Enhanced Growth of Acidocella facilis and Related Acidophilic Bacteria at High Concentrations of Aluminum

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Acidophilic chemoorganotrophic bacteria of the genus Acidocella, Acidiphilium, and Acidobacterium were tolerant to concentrations of monomeric aluminum ion (Al³⁺) of up to 100 mM in an acid medium of pH 3.5 or less. Furthermore, their growth was remarkably enhanced by the presence of aluminum. The maximum amount of Al absorbed was 2.02 mg·g dry cell wt^-1 for Acidocella facilis. Aluminum phosphate as well as aluminum sulfate among aluminum chemicals tested provided for the most luxuriant growth of this bacterium, whereas aluminum acetate and aluminum lactate had inhibitory effects on growth. The enhanced growth seemed to be caused by a chemical buffering action of the aluminum, but could not be explained merely by an alleviation of H⁺ stress or by the scavenging of growth-inhibitory organic acids excreted. The reasons why the acidophiles are genuinely aluminophilic as well as aluminotolerant still remain to be elucidated. Considering that monomeric Al³⁺ is dominant under strongly acid conditions, the possible cultivation of acidophiles at a pH below 4 affords a useful in vitro model system to probe the aluminum toxicity of plants as well as aluminum-microorganism interactions.