Integrated Ecotoxicological Assessment of Metal Nitrates in Lemna minor Based on Growth Inhibition, Biochemical Responses, and Species Sensitivity Comparison.

Heavy metals in water are a global concern due to their persistence, toxicity, and bioaccumulation potential, highlighting the need to assess their effects on aquatic organisms. The ecotoxicity of eight potentially toxic metal nitrates (Cd (0.002 mg/L), Co (0.02 mg/L), Cr (0.02 mg/L), Cu (0.001 mg/L), Mn (0.04 mg/L), Ni (0.005 mg/L), Pb (0.002 mg/L), Zn (0.1 mg/L) up to 1000 mg/L for all metals) was assessed on the aquatic macrophyte Lemna minor, in a multilevel study. Growth responses, measured as frond number and average specific growth rate, were used to generate dose-response curves, but also to calculate EC50 (Median Effective Concentration) and ErC50 (50% Effect Concentration based on growth rate) values. Based on EC50, toxicity followed the order Cd > Co > Ni > Zn > Cu > Cr > Pb > Mn, while ErC50-based ranking was Cd > Ni > Co > Zn > Cu > Cr > Pb > Mn. Metals were classified as highly toxic (Cd, Ni; Co based on EC50), moderately toxic (Co based on ErC50, Zn based on EC50), slightly toxic (Cu, Cr; Zn based on ErC50, Pb based on EC50), or practically non-toxic (Pb based on ErC50, Mn). Biochemical responses regarding photosynthetic pigments, primary metabolites, and oxidative-stress-related enzymes were investigated, revealing physiological effects for most metals. Species sensitivity distribution-based comparison indicated that L. minor sensitivity varies across metals, ranging from typical to relatively low within the broader species sensitivity distribution. Although the toxicity of potentially toxic metals to L. minor has been extensively investigated, data on ErC50 values for several of the metals investigated in the present study remain limited, highlighting the need for further ecotoxicological research.