Phosphorus levels determine changes in growth and biochemical composition of Chlorella vulgaris during cadmium stress

Algae are constantly faced with several physicochemical stresses in the environment that are rarely simultaneously considered during ecotoxicological and ecophysiological studies. We investigated changes in growth and biochemical composition of Chlorella vulgaris during exposure to cadmium (0.02 and 0.1 mu M Cd) under varying phosphorus (0.6, 2.3, and 230 mu M P) concentrations. The cell density of the microalga was significantly lowered with decreasing P and increasing Cd concentrations. Dry weight, chlorophyll a, total carbohydrates, total lipids, and total proteins (pg cell(-1)) were stimulated under P limitation, and the presence of Cd further increased the level of each parameter. P limitation resulted in lower proportions of amino acids (%mol) such as ornithine, leucine, and valine. Furthermore, the exposure to Cd at 230 mu M P significantly lowered the levels of aspartic acid, aminopimelic acid, and asparagine. On the other hand, decreasing P concentrations resulted in significantly higher proportions of amino-n-butyric acid and isoleucine, while the presence of Cd caused a significant decline in the levels of both amino acids at 0.6 mu M P relative to treatments with only the same limited P concentration. At 0.6 mu M P, the presence of Cd resulted in an increase in glutamine and ornithine proportions, and at 2.3 mu M P, the levels of aspartic acid and valine increased in the presence of the metal. Our results showed that changing P concentrations influence the physiological response of C. vulgaris to Cd, which can have significant bottom-up effects in aquatic food chains and community dynamics.