Quantifying the relationship between microbial attachment and mineral surface dynamics using vertical scanning interferometry (VSI)

A major challenge of biogeochemistry is to resolve the precise manner by which microbial activity influences mineral-surface reactions. Although a prerequisite for biological activity at a surface is substrate recognition and attachment, probing the nature of this biological-geological interface is inherently difficult. While atomic force microscopy (AFM) is a powerful high-resolution imaging technique capable of quantifying mineral-surface and microbial cell structure, it suffers from the invasive nature of tip-sample interactions and a limited field of view. A noninvasive imaging technique is needed that can detect the microbe at the surface and quantify any resulting changes in mmeral-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). Here we use AFM and VSI as complementary techniques to evaluate the influence of mineral-surface dynamics on the surface colonization of carbonate surfaces by Shewanella oneidensis MR-1. It was found that 1) surface colonization occurred more slowly on actively dissolving calcite surfaces than on the less dynamic dolomite and magnesite surfaces; 2) cell attachment reduced calcite dissolution rates by more than 40 percent relative to cell-free controls; 3) surface microtopographical features such as etch pits provide high-energy sites that favor microbial attachment. These results indicate the existence of a complex relationship between microbial surface colonization and mineral-surface dynamics that deserves additional study. Further, this study demonstrates that VSI is an effective method for quantifying mineral-reaction rates in the context of microbial attachment. Direct comparisons with atomic force microscope (AFM) measurements also established VSI as a capable technique for making ex situ measurements of cell and biofilm dimensions on the surface. Accordingly, VSI should be considered a valuable complement to those powerful tools already available to the geomicrobiologist for quantification of processes occurring at the microbe-mineral interface.