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Coastal Antarctic marine ecosystems are significant in carbon cycling because of their intense seasonal phytoplankton blooms. Southern Ocean algae are primarily limited by light and iron (Fe) and can be co-limited by cobalamin (vitamin B₁₂). Micronutrient limitation controls productivity and shapes the composition of blooms which are typically dominated by either diatoms or the haptophytePhaeocystis antarctica. However, the vitamin requirements and ecophysiology of the keystone speciesP. antarcticaremain poorly characterized. Using cultures, physiological analysis, and comparative omics, we examined the response ofP. antarcticato a matrix of Fe-B₁₂conditions. We show thatP. antarcticais not auxotrophic for B₁₂, as previously suggested, and identify mechanisms underlying its B₁₂response in cultures of predominantly solitary and colonial cells. A combination of proteomics and proteogenomics reveals a B₁₂-independent methionine synthase fusion protein (MetE-fusion) that is expressed under vitamin limitation and interreplaced with the B₁₂-dependent isoform under replete conditions. Database searches return homologues of the MetE-fusion protein in multiplePhaeocystisspecies and in a wide range of marine microbes, including other photosynthetic eukaryotes with polymorphic life cycles as well as bacterioplankton. Furthermore, we find MetE-fusion homologues expressed in metaproteomic and metatranscriptomic field samples in polar and more geographically widespread regions. As climate change impacts micronutrient availability in the coastal Southern Ocean, our finding thatP. antarcticahas a flexible B₁₂metabolism has implications for its relative fitness compared to B₁₂-auxotrophic diatoms and for the detection of B₁₂-stress in a more diverse set of marine microbes. 

Abstract. Zinc (Zn) is an essential micronutrient for most eukaryotic phytoplankton. Zn uptake by phytoplankton within the euphotic zone results in nutrient-like dissolved Zn (dZn) profiles with a large dynamic range. The combination of key biochemical uses for Zn and large vertical gradients in dZn implies the potential for rapid rates of Zn removal from the surface ocean. However, due to the ease of contamination at sea, direct measurements of dZn uptake within natural environments have not been previously made. To investigate the demand for dZn and for dissolved cadmium (dCd; a closely related nutrient-like element) within Southern Ocean phytoplankton communities, we conducted 67Zn and 110Cd tracer uptake experiments within the Amundsen Sea, Ross Sea, and Terra Nova Bay of the Southern Ocean. We observed a high magnitude of Zn uptake (ρZn > 100 pmol dZn L−1 d−1) into the particulate phase that was consistent with ambient depleted dZn surface concentrations. High biomass and low partial pressure of carbon dioxide in seawater (seawater pCO2) appeared to contribute to ρZn, which also led to increases in ρCd likely through the upregulation of shared transport systems. These high ρZn measurements further imply that only short timescales are needed to deplete the large winter dZn inventory down to the observed surface levels in this important carbon-capturing region. Overall, the high magnitude of Zn uptake into the particulate fraction suggests that even in the Zn-rich waters of the Southern Ocean, high Zn uptake rates can lead to Zn depletion and potential Zn scarcity.