Physiological behavior of Scenedesmus sp. during exposure to elevated levels of Cu and Zn and after withdrawal of metal stress.

A 48 h exposure of Scenedesmus sp. to sublethal concentrations of Cu (2.5 and 10 microM) and Zn (5 and 25 microM) caused a concentration-dependent inhibition of growth, photosynthesis, respiration, NO(3)(-) uptake, and nitrate reductase (EC 1.6.6.1) activity, and a reduction in protein, carbohydrate, and photosynthetic-pigment levels with a concomitant increase in intracellular levels of the test metals. After exposure, algal cells were transferred to the basal medium without the excess level of test metals, to study the recovery of various processes. The growth of the test algae had not recovered up to 12 h after transfer to the basal medium, but some physiological parameters such as photosynthesis and respiration recovered within 6 h. The quicker recovery of photosynthesis and respiration might be used as acclimatory responses as they prepare a background for the recovery of other parameters, including growth, of the test alga by generating energy, forming photosynthate, and establishing the usual catabolism to attain normal conditions. Most of the processes recovered completely or almost completely after being stressed with 2.5 microM Cu or 5 microM Zn. However, the maintenance of a relatively high level of Cu and Zn in the cells previously exposed to 10 microM Cu and 25 microM Zn slowed down the recovery of different processes, which did not fully recover even at the end of the experiment after 96 h. The present study demonstrates that a chain of metabolic events, beginning with respiration and photosynthesis and continuing with assimilation and uptake of nutrients and subsequent restoration of other metabolic processes, is involved in the recovery of the algae from Cu and Zn stress. Each studied parameter seems to play an important role in balancing the cellular homeostasis during recovery from metal stress.