Zinc–Phosphorus Interactions and Variation in Zinc Accumulation during Growth of Chlamydomonas variabilis (Chlorophyceae) in Batch Culture

Abstract

Concentrations of EDTA-extractable zinc ([Zn]a ~ surface-bound zinc) and non-EDTA-extractable zinc ([Zn]c ~ transported zinc) were measured over the growth cycle of Chlamydomonas variabilis Dangeard growing in pH-controlled batch culture in the presence of six concentrations of free ionic zinc. Values of [Zn]a generally decreased throughout the growth cycle in batch culture, the exact pattern of change depending on whether the cells used to inoculate the experiment were obtained from a stock culture in the exponential or stationary growth phase. Concentrations of cellular TCA-insoluble phosphorus (polyphosphate) also declined throughout the growth cycle. Values of [Zn]c increased linearly throughout the exponential phase for growth at all zinc concentrations studied, but during the stationary phase they either reached a plateau for growth at 1.4 and 8.4 μmol Zn2+∙L−1, continued to increase linearly for growth at 13.2, 15.8, and 20.6 μmol Zn2+∙L−1, or declined for growth at 25.4 μmol Zn2+∙L−1. The cell yield of C. variabilis during the stationary phase was significantly decreased (by up to 90%) for growth in the presence of high concentrations of free ionic zinc. These cells also contained significantly more phosphorus than those grown in the presence of low free ionic zinc concentrations, but did not remove all of the initially added phosphorus from the growth medium. We hypothesize that cellular polyphosphate binds the transported zinc. As the concentration of cellular polyphosphate declines with increasing culture age, a portion of the previously bound intracellular zinc is released into the cell and exceeds a critical threshold, disrupting phosphorus metabolism, interfering with cell division, and decreasing cell yield.