Sensor-Based Assessment of Plant–Microbe Energy Interactions

Abstract

The BIONOTE-L voltametric sensor was used in this work to exploit cyclic voltammetry as an operating principle to interact with the plant/soil system. Indeed, in bacteria, the metabolic process oxidizes organic compounds from the soil organic matter with extracellular electron transfer. Plants can also release electrons into the rhizosphere—in what are known as plant microbial fuel cells (PMFCs). The electrons obtained, when appropriately conveyed, generate an electric current. On this basis, the innovation of a sensor system “interrogating” the plant/soil system via cyclic voltammetry is proposed. This work tries to place a new paradigm: Rather than exploiting the energy autonomously produced, the plant-microorganism system and the interaction that occurs between them can be analyzed in energetic terms, which is novel with respect to the standard approach measuring other parameters related to plant growth. An external input is provided, and the response of the system to this solicitation is observed. The energy extracted does not have the purpose of powering other devices or being stored. Instead, it is treated as data on the state of health of the plant-microorganism system. This test has been structured as a step-by-step experimental investigation, with a gradual increase in complexity. The BIONOTE-L was tested to analyze the behavior of four different genera of bacteria in solution: Rhizobium, Bacillus, Enterobacter, and Kosakonia. The behavior of Bacillus was then analyzed in interaction with the soil and with a corn plant (Zea mays L.). The gradual growth of the bacteria in the solution was assessed using a spectrophotometer (optical density: O.D.) and predicting O.D. with BIONOTE-L with root mean square errors in cross validation from 0.004 to 0.2 (depending on the different bacteria). Specific fingerprints were detected for each hour of bacterial growth (therefore specific for the concentration of bacteria in solution) and for each different genus of bacteria. Finally, the electrical response of Bacillus in interaction with the soil and plant was characterized, thus providing a specific electrical signature to be monitored in a real-world scenario.