Oxygen-reducing microbial cathodes monitoring toxic shocks in tap water

Electroactive biofilms (EABs) have recently attracted considerable research interest for their possible use as amperometric biosensors in environmental or bioprocess monitoring, for example for in situ detection of toxic compounds. Almost all related studies have investigated heterotrophic, anodic EABs. The later require sufficiently high organic loads and anoxic conditions to deliver a stable baseline current. Conversely, electroautotrophic O2-reducing EABs have recently been proposed to monitor toxic shocks in oxic solutions poor or devoid of organic substrate. This was done in optimal media and only assessed for formaldehyde as a model toxic compound. Here we show that O2-reducing EABs can grow in unamended tap water on carbon electrodes at + 0.2 V vs. Ag/AgCl. They retained substantial electroactivity for at least seven months without adding exogenous compounds. The most represented operational taxonomic units were two unclassified gammaproteobacteria (25 ± 15 %, n = 5 electrodes). Cyclic voltammograms showed a reproducible nernstian behavior for O2 reduction with a mid-wave potential at + 0.27 V and variable plateau current densities ranging from – 1 to – 22 µA.cm−2 (n = 10 electrodes). The biocatalytic current was substantially impacted by the addition of either of three tested heavy metals (Hg(II), Cr(VI) or Pb(II)) or by organic pollutants (formaldehyde, 2,4 dichlorophenol, benzalkonium chloride), with limits of detection ranging from 0.5 to 10 mg.L−1. Response times were below 1 min. Comparison with previous reports suggests that O2-reducing microbial cathodes may be more sensitive to toxic shocks than anodic, heterotrophic EABs.