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

Abstract

This paper employs a specially designed aggregation-induced emission fluorogen (AIEgen) to in vivo visualize the process of Hg²⁺ bioaccumulation in a common species of freshwater zooplankton Daphnia carinata (D. carinata) by two methods, direct Hg²⁺ absorption and ingestion of Hg²⁺ contaminated food. We analysed the relevance between photoluminescence (PL, I₅₉₅ /I₄₈₀ ) ratios and Hg²⁺ (C_Hg2+ ) and developed the master curve for Hg²⁺ determination based on measuring the PL intensity of the solution. Meanwhile, fluorescent image analysis showed that the major recipient organs of Hg²⁺ 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 Hg²⁺ via uptake from surrounding water differed distinctly from that through food intake of the algae (Euglena gracilis) contaminated by mercury. When Hg²⁺ 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 Hg²⁺ absorption. The D. carinata showed higher mortality by direct Hg²⁺ immersion than via food ingestion. The reason for the high mortality after Hg²⁺ immersion was possibly due to carapace deformity after chemical reaction between chitin and mercury, but the biochemical pathway leading to morality needs further study.

Keywords: Daphnia carinata; aggregation-induced emission fluorogen; bioaccumulation; biological activity; mercury.