Rock weathering in deserts: Mobilization and concentration of ferric iron by microorganisms

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Abstract
Samples of rock coatings from arid and semi‐arid areas in the continental United States, Hawaii, and Australia were found to contain biochemical compounds (siderophores) that are produced by microorganisms to solubilize otherwise insoluble Fe(III) oxides and oxyhydroxides. Pure cultures of most bacteria and microcolonial fungi isolated from the same coatings produced siderophores (catechols and/or hy‐droxamates) in the laboratory. Isolates of common bacteria from the rock coatings concentrated iron oxides/oxyhydroxides on the exteriors of cell walls when exposed to iron‐bearing solutions, including siderophores. These results suggest a mechanism whereby siderophores mobilize ferric iron from source materials in deserts such as wind‐deposited dust under aerobic and alkaline to neutral pH conditions, and the iron extracted in solution by siderophores is concentrated by precipitation onto bacterial cell walls. Upon death and lysis of the microorganisms the iron oxyhydroxides become attached to the rock or grain substrata in the form of coatings. It is suggested that the biochemical activity of microorganisms may control the iron content of such deposits as clay‐rich desert varnish, iron‐stained siliceous crusts, and grain coatings in desert soils.