Feldspars

Are feldspars a source of phosphorus for microorganisms?

Rogers, J.R., jaroberts@mail.utexas.edu, and BENNETT, P.C., Dept. Geological Sciences, UT Austin, Austin, TX 78712; HIEBERT, F.K., RMT/Jones & Neuse, Inc., Austin, TX 78746.

Phosphorus is an essential nutrient for microorganisms. In subsurface aqueous environments phosphorus can be scarce, and tightly cycled within the community, limiting the growth of microbial populations. In P-limited environments some microorganisms can extract phosphorus from detrital minerals however, these detrital phosphates are rarely abundant in aquifers. Phosphorus is a trace or minor constituent in feldspars, however, and pegmatitic feldspars can accomodate P in the crystal matrix, while many igneous feldspars contain apatite inclusions. While these feldspar-bound nutrients have not previously been considered a viable source of phosphorus, in nutrient-limited environments colonizing microorganisms may find no other alternative. We propose that microorganisms extract phosphorus from apatite inclusions in feldspars, scavenging the nutrient while destroying the silicate matrix. We have observed this in carbon-rich, but phosphorus-poor, anaerobic groundwater.

We examined the nutritive value of feldspars by characterizing microbial colonization and weathering of a variety of feldspars in situ. In situ field microcosms containing clean mineral chips were placed into ground water in a petroleum-contaminated aquifer. After microcosm removal the biological tissue was fixed and imaged with SEM to characterize the types of microorganisms present, colonization patterns and feldspar-surface.

Five different feldspars were exposed to microbially active ground water. We observed differential colonization and weathering of the feldspars, with feldspars, containing P, preferentially colonized, and only colonized feldspars weathered. One microcline (S. Dakota) contained 0.24% P₂O₅and was heavily colonized and deeply weathered while, a similar microcline (Ontario) had no detectable P, and was barren of attached organisms and unweathered. Anorthoclase (0.28% P₂O₅) was heavily colonized and weathered, while plagioclase (<0.01% P) was uncolonized and unweathered. We propose that this environment is sensitive to P availability, and that native subsurface microorganisms have developed strategies to aggressively scavenge P from feldspars. The result of this interaction is that only feldspars containing P will be significantly colonized, and these feldspars will be preferentially destroyed, as the subsurface microbial community scavenges a limiting nutrient.