Light exposure interferes with electroactive biofilm enrichment and reduces extracellular electron transfer efficiency.

Light plays a vital role in shaping the structure of natural biofilms, but the effect of light on electroactive biofilm (EAB) has not been systematically studied. Herein, the influence of light on the formation of EAB was investigated. The EAB grown in darkness was more electroactive (EAB-0) with a peak current of ∼4.5 A/m2, which was 196 and 5556 times higher than EABs formed under light intensities of 600 (EAB-600) and 1200 lux (EAB-1200). A thin EAB (30 µm) with spherical pink aggregates was obtained after 13 days in the darkness, comparing to a dense and flat biofilm grown under light conditions. Although the biomass in EAB-1200 (38.5 ± 1.6 mg/L) was 3 times larger than that in EAB-0 (11.4 ± 1.8 mg/L), the degradation of substrate was not sufficient. EAB-0 contained 85% Geobacter species, while the Rhodopseudomonas species made up 66% and 75% of EAB-600 and EAB-1200, respectively. The polysaccharides produced by EAB-1200 was 4801 ± 253 mg/m2, which were 2.3 times higher than 2073 ± 160 mg/m2 of EAB-0, resulting in lower electro-conductivity of the extracellular polymeric substances (EPS) under light conditions. Our findings confirmed that the light exposure affected EAB performance by altering the microbial components, electron transfer capacity, and biofilm morphology, which can be used in predictions of the formation and properties of engineered EAB in outdoor environments.