Variation in Zinc Adsorption and Transport During Growth of Chlamydomonas variabilis (Chlorophyceae) in Batch Culture with Daily Addition of Zinc

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Abstract
C. variabilis Dangeard was grown in batch culture under constant irradiance in the absence (A) or presence (B) of added Zn. Growth parameters were followed for 6 d [day] and aliquots were removed daily from both cultures A and B for the measurement of adsorption and transport of Zn in short-term (10-min) incubations with 65Zn. The concentration of EDTA-extractable zinc ([Zn]a) and the flux (F) of zinc across the cell membrane both decreased with increasing culture age. Values of [Zn]a and F varied depending on when the measurements were made during the growth cycle and on the history of the inoculum. In culture B, previous growth in the presence of 7.0 .mu.mol Zn2+/l resulted in larger values of [Zn]a but smaller fluxes than did growth in the absence of added Zn. Regardless of culture age, the dependence of [Zn]a on the concentration of Zn2+ in solution ([Zn2+]b) could be described by Langmuir adsorption isotherms. Conditional adsorption equilibrium constants, K.theta., were greater for cells previously grown in the presence of added Zn but showed no clear trend with culture age. Rate constants for Zn transport were smaller for cells grown with added Zn and decreased slightly with increasing culture age. The number of binding sites per unit cell surface, Q, decreased markedly with culture age, as did values obtained for [Zn]a. The variation in metal adsorption and transport as a function of the physiological state of the cells should be considered when trace metal uptake is modeled and when phytoplankton are used to assess metal toxicity.
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