A Fumarate Microbiosensor for Use in Biofilms

Abstract

Fumarate is one of the terminal electron acceptors which can be used by microorganisms growing as biofilms. Mass transport in biofilms is dominated by diffusion, which creates a fumarate concentration gradient and could be the limiting factor for the growth of biofilms respiring on fumarate. Bulk solution measurements cannot be used to determine fumarate limitations in fumarate-respiring biofilms. Therefore, new approaches are needed for measuring fumarate concentration in biofilms. The goal of this study was to develop a microbiosensor for in situ depth profiling in biofilms. The working principle of the microbiosensor is the correlation between fumarate concentration and current consumption during fumarate reduction by G. sulfurreducens grown on a carbon microelectrode tip. The microbiosensor consists of a carbon microelectrode with G. sulfurreducens biofilm on a tip with a diameter of ∼30 μm as the working electrode and a built-in Ag/AgCl pseudo reference electrode, enclosed in a glass outer case. The microbiosensor responded in the 0–3.2 mM fumarate concentration range and had a detection limit of 258 ± 25 μM (S/N = 2). We quantified the stirring effect and the interfering effect of alternative substrates on the microbiosensor response. The utility of the fumarate microbiosensor was demonstrated by depth profile measurements of fumarate concentration in cathodic G. sulfurreducens biofilms. We found that fumarate was not a limiting factor for cathodic current generated by G. sulfurreducens biofilms.