RESUMO
Water hardness is known to control the uptake and toxicity of zinc and other metals through either chemical competition, biological acclimation, or both processes. The dominant process controlling zinc uptake has not been previously elucidated, nor has the effect of calcium been investigated independently of other cations. We determined zinc uptake by rainbow trout acclimated and exposed to four calcium treatments: (1) low calcium (6.5 mg Ca/l; 160 µM) acclimation and low calcium exposure (LL); (2) low acclimation and high calcium (131 mg Ca/l; 3300 µM) exposure (LH); (3) high acclimation and low exposure (HL); and (4) high acclimation and high exposure (HH). Trout were exposed to sublethal zinc (100 µg 65Zn/l; 1.5 µM) for 24 h, and whole body and gill 65Zn levels were determined. Zinc uptake was approximately linear during the 24 h exposure period and uptake was calcium dependent. 65Zn uptake by trout was 22 µg/kg per h in the LL treatment and 4.5 µg/kg per h in the HH treatment. 65Zn uptake by trout in the LH and HL treatments was 13 and 10 µg/kg per h, respectively. 65Zn uptake in gills was also significantly reduced by calcium acclimation or exposure. Trout in the LH and HH treatments had five fold lower 65Zn concentrations than LL treatment fish. The results of this study demonstrate that calcium reduces zinc uptake through both biological acclimation and chemical processes, and that the protective effects of calcium are approximately additive.
RESUMO
The toxicity of a water-accommodated fraction (WAF) prepared from weathered oil was assessed in a 7-day static renewal test with Mysidopsis bahia. Weathered oil was collected from the 5x monitoring well at the Guadalupe oil field. Solar ultraviolet and visible light intensities were measured in various habitats in the vicinity of the weathered oil sample collection site, and the resultant measurements were used to produce laboratory light treatments that were representative of the on-site quality and intensity of natural solar radiation. Each of five WAF dilutions and a control without WAF was tested under three different simulated solar radiation intensities. During the test, survival and growth of the mysids, irradiance, and total petroleum hydrocarbon (TPH) concentrations in the test treatments were measured. Significant increases (P
RESUMO
We evaluated the potential cancer risk to adults from ingesting polychlorinated biphenyls (PCBs) in fish and shellfish using an equilibrium partitioning model of PCB bioaccumulation in the aquatic animal. Estimated potential cancer risk to humans increased exponentially with increasing hydrophobicity of the PCB. However, the addition of food-chain sources of PCBs was necessary to cause potential cancer risk to exceed 10(-6). Environmental degradation of the PCB reduced cancer risk by reducing the exposure concentration; 3.3 degradation half-lives were required to reduce cancer risk estimates by one order of magnitude. PCB biotransformation to nongenotoxic metabolites (no increase in the cancer slope factor) by the aquatic animal reduced cancer risk by reducing the steady-state concentration of PCBs in the edible tissue. Even relatively slow biotransformation (e.g., metabolic half-life of 100 days) reduced cancer risk estimates under the default model conditions. Nonequilibrium conditions, such as limited exposure time, reduced potential cancer risk by reducing contaminant concentrations in the aquatic animal. Risk assessment using toxic equivalency factors predicted substantially greater potential risk for specific congeners than for PCB mixtures. Our evaluation demonstrates that deviation from conventional assumptions used in risk assessment (e.g., negligible biotransformation and degradation; steady-state equilibrium) can significantly affect cancer risk estimates.
