Methylmercury bioaccumulation in water flea Daphnia carinata by AIEgen

Mercury ion (Hg²⁺) is a toxic heavy metal ion and Hg²⁺ is convertible to methylmercury (MeHg) by many aquatic microorganisms, leading to bioaccumulation and biomagnification in aquatic organisms, which can interfere with brain development and function in humans. This study employs a newly developed AIEgen (Aggregation-induced emission fluorogen) to quantify and visualise the process of MeHg bioaccumulation in vivo on the species of water flea Daphnia carinata. Two approaches to MeHg absorption were taken, either by direct incubation in a MeHg solution or by indirect consumption of algae contaminated with MeHg. We analysed the relationship between the ratio of photoluminescence (PL) intensities (I₅₈₅/I₄₈₀) and MeHg concentration (C_MeHg) and generated a master curve for determining MeHg concentration based on the measurement of PL intensities. Fluorescent image analysis showed the occurrence of MeHg in D. carinata to be mainly in the compound eyes, optic nerve and carapace. This study indicates that MeHg absorption can be quantified and visualised in the body of zooplankton, and the MeHg transfer to zooplankton is more likely through direct exposure than via indirect food intake. The accumulation of MeHg in the eye and the nervous system could be the cause of the high mortality of D. carinata exposed to MeHg in water.