Role of bacterial siderophores in dissolution of hornblende

Hornblende, a common mineral in granitic soils, may act as a source for a v ariety of metals needed by bacterial species for enzyme function (e.g., Fe, Zn, Mn, Cu, Co, Mo, V, Ni). A species of the bacterial genus Streptomyces was cultured from an Adirondack soil and isolated because of its ability to grow robustly in low Fe medium with hornblende present. Studies with unbuf fered culture medium, to discover whether Streptomyces sp. cultures affecte d solution pH, showed a decrease of 2.0 pH units in 21 d, then an increase of 3.0 pH units at 56 d. Cells that adhered to the hornblende surface at 56 days were difficult to remove, presumably because of mycelial growth deep into pits and cracks. Decreases and increases in pH may have been due to pr oduction of organic acids and ammonia respectively. Increases in pH could a lso have been related to release of components during death of organisms. I n a buffered medium, Streptomyces sp. increased the initial Fe release rate from hornblende approximately fivefold over that of an abiotic control. A catechol derivative, produced by the Streptomyces sp. and characterized by chromatography and mass spectrometry, is presumed to cause this Fe release enhancement. Hornblende dissolution was also analyzed in the presence of a commercially available hydroxamate siderophore, desferrioxamine mesylate (D FAM). DFAM is the methane sulfonate form of one of many siderophores known to be a product of streptomycetes. The rate of Fe release obtained when inc ubating the hornblende with 24 mu m of DFAM was similar to the rate observe d in the presence of the Streptomyces sp. isolate. Higher concentrations of DFAM increased the dissolution rate nonlinearly, described by the rate equ ation R = (7.6 x 10(-13))C-0.47, where R is the release rate of Fe (mol/m(2 )s), and C is the concentration (mol/l) of DFAM. The DFAM also increased re lease of Al and Si from hornblende into solution; however, these release ra tes were not increased by addition of the Streptomyces sp. alone. Preferent ial release of Al and Si in the presence of DFAM, but not in the presence o f bacteria alone, may be related to the difference in selectivity of catech ol vs, hydroxamate siderophores. Addition of Streptomyces sp. in the presen ce of DFAM at three concentrations consistently enhanced Fe release approxi mately two to threefold the rate with siderophore alone. Recycling of sider ophore molecules or enhanced production of one siderophore by microorganism s in the presence of other siderophores makes it difficult to predict a pri ori release rates when both siderophore and bacteria are present, as would be the case in natural soils. Copyright (C) 1999 Elsevier Science Ltd.