Probing the Microbe-Mineral Interface: Towards A Quantitative Treatment of Mineral-Surface Processes in the Context of Microbial Attachment

One of the major challenges of geomicrobiology is to resolve the precise manner by which microbial activity influences mineral-surface reactions. While a prerequisite for biological activity at a surface is substrate recognition and attachment, probing the nature of this biological-geological interface is inherently difficult. Accordingly, direct quantification of microbially-mediated dissolution rates are often complicated by an inability to discern relative contributions from direct microbe-mineral surface interactions (biofilm formation) and changes in the solution environment resulting from biological activity. A noninvasive imaging technique is needed that can both detect the microbe at the surface and quantify any resulting changes in mineral-surface topography, while maintaining both a high spatial resolution and a large field of view. Vertical scanning interferometry (VSI) meets these requirements and enables the measurement of both local dissolution (etch pits) and "global" dissolution rates (surface normal retreat). The novel application of VSI to the study of geomicrobiological problems yields capabilities that are complementary to scanning probe microscopy (SPM) methods in providing quantitative insight into microbial-mineral interactions as well as to the relationship between surface microtopography and biofilm formation. Recently, these coupled techniques have elucidated the mechanistic role of Shewanella oneidensis MR-1 in determining the dissolution rates of carbonate minerals. Our results show that Shewanella surface colonization can either block mineral dissolution through attachment to high-energy sites on the surface or enhance dissolution as a byproduct of irreversible attachment. The relative contribution from these processes to the overall dissolution rate varies with the background abiotic dissolution rate of the mineral. This and other studies by our group are beginning to demonstrate that VSI and SPM are well-suited to provide the critical measurements needed to build quantitative and predictive models of microbial surface recognition and their associated effect on mineral-surface processes.