| The biogeochemical cycling of many metals in aquatic and marine systems is driven primarily by phytoplankton interactions with the dissolved elements. Among the numerous fundamental controls on metal stoichiometry in phytoplankton is the physiological state of the cell, which in many natural systems is strongly affected by availability of the macronutrients N and P. Low P availability is hypothesized to stimulate enhanced uptake and enriched cellular quotas for specific metals, related to up-regulated activity of specific metalloenzymes. This thesis investigates links between P availability and metal uptake in freshwater phytoplankton, using controlled laboratory incubation studies and analyses of the plankton assemblage in Lake Superior (North America), a severely P-depleted system.;Phosphorus limitation had substantial effects on the cellular metal composition of two representative freshwater algae. In Chlorella sp ( UTCC522), P limitation induced a 2-3 fold enrichment of cellular Zn, Cd and Co quotas, while in Microcystis sp (LE3 ) 3-4 fold enrichment of Co and Ni was observed. In contrast, P availability had negligible effect on cellular Fe, Mn, Cu and V in both organisms. The results were inferred to be a response to increased metal requirements of Alkaline Phosphatase (APase), which may include Zn, Cd, and Co co-factors in Chlorella, but be strictly Co-requiring in Microcystis. Elevated Ni in Microcystis suggests increased activity of Ni-requiring superoxide dismutase, resulting from oxidative stress induced by P limitation.;Measurements of seasonal and geographic ranges in the trace metal content of Lake Superior particles revealed that the ambient phytoplankton assemblage is enriched in Zn and Cu by 8-10 fold relative to predictions based on culture studies of marine and Great Lakes species. Upon P amendment, the ambient assemblage Zn and Cd quotas decreased ∼2 fold, supporting the Chlorella result, but failing to explain the magnitude of Zn enrichment in the lake. Further experiment suggested that the unusual Zn enrichment in the natural assemblage results from a combination of P limitation, extracellular adsorption, and high fractional bioavailability of dissolved Zn in the lake, while elevated Cu quotas likely result from unusually high concentrations of inorganic Cu in lake water. |
