Silicates

Bacterial mobilization of phosphate from silicates

Rogers, J.R., jaroberts@mail.utexas.edu, and BENNETT, P.C., Department of Geological Sciences, UT Austin, Austin, TX 78712; HIEBERT, F.K., RMT/Jones & Neuse, Inc., Austin, TX 78746; ULLMAN, W.J., College of Marine Studies, University of Delaware, Lewes, DE, 19958.

In subsurface environments phosphate is often scarce, limiting the growth of the indigenous microbial population. In some cases organisms have developed strategies to extract P from resistant minerals, even silicates, and organisms that can scavenge P from silicates would have a competitive advantage. In this study we examined microbial colonization and weathering of silicate minerals under P-limiting conditions in an anaerobic, petroleum-contaminated aquifer, and in an anaerobic wetland. Using in situ field microcosms in these P-limited, but carbon-rich groundwaters we found that feldspars containing micro-inclusions of P-minerals such as apatite are preferentially colonized over similar feldspars without P. The colonization of these mineral surfaces also directly correlates to accelerated surface weathering, and weathering is only found near attached organisms. In laboratory microcosms containing aquifer sediment and groundwater, but with anorthoclase (Na,K-feldspar) as the sole source of inorganic P the silicate rapidly dissolves with release of silica, while the released P is conserved in the biomass, and analysis by SEM show abundant colonies of rod-shaped microorganisms on the surface. When microcosms are supplied with K as K- feldspar and/or Na as plagioclase, there is no release of silica and no colonization by microorganisms. In microcosms where non-P silicates are mixed with apatite, there is no release of silica and no evidence of silicate weathering but P is released into solution, with the apatite marginally colonized while the silicate is barren of microbial colonization. Microcosms containing anorthoclase degraded toluene five times faster than microcosms containing non-P silicates. We propose that the colonization of silicate surfaces is not random, and is strongly influenced by the presence or absence of P. Further, native subsurface microorganisms preferentially weather silicates, which contain nutrients of value, such as P in apatite inclusions. The P-content of a silicate, therefore, may be a fundamental part of microbial ecology, and therefore a fundamental control on weathering rate.