THE BIOGEOGRAPHY OF ANTARCTIC MARINE DIATOMS AND THEIR RESPONSE TO VARIED IRON CONCENTRATIONS

Abstract

Diatoms are a group of highly diverse photosynthetic eukaryotes that account for about 40% of oceanic primary production. In large areas of the ocean deemed high nitrate, low chlorophyll (HNLC) regions, dissolved macronutrient (nitrogen, phosphorus, silica) concentrations are high in the surface but diatom biomass is consistently low. The Southern Ocean (SO) is the largest of these environments, and when exogenous Fe is supplied to this region, large diatoms blooms and subsequent carbon export are promoted. As such, SO diatoms are major players in the global carbon cycle and were the focus of this dissertation. This work sought to determine SO diatom biogeography and investigate their potential Fe-limitation coping strategies. Chapters 1 and 2 focused on determining diatom biogeography and community structure in the western Antarctic Peninsula (WAP), the Ross Sea Polynya (RSP), and surrounding HNLC Antarctic Circumpolar Current (ACC) waters. Diatom-targeted high-throughput molecular barcoding data and corresponding environmental metadata were analyzed to determine diatom community composition and the factors driving their structure. Chapter 1 details findings from RSP and ACC samples collected during a 2013 austral summer cruise. Fragilariopsis and Pseudo-nitzschia sequences dominated the sampled areas, with Fe and salinity together driving the community patterns, likely due to seasonal sea ice melt water. Chapter 2 analyzed samples collected from two WAP research cruises (austral spring 2014 and 2016) to compare the interannual and spatial variation of the communities. While offshore and inshore WAP communities differed in species composition, there was little interannual variation. In Chapter 3, metatranscriptomic sequencing was used to examine the gene expression response of a SO diatom community and their associated bacteria to Fe-amendment incubations during the 2016 WAP cruise. Fe addition elicited varying responses by different diatom genera, with more prevalent diatoms experiencing more differential regulation of Fe-responsive pathways and proteins. In addition, some diatom genera expressed components of a putative Fe-siderophore uptake system, including an Fe-siderophore outer membrane receptor protein and corresponding ABC transporter components. In the bacterial metatranscriptome, upregulation of Fe-siderophore uptake genes was observed, possibly indicating increased competition for Fe among the stimulated diatom-associated bacterial community. Chapter 4 investigated how three SO diatoms (Fragilariopsis cylindrus, Pseudo-nitzschia arenysensis, and Thalassiosira tumida) interact with associated bacterial communities under Fe stress. On the 2016 WAP cruise, a bacterial addback experiment was completed with these previously antibiotic-treated diatoms under Fe-rich (+Fe) and Fe-limited (-Fe) conditions. The two pennates F. cylindrus and P. arenysensis recruited distinct bacterial