Life Science Journal 2014;11(10) http://www.lifesciencesite.com 1349 Response of the green microalga Chlorella vulgaris to the oxidative stress caused by some heavy metals Amal Hamed El-Naggar 1,2 & Huda Mohammed Sheikh 1 1 Department of Biological Science, Science Faculty for Girls, King Abdulaziz University, Jeddah 21589, Kingdom Saudi Arabia. 2 Botany Department, Faculty of Science, Tanta University, Tanta 31527, Egypt. amalelnagar5@yahoo.com Abstract The present work was carried out to study the response of the microalga Chlorella vulgaris to the oxidative stress caused by Cu, Cd and Zn on growth, total lipids, lipid peroxidation, fatty acids composition, antioxidant enzymes and ultrastructure. The lower concentrations of the three heavy metals stimulated C. vulgaris growth, while the higher ones inhibited the algal growth. The three tested metals could be arranged according to their toxicities to C. vulgaris in the following order: Cu > Cd > Zn. The three metals also induced an increase in total lipid content, lipid peroxidation and activities of peroxidase and catalase; the induction by Cu being stronger than by Cd and Zn. The fatty acids of Chlorella vulgaris were dominated by 16:0, 17:0, 18:1 and 18:2. The three metals caused the appearance of lauric acid , increased significantly the content of 18:0 and decreased the contents of the C14:0, C16:0, and C17:0.Considering unsaturated fatty acids, C. vulgaris responded to the three metals by decreasing the production of 16:1 with considerable increase in the production of 18:1. Cd and Zn increased the production of 18:2 and 18:3, however; Cu decreased their production. The overall effect of the tested metals was to increase the ratio of unsaturated to saturated fatty acids. Cellular damage was studied under transmission electron microscope. The alterations induced by Cd and Cu were invagination of cell envelop, disintegration of thylakoid membranes; increase in the size of inclusion bodies inside the vacuoles, lack of cristae in the mitochondrion, formation of mitochondrial myelin- like structure and dark dots on the cell surface. Zn induced the formation of a dark electron dense layer with an amorphous aspect on the cell surface and numerous plastoglobuli in the cytoplasm. The differences in subcellular effects induced by Cu, Cd and Zn are probably due to specific adaptation mechanisms developed by C. vulgaris. [Amal Hamed El-Naggar and Huda Mohammed Sheikh. Response of the green microalga Chlorella vulgaris to the oxidative stress caused by some heavy metals. Life Sci J 2014;11(10):1349-1357]. (ISSN:1097-8135). http://www.lifesciencesite.com . 197. doi:10.7537/marslsj111014.197 . Keywords: Chlorella, Heavy metals, Lipid peroxidation, antioxidant enzymes, Fatty acids, Ultrastructure. 1. Introduction Toxic effect of heavy metals on living systems is one of the main problems derived from environmental contamination. Microalgae, the key component of the food web in aquatic ecosystems, being seriously affected by metal pollution 1,2 . Increasing levels of heavy metals in the environment affect various physiological and biochemical processes of microalgae. It can cause adverse effects on cell division, growth, photosynthesis, respiration, uptake and assimilation of nitrate and degeneration of the main cell organelles 3,4 . Heavy metals toxicity depends on the metal concentration 5,6 . Heavy metals lead to the formation of reactive oxygen species in algal cells 7,8,9 causing lipid peroxidation 10,11 . The defense mechanisms of algae against oxidative stress can be broadly classified into two types: i) mechanisms that prevent interaction between the metal (s) and their site (s) of actions, and (ii) those that counteract the stress-induced damages. The latter include antioxidant systems consisting of enzymatic and non-enzymatic components. A large amount of convincing evidence demonstrates an increased synthesis of non-enzymatic antioxidants like glutathione and ascorbate as well as enhanced antioxidant enzymes under metal stress 12,13,8 . The amount of oxidized proteins and lipids in the algal cells thus indicates the severity of the stress in a number of microalgal species 3 . Moreover, cell resistance to growth inhibitor can over produce polyunsaturated fatty acids (PUFA) 14 . Algal tolerance to heavy metal is highly dependent upon the defense response against the probable oxidative damages 2 . In this connection, Scenedesmus acutus responded to nickel toxicity by higher activities of antioxidant enzymes as catalase, superoxide dismutase, glutathione reductase and glucose-6- phosphate dehydrogenase 7 . In Cladophora glomerata Lipoperoxides showed positive correlation to heavy metals accumulation sites indicating the tissue damage resulting from the reactive oxygen species and resulted in unbalance to cellular redox status. Also, high activities of ascorbate peroxide and superoxide dismutase, increased dehydroascorbate, decreased glutathione and soluble phenols probably counter balance of this oxidative stress 8 .