Uptake of trace metals and rare earth elements from hornblende by a soil bacterium

Analysis of trace elements released from hornblende between pH 6.5 and 7.5 in the presence of Arthrobacter sp. shows that Fe, Ni, V, Mn, and, to a les ser extent, Co are preferentially released into solution relative to bacter ia-free experiments. This enhanced release into solution could be due to co ntributions from the slightly lowered pH, the presence of low molecular wei ght organic acids (LMWOAs), or the presence of a catecholate siderophore in experiments with bacteria. The best explanation for enhanced metal release is siderophore complexation at the mineral surface followed by release to solution. However, the relative rates of metal release to solution in these experiments do not strictly follow the trend predicted by the relative ord ering of metal hydrolysis, which might be predicted for siderophore-promote d dissolution. For some of these metals, release to solution is fast initia lly in biotic experiments, but concentrations in solution reach a steady st ate value or decrease with time as the bacteria cell numbers increase expon entially. Lack of enhanced release to solution for some metals and decrease s in release rate with time for others may be explained by uptake into bact eria. Many of the metals predicted to strongly complex with siderophore (in cluding Al, Ti, Fe, Cu) are heavily taken up into cellular material. The re lative ordering of organic ligand-element complexation may therefore partia lly explain the relative ordering of uptake of trace metals and rare earth elements into cell material. Fractionation of heavy rare earth elements tak en up into cellular material is also very strong, and increases from Ho to Lu. Strong fractionation in uptake of some elements by bacteria may create biological signatures either in the mineral substrate or in any mineral pre cipitates associated with the cellular material.