Oxidative stress responses in the animal model, Daphnia pulex exposed to a natural bloom extract versus artificial cyanotoxin mixtures

• Similar D. pulex survival percentages were observed in exposure to crude cyanobacterial extract (CE) and artificial toxin mixtures.
• Increased lipid peroxidation was found with exposure to CE, longer recovery time was needed for CE than for the artificial toxin mixtures.
• Increased H2O2 level was seen with all three mixtures after 24 h, but it took longer to decrease back to original level in CE than other exposures.
• Enzyme inhibition was stronger with CE than artificial mixtures, and inhibition lasted for a longer period of time.

In the natural environment, Daphnia spp. are constantly exposed to a complex matrix of biomolecules, especially during cyanobacterial bloom events. When cyanobacterial cells decay, not only are toxic secondary metabolites known as cyanotoxins released, but also multiple other secondary metabolites, some of which act as enzyme inhibitors. The present study examined the effects of such a natural toxin matrix (crude extract from a bloom) versus artificial toxin mixtures in terms of oxidative stress in Daphnia pulex. The results indicate that there is no significant effect on the survival of D. pulex. However, exposure to the bloom extract resulted in increased lipid peroxidation over a shorter exposure period and reduced antioxidative enzyme activities when compared to the artificial mixtures. The daphnids also needed a longer recovery time to reduce the increased cellular hydrogen peroxide concentration associated with the exposure to the crude extract than with the artificial mixtures. The results indicate a significant difference between the bloom crude extract and the two synthetic mixtures for all stress markers tested, indicating enhanced toxicity of the bloom extract.