Aquatic environments are often exposed to various pollutants like heavy metals due to industrial, agriculture and domestic wastewater discharge. Heavy metals can discharged to all ecosystems and make several problems in aquatic living organisms. Microalgae as primary producers are very important in aquatic ecosystems because they provide oxygen and organic matter for other organisms. In this study, the effect of different concentrations (0, 5, 50, 100, 250, 500, and 750 mg/l) of the heavy metals including nickel, copper and lead on density, growth, chlorophyll a contant, dry biomass and cellular morphological changes in green algae Scenedesmus qaudricauda were investigated. The experiments were carried out as completely randomized design for a 14-day at environmental conditions of Bold Basal’s Medium (BBM), temperature of 25 ?C, Photoperiod of 12 hours light: 12 hours dark, and light intensity of 60 µmol photons/m2/s for each set of experiment. The results showed that toxic effects of the metals were different based on metal species and their concentrations. There was significant (P 0.01) decreasing in algal density, growth, biomass and chlorophyll a among different treatments of each set of experiment. The toxic effect of tested heavy metal on biomass and chlorophyll a were in order Cu gt;Ni. Furtheremore, the lowest growth, biomass and chlorophyll a were obtained at concentration of 750 mg/l in the each of metal. Cellular morphology (based on number of cells per colony) of S. qaudricauda were significant changed during each experiment. Treatments at lower concentrations had higher single-cell colonies, but with increasing concentrations of heavy metals (nickel, copper and lead) the cellular morphology tends to four-cell colonies in the S. quadricauda population. This study illustrated that S . quadricauda responses to heavy metals are quick, inexpensive and simple to measure. Therefore, S. quadricauda was recommended for environmental monitoring in freshwater ecosystems, especially in waters exposed to copper, lead and nickel. Key Words Green algae, Scenedesmus quadricauda , Heavy metal, Growth, Biomass, Chlorophyll a
