External resistance modulates anodic biofilm architecture, electrochemical performance, and EET-related gene expression in Shewanella oneidensis MR-1 microbial fuel cells.

External resistance is a key operational variable in microbial fuel cells (MFCs), yet its mechanistic influence on pure-culture electrogenic biofilms remains incompletely understood. Here, we investigated how fixed external resistances (10, 100, 500, and 1000 Ω) applied during start-up modulate electrochemical performance, architecture, membrane-integrity-associated viability, and extracellular electron transfer (EET)-related gene expression in Shewanella oneidensis MR-1 anodic biofilms. After 10 days in dual-chamber MFCs, polarization analysis, cyclic voltammetry, confocal microscopy with LIVE/DEAD staining, epifluorescence viable-cell counting, and reverse-transcription quantitative PCR targeting mtrA and omcA were performed. Low resistance (10 Ω) produced the highest maximum power density (130 mW m-2), current density at maximum power (1609.6 mA m-2), lowest apparent internal resistance, strongest anodic redox activity, and significant mtrA up-regulation. In contrast, intermediate and high resistances promoted thicker biofilms with higher membrane-integrity-associated live signal but lower electrochemical output. omcA expression was not significantly affected. Multivariate correlation and regression analyses identified anodic peak current, peak potential, and external resistance as the main predictors of maximum power density (R2 = 0.978), indicating that external resistance modulates a trade-off between biofilm accumulation and electrochemical output, with mtrA acting as a molecular marker of the anodic electroactive state.