| Summary: | The Southern Ocean is a key driver of biogeochemical processes and the earth’s climate, influencing the global distribution of water masses, nutrients, and drawdown rates of atmospheric carbon dioxide. The physical and geochemical environment of the Southern Ocean influences and is in turn influenced by the plankton of the euphotic zone. One of the key controlling factors which limits the growth and productivity of phytoplankton is the availability of the micronutrient iron. In the first chapter, I characterize a novel high affinity iron binding phytotransferrin in the diatom Phaeodactyulum tricornutum, which reveals that carbonate-coordinated ferric iron binding evolved independently and convergently from metazoan transferrin. Biochemical manipulations of the seawater carbonate system demonstrate that the concentration of the carbonate ion co-limits uptake rates of unchelated ferric iron. As phytotransferrin sequences have broad taxonomic distribution and are abundant in marine environmental genomic datasets, this suggests that ocean acidification will negatively impact a globally important eukaryotic iron acquisition mechanism. In the second chapter, I analyze a metagenomic survey of the distribution and genomic capacity of planktonic organisms in the continental shelf regions of East Antarctica and demonstrate an advective connection between surface and deep populations of ammonia oxidizing Archaea. In the third chapter, I use environmental transcriptomics (metatranscriptomics) to evaluate cellular- and ecosystem-level responses of an ice algae community living and fixing carbon on the underside of Antarctic sea ice. Taken together, these three chapters reveal some of the remarkable adaptations which marine plankton have used to adapt to life in one of the Earth’s most extreme environments, the Southern Ocean.
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