Diel fluctuation of output voltage in sediment microbial fuel cells and its response to light exposure

Controlled indoor sediment microbial fuel cells (SMFCs) were operated under darkness or programmed light exposure, with or without aeration, to validate field-observed diel voltage fluctuations and quantify their temporal relationship with overlying-water dissolved oxygen (DO). Light-exposed non-aerated closed-circuit SMFCs clearly reproduced diel voltage rhythms; in contrast, dark treatments showed only weak fluctuations, and continuous aeration markedly suppressed this rhythm. The daily voltage amplitude and DO amplitude were positively correlated under light exposure, with Spearman correlation coefficients of 0.334-0.551. Peak-time analysis in the main and validation windows revealed a consistent sequence of light, voltage, and bulk DO peaks. Voltage peaks occurred 1.68-2.98 h after light peaks, whereas DO peaks occurred 3.51-3.98 h after light peaks; thus, voltage peaks preceded bulk overlying-water DO peaks by 0.53-2.28 h. Cathode-surface chlorophyll-a and ash-free dry mass were higher under light exposure, indicating greater cathode-attached biomass. Electrochemical, physicochemical, and microbial measurements revealed treatment-dependent differences and were used as supplementary evidence. These results confirmed that the field-observed diel voltage fluctuation could be reproduced under controlled light exposure. The reproducible timing offset between the voltage and bulk overlying-water DO further revealed that voltage dynamics were associated with, but not synchronous with, diel DO dynamics. This study provides a diel-scale basis for evaluating SMFC output in illuminated aquatic environments.