Biological toxicity of fresh and rotten algae on freshwater fish: LC50, organ damage and antioxidant response.

In recent decades, harmful algal blooms (HABs) induced by eutrophication have caused organisms in freshwater ecosystems to become surrounded by toxic cells and dissolved toxins. In this study, the toxic effects of fresh algae solution (FAS) and rotten algae solution (RAS) were investigated. The results showed that the composition of RAS was predominantly organic acids, ketones, polypeptides, esters, phenols, amino acids and intermediate metabolic products. The safety concentrations (SCs) of FAS to Carassius auratus, Ctenopharyngodon idellus and Hypophthalmichthys molitrix were 1.92 × 1010 cells/L, 1.58 × 1011 cells/L and 1.30 × 1011 cells/L, respectively. The SCs of the RAS were significantly lower than those of the FAS (p < 0.05), with the values of 1.25 × 109 cells/L, 8.8 × 109 cells/L and 9.7 × 109 cells/L, for each species, respectively. The toxic algae solutions caused congestion inside the gills, intestinal lesions and high infection rates in the tested fish. FAS and RAS exposure also activated the antioxidant defense system and changed the intestinal microbial structure, resulting in the damage to the microbial balance in the body, and eventually the death of the fish. By studying the acute toxicity to fish, the harm of HABs to aquatic organisms can be predicted.