Experimental observations of the effects of bacteria on aluminosilicate weathering

Mineral dissolution experiments using batch cultures of soil and groundwate r bacteria were monitored with solution chemistry and various microscopic t echniques to determine the effects of these organisms on weathering reactio ns. Several strains of bacteria produced organic and inorganic acids and ex tracellular polymers in culture, increasing the release of cations from bio tite (Si, Fe, Al) and plagioclase feldspar (Si, Al) by up to two orders of magnitude compared to abiotic controls. Microbial colonies on mineral grain s were examined by cryo-scanning electron microscopy (cryo-SEM), confocal s canning laser microscopy (CSLM), and epifluorescence microscopy. Bacteria c olonized all mineral surfaces, often preferentially along cleavage steps an d edges of mineral grains. Low-voltage high-resolution cryo-SEM of high-pre ssure cryofixed and partially freeze-dried colonized minerals showed many b acteria attached by extracellular polymers of unknown composition. These bi ofilms covered much larger areas of the mineral surfaces than bacterial cel ls alone. Mineral surfaces where bacteria and extracellular polymers occurr ed appeared more extensively etched than surrounding uncolonized surfaces. CSLM was used to observe microbial colonization of biotite and to measure p H in microenvironments surrounding living microcolonies using a ratiometric pH-sensitive fluorescent dye set. A strain of bacteria (B0693 from the U.S . Department of Energy Subsurface Microbial Culture Collection) formed larg e attached microcolonies, both on the outer (001) surface and within interl ayer spaces as narrow as 1 mu m. Solution pH decreased from near neutral at the mineral surface to 3-4 around microcolonies living within confined spa ces of interior colonized cleavage planes. However, no evidence of pH micro gradients surrounding exterior microcolonies was noted.