The effects of pH and aluminum on the growth of the acidophilic diatom Asterionella ralfsii var. americana

The effects of pH and Al on the acidophilic diatom Asterionella ralfsii var. americana Körn. 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 µ mol liter −1 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. ralfsii were significantly reduced above 15 µ mol liter −1 total Al at both pH 5 and 6. Al additions increased estimated free ion activities of Al 3+ , Fe 3+ , and Cu 2+ 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 3+ or Cu 2+ free ion activities to toxic levels. Relationships of growth rates to both inorganic monomeric Al (Al r ) and to estimates of pAl were strongly pH‐dependent 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 Al 3+ ions from binding to cell‐surface ligands. The impact of Al on natural phytoplankton populations therefore is likely to depend on a combination of pH‐dependent Al solubility, the protonation of cell‐surface ligands, and chelator‐mediated metal speciation.