New insights into mercury bioaccumulation in deep-sea organisms from the NW Mediterranean and their human health implications

A number of studies have found high levels of mercury (Hg) in deep-sea organisms throughout the world's oceans, but the underlying causes are not clear as there is no consensus on the origin and cycling of Hg in the ocean. Recent findings suggested that Hg accumulation may increase with increasing forage depth and pointed to the deep-water column as the origin of most Hg in marine biota, especially its organic methylmercury (MeHg) form. In the present study, we determined the total mercury (THg) levels in 12 deep-sea fish species and a decapod crustacean and investigated their relationship with the species' nitrogen stable isotope ratio (δ15N) as an indicator of their trophic level, average weight and habitat depth. THg levels ranged from 0.27 to 4.42 μg/g w.w. and exceeded in all, except one species, the recommended 0.5 μg/g w.w. guideline value. While THg levels exhibited a strong relationship with δ15N values and to a lesser extent with weight, the habitat depth, characterized as the species' depth of maximum abundance (DMA), had also a significant effect on Hg accumulation. The fish species with a shallower depth range exhibited lower THg values than predicted by their trophic level (δ15N) and body mass, while measured THg values were higher than predicted in deeper-dwelling fish. Overall, the present results point out a potential risk for human health from the consumption of deep-sea fish. In particular, for both, the red shrimp Aristeus antennatus, which is one of the most valuable fishing resources of the Mediterranean, as well as the commercially exploited fish Mora moro, THg levels considerably exceeded the recommended 0.5 μg/g w.w. limit and should be consumed with caution.

► High total mercury (THg) levels were detected in Mediterranean deep-sea organisms.
► Uniform contamination pattern was observed across the Mediterranean basin.
► All except one species exceeded recommended consumption limit of 0.5 μg/g w.w.
► THg content increases with species' habitat depth.
► Recommendations are made for the consumption of the red shrimp A. antennatus.