Bacteria-water interfaces

Molecular Chemistry of Reactions at the Bacteria-Water Interfaces

Microorganisms and their cell-wall fragments are ubiquitous in soils, sediments, and aquatic systems and strongly influence different geochemical processes such as: mineral dissolution and precipitation, contaminant degradation and transport, metal and organic matter redox reactions, and geochemical kinetics. A number of bacteria species that can degrade a variety of contaminants, and mediate these biogeochemical processes, have been identified. One of the bottlenecks for microbial transformation reactions is the availability of contaminant to microorganisms, which is principally manifested by microorganism-water interfacial chemistry, chemical form and speciation, and the environmental Eh-pH conditions. The same factors also govern the bacterial adhesion to minerals and plant root cells, and biofilm formation processes. Since cell surface chemistry is considered to be one of the primary factors modifying the nature of biochemical transformations, researchers have been trying to study its behavior using acid-base titrations, metal adsorption isotherms, and different surface complexation models. These investigations led to the identification of different organic moieties (e.g. carboxylic, phosphate, hydroxyl) that protonate and complex under different solution chemical conditions, and development of their thermodynamic constants. While these equilibrium constants have immense application in predicting the reactive transport of contaminants and their biotransformation reactions in the subsurface geologic systems, it is difficult to elucidate the mechanisms without molecular information on the contaminant complexes on surfaces. This information is critical in understanding the microbe-contaminant and microbe-mineral surface reactions, and specifically, in predicting the overall biotransformation reactions.

 

Figure 1
Figure 1: Scanning transmission X-ray microscope images of OYS3 and its reactions with Cr(VI). Cr L-edge NEXAFS spectra of solution (Cr(VI)) and the microorganism (Cr(III) along the cell walls) are shown on the right. The spectra at different locations correspond to different points along the line marked "ab." Notice the changes in the spectra at the microorganism-solution interface. The energy scale is not calibrated for Cr. Images and spectra were collected on cells in the presence of solution.

 

Our current research is focused on:
  • Identification of the locus of biotransformation reactions: bacteria surfaces versus the interior of cell-walls,
  • Role of bacteria surfaces in contaminant adsorption and biotransformation, and
  • Influence of solution chemistry and chemical speciation on contaminant availability to bacteria.

Our initial exploratory work was focused on OYS3 species isolated from a DOE site (X-ray images shown in the above figure). Our Lawrence Berkeley Laboratory scientists found that the samples we used have some species of fungi as well. However, these studies indicated that in-situ chemical speciation can be obtained on the chemical variations at the microorganism-water interface using X-ray spectroscopy and microscopy. Current investigations utilize Bacillus subtilis and Pseudomonas aeruginosa bacteria species.