Combined effects of microcystins and anatoxins on lettuce growth, hormone metabolism and safety.

Harmful algal blooms lead to coexistence of microcystins and anatoxins in irrigation water, and the potential risk caused by their coexistence is unknown. To assess the impact of the two cyanotoxins on plant growth and food safety, we used lettuce exposed to single microcystins (0.5, 1, 5, 20, 50 and 100 µg·L-1), single anatoxins (0.5, 1, 5, 20, 50 and 100 µg·L-1) and their combination (0.5 + 0.5, 1 + 1, 5 + 5, 20 + 20, 50 + 50 and 100 + 100 µg·L-1) to compare its growth, hormone metabolism as well as cyanotoxin accumulation. Low concentration single and combination of the two cyanotoxins (0.5 to 20 µg·L-1) increased lettuce growth whereas higher concentration (100 µg·L-1) decreased lettuce growth (P < 0.05). In addition, the increase or decrease in growth caused by the combination of two cyanotoxins was lower than that caused by single microcystins or anatoxins, showing antagonistic effect. The antagonistic effect was also shown on levels of indoleacetic acid and abscisic acid, mainly resulted from regulating activities of tryptophan decarboxylase, tryptophan aminotransferase and 9-cis-epoxycarotenoid dioxygenase in synthesis pathways and activities of indoleacetic acid oxidase and ABA 8'-hydroxylase in catabolism pathways. Meanwhile, it was also related to keeping the balance of conjugates including indoleacetic acid-aspartate and abscisic acid glucose ester. Moreover, lettuce treated with single microcystins or combination at moderate and high concentrations (20, 50 and 100 µg·L-1) had potential health risk although the coexistence of two cyanotoxins could lower their accumulation in lettuce. Therefore, the ecological risk assessment of the two cyanotoxins should consider agricultural productivity and food safety simultaneously.