Importance of Speciation in Understanding Mercury Bioaccumulation in Tilapia Controlled by Salinity and Dissolved Organic Matter

Abstract

We explored the roles of mercury speciation on the bioaccumulation (both aqueous and dietary uptake and elimination) of inorganic mercury (Hg[II]) and methylmercury (MeHg) in tilapia (Oreochromis niloticus) by controlling the mercury binding to inorganic and organic ligands. For the aqueous uptake, we showed that the uptake rates of Hg(II) were significantly higher at 0 psu compared with those at 10 psu and 28 psu. Based on the mercury-Cl complexes distribution, we found a positive relationship between the Hg(II) aqueous uptake rate and the abundance of neutral HgCl(2). Such relationship was further confirmed by the uptake experiments conducted over a lower salinity range (0-6 psu), suggesting that HgCl(2)(0) was the predominant species taken up by tilapia. In the presence of dissolved organic carbon (DOC) from different sources (Suwannee River and natural local waters), mercury uptake rates all decreased dramatically over a wide range of salinity, especially for Hg(II), indicating the overwhelming influence of DOC as opposed to the single effect of salinity. Using the mercury-Cl-DOC model, we demonstrated for the first time that the inhibition of DOC was dependent on the Cl(-), which was less significant at middle salinity level for both mercury forms. In contrast to the complex influence of water conditions on dissolved uptake, we found no significant influence of acclimated salinity on the dietary assimilation and elimination of both mercury species in tilapia. Our results demonstrated the importance of speciation in understanding the mercury bioaccumulation in various natural systems and its broad biogeochemical cycling.