Joint effects of naphthalene and microcystin-LR on physiological responses and toxin bioaccumulation of Landoltia punctata

The co-contamination of naphthalene (NAP) and micmcystin-LR (MC-LR) commonly occurs in eutrophic waters. However, the joint effects of NAP and MC-LR on plants in aquatic environments remain unknown. Landoltia punctata is characterized by high starch yields and high biomass in polluted waters and has been proven to be a bioenergy crop and phytoremediation plant. In this study, L. punctata was cultured in a nutrient medium with environmentally relevant NAP (0.1, 1, 3, 5, and 10 mu g/L) and MC-LR (5, 10, 25, 50, and 100 mu g/L) to determine individual and joint toxic effects. The effects of NAP and MC-LR on physiological responses of L. punctata, including growth, starch accumulation, and antioxidant responses, were studied. Bioaccumulation of MC-LR in L. punctata, with or without NAP, was also examined. The results showed that growth and chlorophyll-a contents of L. punctata were reduced at high concentrations of MC-LR (>= 25 mu g/L), NAP (>= 10 mu g/L) and their mixture (>= 10 + 1 mu g/L) after exposure for 7 d. Starch accumulation in L. punctata did not decrease when exposed to NAP and MC-LR, and higher starch content of 29.8 % + 2.7 % DW could be due to the destruction of starch-degrading enzymes. The antioxidant responses of L. punctata were stronger after exposure to MC-LR + NAP than when exposed to a single pollutant, although not enough to avoid oxidative damage. NAP enhanced the bio- accumulation of MC-LR in L. punctata when NAP concentration was higher than 5 mu g/L suggesting that higher potentials of MC-LR phytoremediation with L. punctata may be observed in NAP and MC-LR co-concomitant waters. This study provides theoretical support for the application of duckweed in eutrophic waters containing organic chemical pollutants.