In vivo Visualization of the Process of Hg2+ Bioaccumulation in Water Flea Daphnia carinata by a Novel Aggregation-Induced Emission Fluorogen

This paper employs a specially designed aggregation-induced emission fluorogen (AIEgen) to in vivo visualize the process of Hg ²⁺ bioaccumulation in a common species of freshwater zooplankton Daphnia carinata (D. carinata) by two methods, direct Hg ²⁺ absorption and ingestion of Hg ²⁺ contaminated food. We analysed the relevance between photoluminescence (PL, I ₅₉₅ /I ₄₈₀ ) ratios and Hg ²⁺ (C _Hg2+ ) and developed the master curve for Hg ²⁺ determination based on measuring the PL intensity of the solution. Meanwhile, fluorescent image analysis showed that the major recipient organs of Hg ²⁺ in D. carinata were the compound eyes and carapace, followed by the intestine and shell gland, but not the brain or heart. The response of D. carinata to Hg ²⁺ via uptake from surrounding water differed distinctly from that through food intake of the algae (Euglena gracilis) contaminated by mercury. When Hg ²⁺ was encapsulated by algae membrane, no fluorescence was detected, and the carapace morphology remained intact after ingesting algae for 80 min, in contrast to the rapid carapace deformation by direct Hg ²⁺ absorption. The D. carinata showed higher mortality by direct Hg ²⁺ immersion than via food ingestion. The reason for the high mortality after Hg ²⁺ immersion was possibly due to carapace deformity after chemical reaction between chitin and mercury, but the biochemical pathway leading to morality needs further study.