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Effects of nitrogen availability on inorganic carbon concentrating mechanisms in marine phytoplankton Corkum, Miranda Elyse


Carbon and nitrogen metabolism are tightly coupled through the enzyme RubisCO, but the effects of nitrogen limitation on cellular carbon acquisition are not well understood. This thesis reports measurements of RubisCO and phosphoenolpyruvate carboxylase (PEPC - an enzyme involved in the C₄ photosynthetic pathway) activity in Thalassiosira weissflogii grown under steadystate nitrogen limitation as well as data on the utilization of dissolved inorganic carbon species by this microalgae. These biochemical and physiological parameters were also examined across a natural nitrogen gradient in a field study of phytoplankton assemblages in the Queen Charlotte Sound coastal region. In nitrogen-limited T.weissflogii cultures, RubisCO activity decreased with decreasing growth rate (r=0.821; p<0.001), while PEPC activity remained unchanged over the range of growth rates tested (1.4 to 0.3d⁻¹). fHC0₃⁻ values, representing the fraction of dissolved inorganic carbon taken up in the form of HCO₃⁻, obtained from ¹⁴C uptake experiments decreased from ~ 0.85 to 0.70 as cells became more severely nitrogen-limited (0.682; p<0.001). Carbon uptake kinetics indicated that the decrease in fHCO₃⁻ was attributed to a decrease in direct HCO₃⁻ transport, as extracellular carbonic anhydrase activity appeared unaffected by changes in growth rate. A down-regulation of the activity of the carbon concentrating mechanism (CCM) may serve to control elemental ratios within the cell and maintain metabolic balance under nitrogen-limited conditions. At the seven coastal field stations sampled, there was no relationship between RubisCO activity and nitrate concentration, but a negative trend was observed between PEPC activity and nitrate concentration. The diatomdominated phytoplankton assemblages studied all utilized HC0₃⁻ as an exogenous inorganic carbon source and HCO₃⁻uptake occurred mainly via a direct transport mechanism. There was no apparent relationship between fHCO₃⁻ values and nitrate concentration at these stations.

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