Double-dose responses of Scenedesmus capricornus microalgae exposed to humic acid

Dissolved organic matter (DOM) has been found to attenuate the ecotoxicity of various environmental pollutants, but research on its own toxic effects in aquatic ecosystems has been very limited. Herein, the toxic effects of humic acid (HA), a represent DOM typically found in natural waters, on the freshwater alga Scenedesmus capricornus were investigated. As result, HA exerted a double-dose effect on the growth of Scenedesmus capricornus. At HA concentrations below 2.0 mgC/L, the growth of Scenedesmus capricornus was slightly promoted, as was the synthesis of chlorophyll and macromolecules in the algae. Moreover, S. capricornus can maintain its growth by secreting fulvic acid as a nutrient carbon source. However, the growth of Scenedesmus capricornus was significantly inhibited when HA was beyond 2.0 mgC/L. The main mechanisms of humic acid's toxicity were membrane damage and oxidative stress. Particularly, when the oxidative stress exceeds the algae's carrying capacity, the synthesis of EPS is greatly inhibited and HA damage results. Taken together, DOM may have both positive and negative effects on aquatic ecosystems.