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 bacter ia. The root nodule bacterium Rhizobium leguminosarum biovar viciae synthes ises 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 bacteri al growth at 25 muM final concentration, whereas the preformed Al-vicibacti n complex had no effect. When aluminum and vicibactin solutions were added separately to growing cultures, growth was partly inhibited at 25 muM final concentration of each, but fully inhibited at 50 muM final concentration o f each. Growth was not inhibited at 50 muM Al and 100 muM vicibactin, proba bly 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 muM Al and vicibactin and fully at 50 muM Al and vicib actin. At 100 muM vicibactin and 50 muM Al, Al was complexed more effective ly 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-vic ibactin complex was not toxic. Several explanations are possible: the Al-vi cibactin complex is not taken up by the cell; the complex is taken up but A l is not released from vicibactin; Al is released in the cell but is precip itated immediately. However, vicibactin reduces the toxicity of Al by compl exing it outside the cell.