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Copper requirements and acquisition mechanisms in marine phytoplankton Guo, Jian


Copper nutrition is essential for the growth of marine diatoms, especially under Fe-limiting conditions. I investigated the physiology of Cu in marine phytoplankton by studying how the Fe and Cu availability interact and control Cu uptake and demand. I used short-lived Cu radioisotopes to determine the Cu requirements and steady-state Cu transport rates (ρssCu) of ten species from three classes of marine phytoplankton, isolated from coastal and oceanic environments. I also determined the Cu uptake kinetics of two marine centric diatoms (Thalassiosira oceanica and T. pseudonana) grown under various Fe and Cu conditions (sufficient and limiting). Furthermore, putative genes encoding potential components of Cu transport and homeostasis were identified in T. pseudonana, and their expression was investigated. Copper had significant effects on growth rates and ρssCu of the oceanic phytoplankton, but not the coastal strains. Although Cu quotas (Cu:C) were not significantly higher in oceanic strains, there were five independent lines of evidence supporting a more important role of Cu in the physiology of oceanic phytoplankton. Distinct biphasic Cu transport rates as a function of Cu concentrations were observed in the centric diatoms, Thalassiosira oceanica and T. pseudonana, suggesting the presence of a high- and a low-affinity Cu transport system. The high-affinity Cu transport system followed Michaelis-Menten saturation kinetics, but was controlled differently by Fe and/or Cu availability. A strong interaction between Fe and Cu nutrition in controlling the expression of genes encoding Cu transport and homeostasis was observed. Most genes, including putative Cu transporters (CTR), Cu transporting P-type ATPases, Cu chaperones and putative Zn transporters in T. pseudonana were up-regulated by low Fe, while low Cu either had no effect or the effect was dependent on Fe availability. These results suggest a complex interaction between Cu and Fe response networks. The function of a putative Cu transporter (CTR) in T. pseudonana was examined using functional complementation of Saccharomyces cerevisiae ctr1Δctr3Δmutant. Though the results were inconclusive, various explanations for these findings were discussed. This thesis highlights a complex interaction between Fe and Cu nutrition in marine phytoplankton at the protein and gene expression level.

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