ROCK WEATHERING IN DESERTS - MOBILIZATION AND CONCENTRATION OF FERRICIRON BY MICROORGANISMS

Samples of rock coatings from arid and semi-arid areas in the continen tal United States, Hawaii, and Australia were found to contain biochem ical compounds (siderophores) that are produced by microorganisms to s olubilize otherwise insoluble Fe(III) oxides and oxyhydroxides. Pure c ultures of most bacteria and microcolonial fungi isolated from the sam e coatings produced siderophores (catechols and/or hydroxamates) in th e laboratory. Isolates of common bacteria from the rock coatings conce ntrated iron oxides/oxyhydroxides on the exteriors of cell walls when exposed to iron-bearing solutions, including siderophores. These resul ts 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 solut ion by siderophores is concentrated by precipitation onto bacterial ce ll walls. Upon death and lysis of the microorganisms the iron oxyhydro xides become attached to the rock or grain substrata in the form of co atings. It is suggested that the biochemical activity of microorganism s may control the iron content of such deposits as clay-rich desert va rnish, iron-stained siliceous crusts, and grain coatings in desert soi ls.