Seasonal control of Caribbean mangrove MEC performance: a metabarcoding and chronoamperometry study

Microbial electrolysis cells (MECs) can couple waste valorization with hydrogen generation, yet performance with natural consortia is variable. We assessed how the season of sediment sampling and the running electrolyte shape electrochemical activity and community assembly in single-chamber, two-electrode MECs (ΔV = 0.8 V) inoculated with mangrove sediment and operated with either natural mangrove water (MW) or Marine Broth (MB). Electrochemical performance (current density, Faradaic efficiency) was linked to community profiles from full-length 16S rRNA metabarcoding.Rainy-season MW consistently outperformed dry-season MW and MB, reaching up to 4.8 A·m⁻² and sustaining higher FE. High-performing biofilms were enriched in canonical exoelectrogens—Geobacter spp. (24.6%) and Shewanella spp. (14.8%)—whereas dry-season systems shifted over acetate cycles toward Desulfuromonas spp. and green-sulfur bacteria. MB yielded biofilms lacking these EET lineages and produced lower electrochemical output.These results indicate that seasonal hydrology (salinity/ionic strength) and electrolyte choice are primary levers for assembling electroactive biofilms that deliver A·m⁻²-level currents. Lower-salinity, rainy-season MW emerges as a practical electrolyte for sediment-inoculated MECs in tropical island contexts. Given the detection of exoelectrogenic taxa at low abundance in MW, using MW alone as a future inoculum is plausible, but was not tested here.