Applicable New Technologies

Scanning transmission x-ray microscopy (STXM) is a promising new technology for the analysis of flocs and biofilms, as shown by recent progress in mapping the three-dimensional disposition of diverse exopolymeric matrix materials in a biofilm92 developed from water of the South Saskatchewan River (Canada). STXM represents a new frontier in the use of synchrotron radiation to characterize heterogeneous aggregated materials, yielding nanoscale structural resolution accompanied by data on the atomic environment of selected elements within a preselected colloid. It exploits the fact that soft x-rays interact with almost all elements to allow mapping of chemical species based on bonding structure. With STXM analysis, there is a potential to follow the evolution of biofilm and floc architecture over time, while relating the chemical transformations of specific toxic metals to the specific colloids involved in the transformations. Such research has already begun on freshwater flocs using x-ray absorption spectroscopy (XAS), a precursor of STXM, in combination with electron microscope-based, energy-dispersive spectroscopy. Gaillard et al.93 showed for Lake DePue (a lake contaminated by zinc in Illinois, United States) that, close to the contaminant source, zinc is bound to relatively labile phosphate and carbonate ligands associated spatially with aggregated microbiota. Far from the source, however, the zinc is predominantly coordinated with sulfides.

Atomic force microscopy (AFM) is a promising new tool for measuring the interaction forces between one colloid and another, and also between a colloid and a surface.94 AFM is applicable to the analysis of colloids found in flocs,95,96 as is the related technique of biological force microscopy, or BFM.97 Camesano and Logan95 used AFM to probe the effects of pH, ionic strength, and the presence of bacterial surface polymers on interaction forces between individual bacteria and silicon nitride surfaces, finding that the bacterial surface polymers dominated interactions between bacteria and AFM silicon nitride tips. Continuation of this work could provide insight into specific roles for individual kinds of EPS molecules. Currently, Muirhead and Lead98 are using AFM to measure the size and nanostructure of natural aquatic colloids in river waters, including humic substances and fibrillar EPS of probable microbial origin.

Environmental genomics is a genetics-based, interdisciplinary field of research that seeks to understand external factors affecting organisms when they are exposed to environmental stresses, such as contaminants and pathogens. Host responses to these stresses include changes in gene expression and genetic products, changes that culminate with alterations in host phenotype. Applications to floc research are imminent, including the development of labeled probes which will identify which microbe is which in a floc or biofilm. In fact, at the level of light microscopy, this can now be done quite well for the direct identification of individual microbes in mixed communities using FISH, or fluorescence in situ hybridization, with rRNA-targeted nucleic acid probes.99

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