Determining limiting steps for respiration of Geobacter sulfurreducens bacteria grown on biofilms, by numerical modeling

BONANNI P. S.; BRADLEY DAN F.; SCHROTT GERMÁN D.; SCHIFFRIN DAVID J.; BUSALMEN J.P.

Mar del Plata

Congreso; VIII Congreso argentino de microbiología general; 2012

SAMIGE

Geobacter Sulfurreducens is capable of respiring insoluble minerals in its natural subsurface environment. More interestingly, cells of this strain can be grown on electrodes forming biofilms that use the polarized surface as their final electron acceptor, which gives the unique possibility of studying their respiratory metabolism under controlled conditions by means of electrochemistry. Interestingly, for respiring electrodes, these cells manage to transport the electrons from their respiratory chain through tents of microns, from their outer membrane to the electrode. The way they make such long distance connection is a topic under intense discussion. Two main mechanisms are being proposed: the electron hopping (super-exchange) model, stating that electrons are transported through a sequence of redox reactions between outer membrane cytochromes in the biofilm extracellular matrix, and the metallic-like conduction model in which electron transport occurs through conductive filaments (pili) extending from the cell external membrane to the electrode. In this work we gain insight into the mechanism of electron transport by mathematically modeling the electrochemical response of biofilms grown on electrodes to changes in experimental conditions such as substrate concentration, applied potential and substrate transport rate to the biofilm. By considering the electron hopping mechanism we successfully modeled the current produced under each condition, determining at the same time the limiting step for respiration in each case. Results indicate that cells located close to the electrode are limited by substrate availability, in particular under low substrate concentrations. On the other hand, cells located close to the biofilm-solution interface, are limited by the concentration of external cytochromes able to accept electrons from the respiratory chain. In consequence, cells at the biofilm-solution interface are thought to respire at a rate that only allows the maintenance of their basal metabolism, impeding growth and division, and probably determining the limit for biofilm thickness.