Effect of humic acid on copper uptake and metallothionein induction in Daphnia magna

Daphnia magna was used to study the influence of humic acid (HA) on the biological effect of copper (Cu) in water. Cu species in reconstituted water with different HA concentrations was calculated using MINTEQA2. The lethal concentration 50 (LC50) was predicted by the biotic ligand model (BLM). Mortality, Cu bioaccumulation, and metallothionein (MT) in D, magna exposed to Cu and HA for 3 d were measured. All indicators were proven using LC50. As the HA concentration increased from 0 mg/L to 6 mg/L, the content of free Cu ²⁺ decreased and the content of Cu and HA complexes (Cu-HA) increased. Mortality, Cu bioaccumulation, and MT decreased significantly, indicating that the existence of HA in the water reduced the uptake and toxicity of Cu. The uptake of Cu-HA complexes was significantly lower than that of free Cu ions. Their mechanisms of bioavailability were different because the Cu-HA complexes were not involved in the synthesis of MT in vivo.