Mass Transfer and Metabolic Variability in Electrochemically Active Biofilms

Electrochemically active biofilms (EABs) are complex communities of microorganisms that have a unique form of respiration that allows them to utilize insoluble extracellular materials as their terminal electron acceptor for metabolism. Cellular phenotypes and metabolic activities in EABs are largely governed by restricted mass transport and by the formation of microscale gradients, often referred to as microenvironments. A detailed quantification of the microenvironments and the chemical and structural variability inside living biofilms is necessary to fully understanding EABs and the limitations to scaling them up in full-scale industrial biofilm reactors. Furthermore, measuring the variation of gradients inside of the biofilm matrix is important for determining the limitations of electron transfer processes and for developing accurate mathematical models for engineering EABs.

NMR is ideally suited for studying microenvironments and variability in biofilms. NMR has the ability to measure chemical, physical, anatomical, and transport information, live and in situ, over extended time periods. Furthermore, these measurements can be done noninvasively and nondestructively without signal loss through the sample, and at high spatial resolution in multiple dimensions. Although NMR has been used in biofilm research it has had limited use in the study of EABs specifically. EABs require a conducting substratum available as an electron acceptor for respiration and the presence of a conductive electrode constrains NMR use in these systems. Without careful geometric design, conductive objects proximal to the sample can shield the sample from radio frequency excitation and subsequent emission. In addition, a conductive substratum can serve as a conduit for external noise as part of an external circuit. The controlled growth environments provided by microscale reactors are essential to fully understand the dynamics of metabolic reactions in EABs over time. A small volume would allow detection of rapid concentration and metabolic changes required for kinetic studies. A microscale NMR-compatible biofilm reactor with simultaneous electrochemical monitoring would enable experiments that allow researchers to concurrently determine nutrient balances (via NMR spectroscopy), energy balances (electron monitoring via electrochemical experiments), and biofilm structure (via NMR imaging).

Our group has recently developed a set of microreactors for simultaneous electrochemical and nuclear magnetic resonance (EC-NMR) techniques with an electrode acting as the sole electron acceptor ^{1, 2}. We applied the microreactors to measure diffusion coefficients in Shewanella oneidensis and Geobacter sulfurreducens biofilms respiring on electrodes, as well as acetate (electron donor) concentration profiles in the G. sulfurreducens biofilms. We found that diffusion coefficients were highly variable by depth and showed a significant decrease in diffusivity near the electrode-biofilm interface. G. sulfurreducens biofilms had average diffusion coefficients that were nearly 10 times lower than S. oneidensis biofilms, which likely reflects the difference between electron transfer mechanisms utilized by each species. Finally, we found that acetate was entirely consumed in the depth of G. sulfurreducens biofilms and that the top of these biofilms were metabolically active and capable of transferring electrons across hundreds of microns to the electrode.

REFERENCES

1.     R. S. Renslow, J. T. Babauta, A. C. Dohnalkova, M. I. Boyanov, K. M. Kemner, P. D. Majors, J. K. Fredrickson and H. Beyenal, Energy Environ. Sci., 2013, 6, 1827-1836.

2.     R. S. Renslow, J. T. Babauta, P. D. Majors and H. Beyenal, Energy Environ. Sci., 2013, 6, 595-607.