Influence of aluminum and pH on the physiological ecology and cellular morphology of the acidophilic diatom Asterionella ralfsii var. Americana.

Abstract

The biological effects of Al and pH were examined on a clone of the acidophilic diatom Asterionella ralfsii var. americana Korn. To assess predictions of in vitro Al bioavailability, the pH-dependent Al chemistry of the algal growth medium FRAQUIL was compared to output from the chemical speciation model MINEQL. Overall relationships between analytical measurements and predictions of dissolved Al concentrations suggested that as in natural waters, some form of Al(OH)$\\sb3$ controls the solubility of Al in vitro. Models based on the solubility of individual forms of Al(OH)$\\sb3$ and AlPO$\\sb4$, however, were not consistently the best predictors of dissolved Al concentrations, particularly at high Al concentrations. Therefore, analytical measurements of dissolved Al were chosen as input parameters for chemical speciation models. Growth rates of A. ralfsii in batch culture were significantly reduced above 15 $\\mu$mol $\\cdot$ L$\\sp{-1}$ total Al at both pH 5 and 6. Aluminum additions were predicted to increase ion activities of Al$\\sp{+3}$, Fe$\\sp{+3}$, and Cu$\\sp{+2}$ at pH 5 and 6, therefore all three were significantly correlated to growth rate reductions. Independent manipulations of Fe and Cu eliminated the possibility that Al was indirectly increasing either Fe$\\sp{+3}$ or Cu$\\sp{+2}$ free ion activities to toxic levels. Aluminum toxicity was strongly pH-dependent with toxicity being greater per unit dissolved Al concentration at pH 6 than at pH 5. In Si-limited semicontinuous cultures, Al decreased mean cell size, area, and biovolume of A. ralfsii up to 40-50% at pH 6. The effects of Al were dependent on growth rate with the magnitude of size reduction increasing at higher growth rates. Aluminum had a significant impact on the relationship between Si cell quota and steady-state growth rates, with cells becoming 30-40% more heavily silicified at higher concentrations. Although the mechanisms for these size reductions are unclear, the relationship between metal concentration and frustule morphology may be useful as an indicator of metal loading to acidified lakes.