Mercury in the blood of free ranging pinnipeds: levels, sources of variation, toxicocinetic and potential impact using an in vitro model

Despite 30 years of international regulations, Hg levels in marine mammals have not decreased. Various environmental models even suggest a rise of mercury in the biota during the next decades, linked to climate change. The objective of this study is (1) to assess Hg levels in blood samples of free-ranging pinnipeds, (2) to understand level variation during different periods of life (gestation, lactation, fasting) and potential impact on lymphocytes using a preliminary in vitro model (peripheral blood mononuclear cells, PBMC). Blood samples were collected from harbour seals (Phoca vitulina n= 22) from the North Sea and from elephant seals from the Californian coast (Mirounga angustirostris 12 females and 12 pups). Harbour seal PBMC were isolated, set in medium culture and contaminated with methyl-Hg (1 µM). Biological tests and proteomic assay were realized on control and contaminated PBMC. Hg levels were from the same order of magnitude for the two species despite two different lifestyles and habitats: from 40 to 590 ng.g-1 fw in harbour seal and from 63 to 919 ng.g-1 fw in elephant seal. Hg concentrations in the blood depend upon several factors such as body weight, fasting and lactation duration for mothers and pups. After 21 days of lactation, female elephant seals doubled their blood Hg levels (from 308 to 593 ng.g-1fw) while a decrease is observed for pups. This increase is linked to mobilization from blubber and muscle during fasting associated to lactation. Cell model revealed an in vitro effect of Hg even at low concentration (1µM). Number of PBMC, viability, metabolic activity, DNA and RNA synthesis were reduced in vitro suggesting deleterious effects of Hg in concentrations encountered in free-ranging pinnipeds. Knowing that Hg methylation in the ocean is linked to temperature, one can wonder on Hg levels (and effects) in pinnipeds during the next decades.