In Situ Search for Extraterrestrial Life: A Microbial Fuel Cell-Based Sensor for the Detection of Photosynthetic Metabolism

Review: can bioelectrochemical sensors be used to monitor soil microbiome activity and fertility?

This review presents current knowledge on applying bioelectrochemical sensors to monitor soil fertility through microbial activity and discusses future perspectives. Soil microbial activity is considered an indicator of soil fertility due to the interconnected relationship between soil nutrient composition, microbiome, and plant productivity. Similarities between soils and bioelectrochemical reactors provide the foundation for the design of bioelectrochemical sensors driven by microorganisms enriched as electrochemically active biofilms on polarized electrodes. The biofilm can exchange electrons with electrodes and metabolites with the nearby microbiome to generate electrochemical signals that inform of microbiome functions and nutrient bioavailability. Such mechanisms can be harnessed as a bioelectrochemical sensor for proxy monitoring of soil fertility to address the need for real-time monitoring of soils.

Recent Implementations of Hydrogel-Based Microbial Electrochemical Technologies (METs) in Sensing Applications

Hydrogel materials have been used extensively in microbial electrochemical technology (MET) and sensor development due to their high biocompatibility and low toxicity. With an increasing demand for sensors across different sectors, it is crucial to understand the current state within the sectors of hydrogel METs and sensors. Surprisingly, a systematic review examining the application of hydrogel-based METs to sensor technologies has not yet been conducted. This review aimed to identify the current research progress surrounding the incorporation of hydrogels within METs and sensors development, with a specific focus on microbial fuel cells (MFCs) and microbial electrolysis cells (MECs). The manufacturing process/cost, operational performance, analysis accuracy and stability of typical hydrogel materials in METs and sensors were summarised and analysed. The current challenges facing the technology as well as potential direction for future research were also discussed. This review will substantially promote the understanding of hydrogel materials used in METs and benefit the development of electrochemical biosensors using hydrogel-based METs.

Electrochemically Active Biofilms as an Indicator of Soil Health

Soil health is a complex phenomenon that reflects the ability of soil to support both plant growth and other ecosystem functions. To our knowledge, research on extracellular electron transfer processes in soil environments is limited and could provide novel knowledge and new ways of monitoring soil health. Electrochemical activities in the soil can be studied by inserting inert electrodes. Once the electrode is polarized to a favorable potential, nearby microorganisms attach to the electrodes and grow as biofilms. Biofilms are a major part of the soil and play critical roles in microbial activity and community dynamics. Our work aims to investigate the electrochemical behavior of healthy and unhealthy soils using chronoamperometry and cyclic voltammetry. We developed a bioelectrochemical soil reactor for electrochemical measurements using healthy and unhealthy soils taken from the Cook Agronomy Farm Long-Term Agroecological Research site; the soils showed similar physical and chemical characteristics, but there was higher plant growth where the healthy soil was taken. Using carbon cloth electrodes installed in these soil reactors, we explored the electrochemical signals in these two soils. First, we measured redox variations by depth and found that reducing conditions were prevalent in healthy soils. Current measurements showed distinct differences between healthy and unhealthy soils. Scanning electron microscopy images showed the presence of microbes attached to the electrode for healthy soil but not for unhealthy soil. Glucose addition stimulated current in both soil types and caused differences in cyclic voltammograms between the two soil types to converge. Our work demonstrates that we can use current as a proxy for microbial metabolic activity to distinguish healthy and unhealthy soil.

Detecting Endogenous Microbial Metabolism and Differentiating Between Abiotic and Biotic Signals Observed by Bioelectrochemical Systems in Soils

Unambiguous detection of chemical and physical signatures of microbial life on Mars or other solar system bodies requires differentiation between signals produced by biotic and abiotic processes; instruments aimed at generalized in situ extant life detection would therefore increase the science return of a life-detection mission. Here, we investigate Bioelectrochemical Systems (BES) as a technique to measure microbial metabolism (which produces electrical current and redox changes) and distinguish between potential abiotic and biotic responses in environmental samples. Samples from inhabited niches should contain everything necessary to produce current, that is, catalysts (microorganisms) and fuel (nutrients). BES can also probe for inactive organisms in less energetically rich areas by adding a fuel to drive metabolism. A commercial potting soil and a Mars simulant soil were inoculated in the anodic chamber of microbial fuel cells, and current was monitored over time. Addition of a fuel (electron donor) source was tested for metabolic stimulation of endogenous microbes. Redox reactions between Mars simulant soil and the introduced electron donor (lactate) produced false-positive results, emphasizing the importance of careful interpretation of signals obtained. The addition of lactate to both soils resulted in enhanced biologically produced current, allowing stimulation and detection of dormant microbes. Our results demonstrate that BES provide an approach to detect metabolism in samples without prior knowledge of the organisms present, and that thorough electrochemical analyses and experimental design are necessary to determine if signals are biotic.

Characterization of a new composite membrane for point of need paper-based micro-scale microbial fuel cell analytical devices

Microbial fuel cells (MFCs) can evolve in a viable technology if environmentally sound materials are developed and became available at low cost for these devices. This is especially important not only for the designing of large wastewater treatment systems, but also for the fabrication of low-cost, single-use devices. In this work we synthesized membranes by a simple procedure involving easily-biodegradable and economic materials such as poly (vinyl alcohol) (PVA), chitosan (CS) and the composite PVA:CS. Membranes were chemical and physically characterized and compared to Nafion^®. Performance was studied using the membrane as separator in a typical H-Type MFCs showing that PVA:CS membrane outperform Nafion^® 4 times (power production) while being 75 times more economic. We found that performance in MFC depends over interactions among several membrane characteristics such as oxygen permeability and ion conductivity. Moreover, we design a paper-based micro-scale MFC, which was used as a toxicity assay using 16 μL samples containing formaldehyde as a model toxicant. The PVA:CS membrane presented here can offer low environmental impact and become a very interesting option for point of need single-use analytical devices, especially in low-income countries where burning is used as disposal method, and toxic fluoride fumes (from Nafion^®) can be released to the environment.

New insights in photosynthetic microbial fuel cell using anoxygenic phototrophic bacteria

Anoxygenic phototrophic bacteria (APB) pay a key role in biogeochemical cycles, and it can convert light energy to chemical energy by photosynthesis process. Photosynthetic microbial fuel cell (photo-MFC) is regarded as a promising energy-harvesting technology, which is also applied to environment treatment in recent years. The previous studies show that photo-MFC with APB have higher power putout than other bioelectrochemical systems. However, photo-MFC with APB is not reviewed due to some limited factors in the development process. In this review, photo-MFC with APB is treated according to its electron transfer pathways, the current understanding, APB strains, application, influence of substrates, and economic assessment. Meanwhile, knowledge of photosynthesis components and electron transfer pathways of APB is crucial for developing new energy and easing the serious energy crisis. Moreover, some new insights (the optimization of light source and self-sustaining bioelectricity generation) are proposed for the future explorations.