Combined toxicity of cyanotoxins and anthropogenic contaminants on Chlorella vulgaris: insights from single and binary mixture exposures.

Aquatic organisms in eutrophic brackish and freshwater systems are chronically exposed to natural and anthropogenic contaminants, including cyanotoxins, metals and pesticides, mainly driven by cyanobacterial blooms and human activities. However, the environmental risks posed by their combinations to the aquatic environment remain poorly understood. This study investigated the growth response of Chlorella vulgaris exposed to four single-chemical compounds (microcystin-LR, cylindrospermopsin, terbuthylazine, and cadmium) and five binary mixtures for 7 days. In single-chemical exposures, terbuthylazine was the most toxic to C. vulgaris, followed by cadmium, cylindrospermopsin and microcystin-LR. The obtained growth data were compared with concentration addition predictions based on individual toxicities using the MIXTOX tool, testing deviations for synergistic/antagonistic interactions, dose ratio and dose level dependencies. Mixture effects varied with composition and exposure time. Antagonism was the prevailing type of interaction between the chemical compounds after 4 days of exposure, while dose level dependency was the only deviation observed after 7 days. Regarding dose level dependency, synergism was observed at low doses of each chemical in three of the five tested binary mixtures. Time was an important factor in the change in the toxicity pattern. This synergistic effect poses a significant risk to aquatic organisms exposed simultaneously to cyanotoxins and anthropogenic contaminants in the environment.