Ecotoxicological Evaluation of Azolla pinnata and Lemna minor for Biomonitoring Cd and Pb Contamination in Aquatic Environments

Abstract

Heavy metal pollution in aquatic environments has led to devastating long-term effects in nature. Therefore, immediate action is required to develop ecotoxicological monitoring strategies to combat this environmental issue. This study evaluated the response of two aquatic plants, Azolla pinnata (Mosquito fern) and Lemna minor (Duckweed), to a concentration series (0.1-10.0 mg L-1

) of two heavy metals, Cadmium (Cd) and Lead (Pb), over a 3-day and 7-day microcosm exposure. A multiparameter approach was used where vegetative growth (frond number, wet weight, root length and plant width), biochemical changes related to oxidative stress (glutathione (GSH), peroxidase (POD), superoxide dismutase (T-SOD) activity), as well as chlorophyll a content were measured. Both plants showed an increase in growth at lower metal concentrations (up to 1 mg L-1 ), but growth decreased dose-dependently with Cd being more toxic than Pb (p<0.05), shown by severe necrosis and browning in plants. Results of the biochemical tests revealed more complex defense mechanisms at play. Elevated POD activity (112-119 U mg-1 ) and T-SOD activity (2.4-14.24 U mg-1 ) at higher concentrations of Cd exposure, coupled with a significant decrease in chlorophyll a (8.92 to 3.93 units, p<0.05), indicating targeting of the photosynthetic apparatus, were some of the trends observed. Both plants demonstrated consistent responses to heavy metal exposure within relevant environmental ranges, indicating their suitability to be used as ethical plant models for ecotoxicity testing and environmental monitoring.