Bioaccumulation and toxicity of hexachlorobenzene in Chlorella vulgaris and Daphnia magna

Abstract

Algae play a major role in the accumulation of hydrophobic pollutants. The accumulation of hexachlorobenzene (HCB) in algae is very rapid, reaching equilibrium after a few hours. The elimination of HCB from algae has been described by a two-compartment nonequilibrium model. This paper describes the elimination kinetics of HCB from the second (slow rate) compartment in Chlorella vulgaris, and the bioavailability of the HCB bound to algae on Daphnia magna. First-order elimination kinetics was observed. The elimination rate constant and the half life were 0.43 day −1 and 1.6 days, respectively. The algal growth curve showed a growth rate constant of 0.27 day −1 and a duplication time of 2.56 days. Comparisons among these values indicate that the dilution effect associated with algal growth accounts for more than 60% of the observed reduction. Daphnia magna fed for 6 days with contaminated Chlorella, 4.4 μg HCB kg −1 dry weight, accumulated 1.7 μg HCB kg −1 dry weight. Feeding with HCB-contaminated Chlorella led to 49% and 41% reductions, respectively in the number and biomass of juvenile daphnids produced during the time span of the experiment. The HCB concentration in the juveniles was 0.28 μg HCB kg −1 dry weight. Calculations based on these results and published bioconcentration factors for Chlorella, 124000 on a dry-weight basis, indicated that Chlorella growing in an HCB concentration equal to the actual European Water Quality Objective for HCB (0.01 μg 1 −1), will accumulate HCB at a level within the same order of magnitude as that found toxic for Daphnia feeding on the algal population.