Aluminum is the most abundant metal in the Earth's crust, but it is usually bound in insoluble oxides and silicates (Macdonald and Martin, 1988). Environmental acidification can increase levels of soluble aluminum (Al(III)), which is of concern as Al(III) is known to be toxic to many organisms (Williams, 1999). No biological role has been identified for aluminum, but aluminum can replace hydrogen with limited efficiency as an electron donor for methane production by methanogenic archaea (Belay and Daniels, 1990). Research into microbial interactions with aluminum has focused on corrosion of aluminum alloys, leaching of aluminum from materials, and aluminum accumulation, toxicity and detoxification.

Organic and inorganic acids produced by bacteria and fungi can mobilize aluminum from a variety of materials. Corrosion of aluminum alloys due to the growth of microorganisms was observed in aircraft fuel tanks in the 1960's (reviewed by Iverson, 1987), and remains a current area of research (Yang et al, 1998). Researchers have documented microbially-mediated aluminum leaching from the mineral spodumene (Karavaiko et al, 1980), red mud waste remaining after the alkaline extraction of alumina from bauxite ores (Vachon et al, 1994), incinerator fly ash (Brombacher et al, 1998), and electronic scrap (Brandl et al, 2000).

Phosphate has been shown to influence the mechanisms of aluminum tolerance in Pseudomonas fluorescens grown with aluminum-complexed citrate as a sole carbon source. In a phosphate-rich medium, P. fluorescens deposited aluminum in an insoluble extracellular residue composed partly of phosphatidylethanolamine (Appanna and St. Pierre, 1996). When phosphate was limiting, aluminum was found complexed with soluble extracellular metabolites (Appanna and St. Pierre, 1994). Iron concentration can determine which aluminum detoxification mechanism is used by P. fluorescens (Appanna and Hamel, 1996). Cellular phosphate concentration can also modulate aluminum toxicity in Bradyrhizobium japonicum (Mukherjee and Asanuma, 1998).

Aluminum binding and accumulation has been described in a variety of microorganisms. Isolated cell walls of Staphylococcus aureus were found to bind aluminum ions (Bradley and Parker, 1968), and Anabaena cylindrica was found to accumulate aluminum in phosphate granules in the cell wall (Pettersson et al, 1985). Cell surface and intracellular aluminum accumulation was observed in Eschericia coli (Guida et al, 1991). Aluminum uptake via hydroxamate siderophores can occur in the absence of iron in Bacillus megaterium (Hu and Boyer, 1996).

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Appanna VD, St. Pierre M. Influence of phosphate on aluminum tolerance in Pseudomonas fluorescens. FEMS Microbiol Lett. 1994 Dec;124(3):327-32.

Appanna VD, St. Pierre M. Aluminum elicits exocellular phosphatidylethanolamine production in Pseudomonas fluorescens. Appl Environ Microbiol. 1996 Aug;62(80):2778-82.

Brandl H, Bosshard R, Wegmann M. Computer-munching microbes: metal leaching from electronic scrap by bacteria and fungi. Hydrometallurgy. 2001 Feb;59(3):319-26.

Brombacher C, Bachofen R, Brandl H. Development of a laboratory-scale leaching plant for metal extraction from fly ash by Thiobacillus strains. Appl Environ Microbiol. 1998 Apr;64(4):1237-41.

Flis SE, Glenn AR, Dilworth MJ. The interaction between aluminium and root nodule bacteria. Soil Biol Biochem. 1993 Apr;25(1):403-17.

Hu X, Boyer GL. Siderophore-mediated aluminum uptake by Bacillus megaterium ATCC 19213. Appl Environ Microbiol. 1996 Nov;62(11):4044-48.

Iverson WP. Microbial corrosion of metals. Adv Appl Microbiol. 1987;32:1-36.

Mukherjee SK, Asanuma S. Possible role of cellular phosphate pool and subsequent accumulation of inorganic phosphate on the aluminum tolerance in Bradyrhizobium japonicum. Soil Biol Biochem. 1998 Oct;30(12):1511-16.

Pettersson A, Kunst L, Bergman B, Roomans GM. Accumulation of aluminium by Anabaena cylindrica into polyphosphate granules and cell walls: an X-ray energy-dispersive microanalysis study. J Gen Microbiol. 1985;131:2545-48.

Rogers NJ, Carson KC, Glenn AR, Dilworth MJ, Hughes MN, Poole RK. Alleviation of aluminum toxicity to Rhizobium leguminosarum bv. viciae by the hydroxamate siderophore vicibactin. Biometals. 2001 Mar;14(1):59-66.

Vachon P, Rajeshwar DT, Auclair J-C, Wilkinson KJ. Chemical and biological leaching of aluminum from red mud. Environ Sci Technol. 1994;28(1):26-30.