Alleviation of aluminum toxicity to Rhizobium leguminosarum bv. viciae by the hydroxamate siderophore vicibactin

Acid rain solubilises aluminum which can exert toxic effects on soil bacteria. The root nodule bacterium Rhizobium leguminosarum biovar viciae synthesises the hydroxamate siderophore vicibactin in response to iron limitation. We report the effect of vicibactin on the toxicity of aluminum(III) to R. leguminosarum and kinetic studies on the reaction of vicibactin with Al(III) and Fe(III). Aluminum (added as the nitrate) completely inhibited bacterial growth at 25 μM final concentration, whereas the preformed Al-vicibactin complex had no effect. When aluminum and vicibactin solutions were added separately to growing cultures, growth was partly inhibited at 25 μM final concentration of each, but fully inhibited at 50 μM final concentration of each. Growth was not inhibited at 50 μM Al and 100 μM vicibactin, probably reflecting the slow reaction between Al and vicibactin; this results in some aluminum remaining uncomplexed long enough to exert toxic effects on growth, partly at 25 μM Al and vicibactin and fully at 50 μM Al and vicibactin. At 100 μM vicibactin and 50 μM Al, Al was complexed more effectively and there was no toxic effect. It was anticipated that vicibactin might enhance the toxicity of Al by transporting it into the cell, but the Al-vicibactin complex was not toxic. Several explanations are possible: the Al-vicibactin complex is not taken up by the cell; the complex is taken up but Al is not released from vicibactin; Al is released in the cell but is precipitated immediately. However, vicibactin reduces the toxicity of Al by complexing it outside the cell.