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Diatoms are an ecologically important group of Southern Ocean phytoplankton forming the base of polar marine food webs and contribute significantly to carbon and silica export. Changes in their physiological status can alter their growth, productivity and elemental stoichiometry, consequently affecting food web dynamics and the efficiency of the biological carbon pump. Iron and light availability are predicted to be two abiotic factors that control diatom physiology and productivity. Therefore, a better understanding of the ecological importance of iron and light limitation on Southern Ocean diatom physiology is needed to decipher the role of diatoms in polar ocean ecology and biogeochemistry. By coupling culturing methods and ‘omic techniques, my dissertation sheds light on the physiology and metabolism of ecologically relevant diatom isolates and natural diatom assemblages from the Western Antarctic Peninsula. The focus of the first two chapters of my dissertation are 1) to investigate the molecular underpinnings of the response of a Southern Ocean diatom, Fragilariopsis kerguelensis, to changing iron and light levels, and 2) to understand how the physiology and elemental composition of four Southern Ocean diatoms change in low iron conditions, focusing on a common bloom forming diatom species, Pseudo-nitzschia subcurvata. The focus of the third chapter is 3) to explore diatom cellular metabolism along the Western Antarctic Peninsula to gain insight into the environmental factors influencing diatom growth and molecular physiology. My findings suggest the physiological responses of polar diatoms to low iron availability are diverse and that certain molecular mechanisms may underpin the ecological success of polar diatoms under low iron and low light conditions, highlighting the important role of diversified photosynthetic isoforms, iron acquisition and metabolic shifts in amino acid and carbon metabolism. Furthermore, diatom molecular physiology varied across the Western Antarctic Peninsula, indicating blooming centric diatoms in cold and fresher waters in the southern coastal regions, and iron-limited diatom communities in shelf and slope waters. This research provides new insights into the unique molecular physiological response of Southern Ocean diatoms to variations in iron and light availability, and the environmental factors influencing diatom metabolism and their predicted responses to future ocean conditions.

Iron and light are two important abiotic factors that influence diatom growth and distribution in the Southern Ocean (SO). Through a combination of physiological and transcriptomic approaches, I have explored the molecular underpinnings of nine SO diatoms that allow for adaptation and/or acclimation to low iron and light conditions. SO diatoms used in this study ranged across five orders of magnitude in size and displayed various degrees of resistance to iron and light limitation. Specifically, we investigated the presence or absence of 22 key genes involved in iron acquisition and homeostasis, photosynthesis, and nitrogen assimilation. SO diatoms have a variety of unique resource utilization strategies coupled with gene repertoires that allow them to take advantage of ecological niches or play important roles in phytoplankton blooms. Certain diatom genes, such as B12-independent methionine synthase (MetE) and flavodoxin, were found to exhibit biogeographical patterns in distribution that favor high-nutrient, low-chlorophyll regions.

Despite our current realization of the tremendous diversity that exists in plankton communities, we have little understanding of how this biodiversity influences the biological carbon pump other than broad paradigms such as diatoms contributing disproportionally to carbon export. Here we combine high-resolution underway O2/Ar, which provides an estimate of net community production, with high-throughput 18 S ribosomal DNA sequencing to elucidate the relationship between eukaryotic plankton community structure and carbon export potential at the Western Antarctica Peninsula (WAP), a region which has experienced rapid warming and ecosystem changes. Our results show that in a diverse plankton system comprised of ~464 operational taxonomic units (OTUs) with at least 97% 18 S identity, as few as two or three key OTUs, i.e. large diatoms, Phaeocystis, and mixotrophic/phagotrophic dinoflagellates, can explain a large majority of the spatial variability in the carbon export potential (76-92%). Moreover, we find based on a community co-occurrence network analysis that ecosystems with lower export potential have more tightly coupled communities. Our results indicate that defining plankton communities at a deeper taxonomic resolution than by functional groups and accounting for the differences in size and coupling between groups can substantially improve organic carbon flux predictions.