Uptake and biological response to zinc by the Diatom Thalassiosira pseudonana
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The uptake and incorporation of zink by a Si-, Co- , and Zn starved Thalassiosira pseudonana culture was examined. Two experiments were conducted in which a pre-determined amount of a zinc sulfate solution and sodium metasilicate nonahydrate solution were delivered to an exponentially grown fed-batch algae culture yielding the wanted start concentrations in the medium. Experiment 1 and 2 had final zinc concentration of 8.3 μM (high dose) and 1.7 μM (low dose), respectively. The experiments were conducted in batch mode for 48 hours. Frequent sampling during the first part of the experiments were performed in order to examine the biological uptake of zinc and silicate. The biological response to zinc addition was followed throughout the experiment by measuring photo-physiological parameters (Ft og QY), organic carbon and nitrogen content, cell density, optical density, and nutrient concentrations (PO43- and NO2-). The concentration development of silicate and zinc was followed in the uptake assay samples through ICP-MS analysis. The structural and elemental composition of the diatom frustules were later examined through the use of Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy, respectively. The cellular zinc development followed a two-phase sorption process in which first phase consisted of rapid metal binding to cellular surfaces followed by a gradual desorption of zinc from the cells in the second phase. High initial pH-increase followed by precipitation of zinc as hydroxides is assumed to be the main reason explaining the observed sorption trend, explicitly through the attachment of particulate zinc to cellular surfaces. Other possible explanations are assigned to complexation of zinc by organic exudates excreted from actively photosynthesizing and growing cells. Reduced biological availability of zinc may have led to decreased metal uptake by the algae. However, a higher fraction of zinc was present in the experimental frustules as compared with frustules from untreated algae culture. The highest amount of zinc was detected in the low-dose experiment corresponding to a weight-fraction relative to silicate of 5.16% for the 24 h-sample and 0.146% for the 48 h-sample. Due to the high inter-sample difference in zinc fraction combined with possible sample-contamination from chemicals or equipment through the cleaning process (H2O2) used on the frustule samples from this experiment, these fractions are connected with high uncertainty. No explicit biological response to zinc addition was observed in the algae cultures in either of the two experiments, and the reduction of the photo-physiological condition observed by the end of the experiments were related to silicate-depletion of the culture medium.