The Role of Motility and Chemotaxis in Biodegradation of Nitrate by Shewanella oneidensis MR-1 in Diffusion Dominated Porous Media

Abstract:

Biodegradation is one of the major attenuation processes for nitrate in the environment. In low permeability zones (LPZ) of porous media, microorganisms can reduce nitrate and thereby gain energy for growth and metabolism. Many such organisms have chemical receptors for nitrate, allowing them to migrate toward nitrate source zones to maximize their energy yield. Hence, motile and chemotactic motions can facilitate cell growth in these diffusion-dominated areas. The process of nitrate removal in LPZs is important in agricultural sites, where antibiotics have also infiltrated into the soil along with nitrate. Antibiotics inhibit microbial function and can therefore negatively affect nitrate reduction.

In a recent publication, high concentrations of Shewanella oneidensis MR-1 were observed in nitrate rich areas, also containing high concentrations of the antibiotic ciprofloxacin (Alcalde et al. (2019)). To interpret this observation, a 1D finite difference model was developed in which sequential reduction of nitrate to nitrite and nitrite to ammonium were solved, along with bacterial transport and growth. A genetic algorithm was used to fit the model to the experimental data by adjusting the bacterial motility coefficient, chemotaxis sensitivity coefficient, and inhibition constant of ciprofloxacin. Model results indicate that accumulation of cells in nitrate and ciprofloxacin rich zones occurs due to chemotactic-driven motility toward nitrate, not cell growth in these zones.

Alcalde, R.E., Michelson, K., Zhou, L., Schmitz, E.V., Deng, J., Sanford, R.A., Fouke, B.W., Werth, C.J., 2019. Motility of Shewanella oneidensis MR-1 Allows for Nitrate Reduction in the Toxic Region of a Ciprofloxacin Concentration Gradient in a Microfluidic Reactor. Environ. Sci. Technol. 53, 2778–2787.