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The influence of a commercial humic acid (Aldrich; AHA) on the development of anthracene photoinduced toxicity to the duckweed Lemna gibba was examined using both room- and low-temperature (77°K) chlorophyll fluorescence assays. Plants were exposed to 2 mg liter -1 anthracene both with and without 6.2 mg liter -1 of AHA and grown under either visible light or simulated solar radiation that mimics the natural abundance of UV radiation. Exposure periods ranged from 1 to 48 h to examine temporal changes in chlorophyll degradation and chlorophyll a fluorescence induction in response to these light and HA treatments. The onset of anthracene photoinduced toxicity followed a definite sequence; chlorophyll a fluorescence induction parameters (F v /F m , and t 1 /2) responded earliest to anthracene exposure, with observable chlorophyll degradation requiring up to 24 to 48 h. Of these, t 1 /2 was the most sensitive, with significant inhibition apparent within I h of exposure. Throughout the entire 48-h exposure, 6.2 mg liter 1 AHA ameliorated the photoinduced toxicity of anthracene, in terms of both chlorophyll degradation and F v / F m inhibition. In contrast, AHA delayed the complete inhibition of t 1 /2 by only I to 24 h rather than permanently protecting the plants from anthracene damage to PS2. This suggests that AHA may slow, but not prevent, the entrance of either intact anthracene or its photooxidized byproducts under these exposure conditions.

The effects of pH and Al on the acidophilic diatom Asterionella ra[fsii var. americana K6m. were examined in axenic batch cultures. Experiments were performed under conditions of both high (pH 5) and low (pH 6) Al solubility over a range of concentrations from 0 to 30 firno liter-’ total Al. Growth rates were analyzed with respect to analytically determined Al concentrations and to predicted changes in dissolved metal ion speciation in response to Al additions. Growth rates of A. rufiii were significantly reduced above 15 pmol liter-’ total Al at both pH 5 and 6. Al additions increased estimated free ion activities of Al 3+, Fe3+, and Cu*+ through indirect chelator interactions at pH 5 and 6; therefore, all three were significantly correlated with growth rate reductions, Independent manipulations of total Fe and Cu, however, suggested that Al was not indirectly increasing either Fe’+ or Cu2+ free ion activities to toxic levels. Relationships of growth rates to both inorganic monomeric Al (A13 and to estimates of pA1 were strongly pHdependent with toxicity being greater per unit of dissolved Al concentration at pH 6 than at pH 5. These results are consistent qualitatively with predictions that H.’ ions can ameliorate dissolved metal toxicity by competitively excluding AP+ ions from binding to cell-surface ligands. The impact of Al on natural phytoplankton populations therefore is likely to depend on a combination of pHdependent Al solubility, the protonation of cell-surface ligands, and chelator-mediated metal speciation.