Evidence towards a higher genotoxicity of inorganic mercury in fish (Diplodus sargus) comparing to the organic form - a paradigm shift?

The genotoxicity of mercury in fish have been scarcely examined and most of the studies were focused on the effects of methylmercury (MeHg), while other Hg forms were not investigated yet. The preponderance of studies on MeHg... [ view full abstract ]

The genotoxicity of mercury in fish have been scarcely examined and most of the studies were focused on the effects of methylmercury (MeHg), while other Hg forms were not investigated yet. The preponderance of studies on MeHg is probably due to the assumption of its higher toxicity based on findings in humans and other mammals, mostly exposed to MeHg through food consumption, as well as to its higher propensity for bioaccumulation. However, fish exposure to inorganic Hg (iHg) is relevant considering that it represents the main Hg form in the water compartment. While some studies pointed out the higher toxicity of MeHg in fish relatively to iHg, recent findings advocated the opposite. The current research was designed under this lack of scientific consensus and it aims to clarify the genotoxicity of dietary MeHg and waterborne iHg in fish by covering Hg levels in the blood and the occurrence of erythrocytic nuclear abnormalities (ENA). Concomitantly, the erythrocytic maturity index (EMI) was assessed to detect disturbances on the haematological dynamics. This is a time-course experiment that incorporates 4 exposure (1, 3, 7 and 14 days) and 2 post-exposure (14 and 28 days without Hg exposure) periods, in which juveniles of white seabream (Diplodus sargus) were exposed to waterborne Hg²⁺ (2 µg L^-1) and dietary MeHg (8.7 µg g^-1), corresponding to a similar daily intake rate (around 30 µg Hg/day/fish). The incorporation of MeHg in the blood (day 1) was faster than that recorded for iHg (day 3). After that, time-course of MeHg and iHg incorporation in the blood was identical over time, with significantly higher levels in exposed fish than in control at all exposure and post-exposure time-points. MeHg in the blood was about 2-fold higher than iHg, indicating the highly efficient transport of the former in fish body. Levels of MeHg and iHg showed a tendency to decrease in the blood over time, as expressed by the relationships between accumulation and time (m = −0.07 μg g⁻¹/day and −0.04 μg g⁻¹/day, respectively), denoting an elimination rate slightly higher for MeHg. Moreover, fish exposure to iHg was on the basis of a significant increase of total ENA frequency at all experimental times (including exposure and post-exposure periods), unveiling a genotoxic impact. Total ENA frequency decreased significantly between the end of the exposure period (day 14) to iHg and 14 days after exposure, following the decrease of levels in the blood. Moreover, EMI data denoted alterations on the haematological dynamics in iHg-exposed fish, translated into an erythropoiesis increment. Contrastingly, MeHg exposure was not on the basis of an enhancement of the total ENA frequency for any of the experimental times, suggesting the lack of genotoxic effects. To conclude, waterborne iHg displayed a higher potential to induce chromosomal damage as compared to dietary MeHg, according to the occurrence ENA. In order to prevent an underestimation of risk for wild fish populations, both Hg forms need to be considered in the design of environmental health assessment plans. Current findings should be taking into out in future plans of environmental and biodiversity conservation, as well as for the safeguard of human health.

Keywords: Inorganic mercury; Methylmercury; Genotoxicity; Haematological dynamics; Fish