The influence of Pseudomonas aeruginosa biofilm microenvironments on metal-microbe interactions
It is only over the past two decades that microbes have been recognized as important mediators of Earth's geochemistry. Similarly, the recognition of 'biofilms' as the predominant mode of bacterial growth has only been appreciated for the same amount of time. Because these two fields are in their infancy, the relationship between the two is poorly understood. Biofilm microbiology has been extensively characterized, however the interplay between chemically driven and microbiologically-mediated processes is a complicating factor in understanding these communities and the metal interactions that occur within their confines. A better understanding of so-called biofilm 'microenvironments' (conditions that differ from the surrounding phase) is required as they have been implicated in the complexity of metal interactions. This research investigates biofilm microenvironments and their influence on geomicrobiological phenomena. Initially, confocal microscopy was used to characterize 'Pseudomonas aeruginosa' biofilm development using a flow-cell system. Once biofilms matured, multiple fluorophores were applied which revealed that fully mature biofilms are oxygen-saturated throughout, but feature remarkable pH changes (~2 units) over micron-sized areas. Physiological probes also revealed that clusters of metabolically inactive cells are strewn throughout the community. Cryo-electron microscopy was then used to map the spatial organization of cells and their exopolymers (EPS), which revealed an extraordinary complexity at both the cellular and extracellular levels. These microenvironments were expected to have an impact on the geochemistry of bacterial communities. Accordingly, acid/base titrations and equilibrium dialysis were used to elucidate how a shift in environmental chemistry influences the metal sorption capacity of the bacterial surface. This revealed that ambient growth conditions can influence surface hydrophobicity and thus metal sorption behaviour. The same methods were used to analyze the metal reactivity of EPS with and without extracellular DNA (eDNA), which implicated this macromolecule as a major contributor to metal sorption capacity of the biofilm matrix. Finally, because certain minerals form only under discrete geochemical conditions, the production of fine-grained minerals was analyzed by electron microscopy and synchrotron radiation to determine that chemical microenvironments encourage extracellular mineral formation. These studies emphasize the complexity of microbial communities, and the influence of these systems on geochemical processes in nature.