Biodynamic understanding of mercury bioaccumulation in freshwater tilapia (Orechromis niloticus).

摘要

The bioaccumulation and biomagnification of mercury in fish has raised great concerns as it can cause high health risk to humans via fish consumption. Fish could accumulate inorganic [as Hg(II)] and organic methymercury (as MeHg) from both waterborne and dietborne sources. However the underlying mechanism of bioaccumulation has seldom been quantitatively studied. In this thesis, we tried to give a comprehensive description of mercury bioaccumulation in freshwater tilapia Orechromis niloticus, using biokinetic model and isotopic techniques (both stable and radio isotopes) as major tools. Firstly, the species-specific bioaccumulation process (including aqueous uptake, dietary assimilation and elimination) was clearly revealed. MeHg was the greater contributor to the overall mercury bioaccumulation due to its high bioavailability. Dietary exposure was the dominant accumulation pathway for both mercury species. Secondly, several biological and physic-chemical key factors on bioaccumulation have been identified. Naturally, the aqueous uptake was largely depending on the mercury speciation in water, which is commonly under the control of inorganic Hg-Cl and organic Hg-DOC complexes. Food conditions (quantity and quality) could directly influence the dieborne accumulation process, by affecting the Hg influx, fish growth as well as the biokinetic process of mercury (assimilation efficiency and efflux). Thirdly, physiological processes of fish (water pumping and respiration activities) exhibited great effects on MeHg aqueous uptake. Coupling relationships among MeHg, water and oxygen uptakes were observed, suggesting that MeHg uptake might be a rate-limiting and energy-involved process. Finally, the in vivo transformation and inter-organ transportation was explored in tilapia, together with biodynamic studies, it was then be able to fully understand why MeHg is the dominate form in fish. Biodynamic study showed its great power to explain mercury bioaccumulation process and the related ecotoxicological issues.