PHYSIOLOGICAL AND PHOTOSYNTHETIC RESPONSES OF SYNECHOCYSTIS AQUATILIS F. AQUATILIS (CYANOPHYCEAE) TO ELEVATED LEVELS OF ZINC1

Physiological and structural changes in cells of Synechocystis aquatilis f. aquatilis acclimated to grow in the presence of high zinc levels (2.20–3.30 mg·L^?1) were investigated. Growth of these cells showed a decreased specific growth rate and final yield of about 60% and 50%, respectively, of the values found for cells grown in the presence of 0.21 mg zinc·L^?1 (control culture). The higher the zinc concentration in the culture medium, the more pronounced the reduction in the chl a content. Regardless of zinc concentration, S. aquatilis possessed three distinct carotenoids. A decrease in carotenoid content accompanied the decrease of chl a, and the proportions of the pigments to each other were not affected by zinc. The photosynthetic performance of cells cultured in the presence of high zinc levels showed a decline in both the apparent photosynthetic efficiency and the photosynthetic maximal rate. In these cells the PSII reaction centers became partially closed, and the electron transport activity around PSII and PSI was reduced to 61% and 38% of the control values, respectively, which may indicate an altered PSII/PSI stoichiometry. In addition, electron micrographs revealed a reduced amount of thylakoid membranes, indicating that acclimation to high zinc levels led to a decrease in the overall number of photosynthetic units. On the other hand, light microscopic observation of negative‐stained cells revealed the presence of a thick mucilaginous layer surrounding the high zinc‐acclimated cells. This extracellular material could retain high amounts of metal ions from the medium, thus providing the Synechocystis cells a mechanism to circumvent toxic levels of zinc.