revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 404
5. POTENTIAL FOR HUMAN EXPOSURE
concentrations of both the inorganic and methylated mercury in phytoplankton. However, differences in
partitioning within phytoplankton cells between inorganic mercury (which is principally membrane-
bound) and methylmercury (which accumulated in the cytoplasm) lead to a greater assimilation of
methylmercury during zooplankton grazing.
Most of the discrimination between inorganic and methylmercury thus occurs during trophic transfer,
while the major enrichment factor is between water and the phytoplankton. This also has been reported for
the diatom Thalassiosura weissflogii in a marine food chain (Mason et al. 1996). Methylmercury was
accumulated in the cell cytoplasm, and its assimilation by copepods was 4 times more efficient than the
assimilation of inorganic mercury. Bioaccumulation has been demonstrated for predator fish in both
freshwater and marine systems and in marine mammals (see Section 5.4.4). Bioaccumulation of
methylmercury in aquatic food chains is of interest, because it is generally the most important source of
nonoccupational human exposure to this compound (EPA 1984b; WHO 1990, 1991).
Aquatic macrophytes have been found to bioconcentrate methylmercury in almost direct proportion to the
mercury concentration in the water (Ribeyre et al. 1991). Mortimer (1985) reported bioconcentration
factors (BCFs) for several species of submerged aquatic plants exposed to inorganic mercury in laboratory
aquaria of 3,300, 1.3, 0.9, and 1.3 for Utricularia, Ceratophyllum, Najas, and Nitella, respectively. The
concentrations factor used by this author was based on µg g -1 dry weight in the plant/µg mL-1 water day -1 .
The potential for bioaccumulation in terrestrial food chains is demonstrated by the uptake of mercury by
the edible mushroom Pleurotus ostreatus, grown on compost containing mercury at concentrations of up to
0.2 mg/kg (ppm). The bioaccumulation factors reported ranged from 65 to 140, indicating that there are
potential risks to human health if these mushrooms are eaten in large quantities (Bressa et al. 1988).
Elevated concentrations of mercury in 149 samples of mushrooms representing 11 different species were
reported by Kalcac et al. (1991). These authors collected mushrooms within 6 km of a lead smelter in
Czechoslovakia in operation since 1786. Mercury was accumulated by Lepista nuda and Lepiota rhacodes
at 11.9 mg/kg (ppm) and 6.5 mg/kg (ppm) (dry weight), respectively. The mean concentration of other
species ranged from 0.3 to 2.4 mg/kg (ppm). Concentrations of mercury in most of the mushroom species
collected in that location were higher than in mushrooms collected in other parts of the country.
Data from higher plants indicate that virtually no mercury is taken up from the soil into the shoots of plants
such as peas, although mercury concentrations in the roots may be significantly elevated and reflect the