salinity change (Chapter 2), whereby the solutes are strongly accumulated under cold and salty conditions compared to warmer and fresher ...
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