Mercury Accumulation Pathways in a Model Marine Microalgae: Sorption, Uptake, and Partition Kinetics.

The accumulation of dissolved mercury (Hg) by phytoplankton is the largest concentration step along aquatic food chains. However, the cell uptake mechanisms remain unclear. In this study, the marine haptophyteTisochrysis lutea, a model phytoplankton species, was examined for its interactions with picomolar levels of dissolved inorganic divalent Hg (iHg) and monomethyl Hg (MMHg). For both these Hg species, the study observed their successive sorption and internalization over time, yielding Hg partition coefficients as well as sorption, uptake, and release rates. These results were integrated into a time-dependent, three-compartment model for marine cellular Hg accumulation that included exposure medium, phycosphere, and internalized mercury. Assuming equilibria and pseudo-first-order kinetics between compartments, this study obtained transfer rates of Hg between compartments. The results provide insight into the phycosphere as an intermediate compartment for Hg species accumulation and quantify its role in the internalization of Hg. Ultimately, the new model and its parametrization were successfully applied to literature data showing Hg cellular accumulation in different groups of marine phytoplankton, lending confidence in its robustness and potential contributions to help model the uptake of Hg in the aquatic food web.

Bioconcentration, bioaccumulation and biomagnification of mercury in plankton of the Mediterranean Sea.

Plankton plays a prominent role in the bioaccumulation of mercury (Hg). The MERITE-HIPPOCAMPE campaign was carried out in spring 2019 along a north-south transect including coastal and offshore areas of the Mediterranean Sea. Sampling of sea water and plankton by pumping and nets was carried out in the chlorophyll maximum layer. Two size-fractions of phytoplankton (0.7-2.7 and 2.7-20 µm) and five of zooplankton (between 60 and >2000 µm) were separated, and their total mercury (THg) and monomethylmercury (MMHg) contents were measured. Bioconcentration of THg was significantly higher in the smallest phytoplankton size-fraction dominated by Synechococcus spp. The bioaccumulation and biomagnification of MMHg in zooplankton was influenced by size, food sources, biochemical composition and trophic level. MMHg was biomagnified in the plankton food web, while THg decreased toward higher trophic levels. Higher MMHg concentrations were measured in oligotrophic areas. Plankton communities in the Southern Mediterranean Sea had lower MMHg concentrations than those in the Northern Mediterranean Sea. These results highlighted the influence of environmental conditions and trophodynamics on the transfer of Hg in Mediterranean plankton food webs, with implications for higher trophic level consumers.

Patterns of trace metal bioaccumulation and trophic transfer in a phytoplankton-zooplankton-small pelagic fish marine food web.

Trace metal contamination in the European sardine and anchovy food web was investigated in the Gulf of Lions, NW Mediterranean Sea, including seawater and size fractions of plankton. The results highlighted: i) higher and more variable concentrations in the smaller plankton size classes for all metals except cadmium; ii) higher concentrations in anchovy versus sardine for all elements except lead; iii) different patterns of metal bioaccumulation through the food web: cobalt, nickel, copper, silver, lead and zinc displayed continuously decreasing concentrations (with the exception of increased zinc in fish only), while mercury concentrations dropped considerably in larger plankton size classes and rose significantly in fish. Lastly, cadmium concentrations were found to be highest in intermediate plankton size classes, with very low levels in fish. The need to efficiently characterize the biological composition of plankton in order to fully identify its role in the mobilization and transfer of metals was highlighted.

Oligotrophy as a major driver of mercury bioaccumulation in medium-to high-trophic level consumers: A marine ecosystem-comparative study.

Mercury (Hg) is a global contaminant of environmental concern. Numerous factors influencing its bioaccumulation in marine organisms have already been described at both individual and species levels (e.g., size or age, habitat, trophic level). However, few studies have compared the trophic characteristics of ecosystems to explain underlying mechanisms of differences in Hg bioaccumulation and biomagnification among food webs and systems. The present study aimed at investigating the potential primary role of the trophic status of systems on Hg bioaccumulation and biomagnification in temperate marine food webs, as shown by their medium-to high-trophic level consumers. It used data from samples collected at the shelf-edge (i.e. offshore organisms) in two contrasted ecosystems: the Bay of Biscay in the North-East Atlantic Ocean and the Gulf of Lion in the North-West Mediterranean Sea. Seven species including crustaceans, sharks and teleost fish, previously analysed for their total mercury (T-Hg) concentrations and their stable carbon and nitrogen isotope compositions, were considered for a meta-analysis. In addition, methylated mercury forms (or methyl-mercury, Me-Hg) were analysed. Mediterranean organisms presented systematically lower sizes than Atlantic ones, and lower δ13C and δ15N values, the latter values especially highlighting the more oligotrophic character of Mediterranean waters. Mediterranean individuals also showed significantly higher T-Hg and Me-Hg concentrations. Conversely, Me-Hg/T-Hg ratios were higher than 85% for all species, and quite similar between systems. Finally, the biomagnification power of Hg was different between systems when considering T-Hg, but not when considering Me-Hg, and was not different between the Hg forms within a given system. Overall, the different parameters showed the crucial role of the low primary productivity and its effects rippling through the compared ecosystems in the higher Hg bioaccumulation seen in organisms from oligotrophic Mediterranean waters.

Chemical contaminants (trace metals, persistent organic pollutants) in albacore tuna from western Indian and south-eastern Atlantic Oceans: Trophic influence and potential as tracers of populations.

Albacore tuna (Thunnus alalunga) is a highly commercial fish species harvested in the world's Oceans. Identifying the potential links between populations is one of the key tools that can improve the current management across fisheries areas. In addition to characterising populations' contamination state, chemical compounds can help refine foraging areas, individual flows and populations' structure, especially when combined with other intrinsic biogeochemical (trophic) markers such as carbon and nitrogen stable isotopes. This study investigated the bioaccumulation of seven selected trace metals - chromium, nickel, copper (Cu), zinc (Zn), cadmium (Cd), mercury (Hg) and lead - in the muscle of 443 albacore tunas, collected over two seasons and/or years in the western Indian Ocean (WIO: Reunion Island and Seychelles) and in the south-eastern Atlantic Ocean (SEAO: South Africa). The main factor that explained metal concentration variability was the geographic origin of fish, rather than the size and the sex of individuals, or the season/year of sampling. The elements Cu, Zn, Cd and Hg indicated a segregation of the geographic groups most clearly. For similar sized-individuals, tunas from SEAO had significantly higher concentrations in Cu, Zn and Cd, but lower Hg concentrations than those from WIO. Information inferred from the analysis of trophic markers (δ13C, δ15N) and selected persistent organic pollutants, as well as information on stomach contents, corroborated the geographical differences obtained by trace metals. It also highlighted the influence of trophic ecology on metal bioaccumulation. Finally, this study evidenced the potential of metals and chemical contaminants in general as tracers, by segregating groups of individuals using different food webs or habitats, to better understand spatial connectivity at the population scale. Limited flows of individuals between the SEAO and the WIO are suggested. Albacore as predatory fish also provided some information on environmental and food web chemical contamination in the different study areas.