Switchover of electrotrophic and heterotrophic respirations enables the biomonitoring of low concentration of BOD in oxygen-rich environment

Biochemical oxygen demand (BOD) is a key indicator of water quality. However, there is still no technique to directly measure BOD at low concentrations in oxygen-rich environments. Here, we propose a new scheme using facultative electrotrophs as the sensing element, and confirmed aerobic Acinetobacter venetianus RAG-1 immobilized on electrode was able to measure BOD via the switchover between electrotrophic and heterotrophic respirations. The hybrid binder of Nafion and polytetrafluoroethylene (PTFE) maximized the baseline current (127 ± 2 A/m2) and sensitivity (2.5 ± 0.1 (mA/m2)/(mg/L)). The current decrease and the BOD5 concentration fitted well with a linear model in the case of known contaminants, verified with both lab samples of acetate and glucose (R2>0.96) and in standard curves of real environmental samples collected from the lake and the effluent of wastewater treatment plant (R2>0.98). Importantly, the biosensor tested actual contaminated water samples with an error of 0.4∼10% compared to BOD5 in the case of unknown contaminants. Transcriptomics revealed that reverse oxidative TCA may involve in the electrotrophic respiration of RAG-1 since citrate synthase (gltA) was highly expressed, which was partly downregulated when heterotrophic metabolism was triggered by BOD. This can be returned to electrotroph when BOD was depleted. Our results showed a new way to rapidly measure BOD in oxygen-rich environment, demonstrating the possibility to employ bacteria with two competitive respiration pathways for pollution detection.