Microbial fuel cell biosensors for converting hydroponic root-exudate-derived carbon into electrical signals

Root exudates (RE) represent an important but often overlooked fraction of photosynthetically fixed carbon released into hydroponic nutrient solutions, creating a need for monitoring. Here, we developed adaptive microbial fuel cell (MFC)-based biosensors for translating the total COD of RE-organic acid mixtures into electrical signals. Anode biofilms were selectively acclimated with RE-associated organic acids, including malate, citrate, and pyruvate, with acetate as a control. After screening with real lettuce RE, citrate- and malate-acclimated MFCs were operated with RE-containing substrates for 88 days to assess sensing stability. Malate-acclimated MFCs showed consistent concentration-dependent responses, with a mean in-sample absolute relative error of 7.2%. Predictive performance was subsequently evaluated over 15 days using three parallel ma-MFCs and a previously established calibration curve. The mean absolute relative error across 45 repeated observations was 10.63% (SD, 7.66%). Multi-omics analysis showed that citrate- and malate-acclimated biofilms shared a fermentative-electrogenic route for RE sensing, but differed in auxiliary metabolic functions. Omics analyses suggested differences between citrate- and malate-acclimated biofilms in nitrogen-related metabolic potential and possible sulfur- and denitrification-associated pathways. These findings provide a basis for further investigation of the sensing mechanism. Overall, acclimated MFC biosensors offer a low-cost approach to estimating total COD of RE-organic acid mixtures during hydroponics cultivation through sequential batch measurements and support further development toward in situ, real-time, continuous monitoring.