Microscale Mineralogic Controls on Microbial Attachment to Rock Surfaces: Implications for Microbial Mineral Dissolution

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Microorganisms are ubiquitous in the shallow subsurface, attached to sediment surfaces and in the aqueous phase. When attached, microorganisms can alter the geochemical microenvironments at the point of attachment, shifting mineral equilibria and driving weathering reactions. However, microbial attachment on sediment grains is heterogeneous and the variability of microbial occurrence cannot be simply explained by electrostatic interactions between the cell and the mineral surface. Several studies have shown that other factors such as, solution chemistry, surface coatings, and surface roughness can change attachment behavior.Previous research by the PI on subsurface microbial colonization in a petroleum-contaminated aquifer suggests that the distribution of microorganisms on different mineral surfaces in situ is related, in part, to the nutrient content of the mineral. Microorganisms preferentially colonize and destroy silicate minerals that contain limiting trace nutrients, such as P and Fe, occurring as trace apatite and iron oxyhydroxides (Rogers et al., 1998; 2001). It is still unclear, however, whether the mineralogy of the point of attachment is nutrient-rich inclusion or silicate matrix. Nor is it known if these inclusions impact only initial attachment, or if they influence permanent attachment, subsequent growth and colonization, and surface etching.The goal of the proposed research is to study the influence of nutrient-bearing mineral inclusions on microbial surface attachment. Specifically, this study will examine how apatite and iron oxide inclusions in feldspars influence microbial attachment. It is hypothesized that microorganisms will preferentially attach to surface expressions of these inclusions leaving the silicate groundmass barren. Inclusions may increase attachment through increased surface charge or chemotactic behavior to nutrients or terminal electron acceptors.The proposed research will use laboratory experiments to characterize microbial attachment to silicate rocks that are compositionally heterogeneous. Laboratory experiments will be used to examine attachment under static (no-flow) conditions and will approximate "maximum" initial attachment. Pure strain cultures, including Geobacter metallireducens and Methanosaeta concilii and native anaerobic consortia will be used in experiments to characterize any changes in attachment behavior due to type-specific interactions, competition, and/or symbiosis. The solid phases used in these experiments will consist of feldspars containing inclusions of P and Fe minerals, as well as controls without these nutrient phases. Pyrex glasses will also be manufactured containing dispersed P and Fe as well as, included P and Fe minerals. Glasses will serve as "artificial" rocks to test whether cells preferentially attach to inclusions or if patchy behavior is prevalent when nutrients are dispersed. For all attachment experiments the underlying surface mineralogy/composition will be mapped and correlated to attachment patterns. Finally, field microcosms using the same suite of solids will be reacted with the native consortia in situ. It is assumed that attachment of microbes to mineral surfaces is necessary for substantial mineral etching to occur, and field results will be used to link attachment to microbial weathering processes observed in situ.