Spirulina-Based Multispecies Phototrophic Biofilm Anodic Biocatalyst Endures a Prolonged Dark Phase within Light–Dark Cycle Operations and Enhances Anodic Performance in Biophotovoltaic Cells

Abstract

Phototrophs with heterotrophic bacterial consortium as an electrode biocatalyst are an emerging concept for developing naturally sustained biophotovoltaic systems. Herein, Spirulina subsalsa-based mixed heterotrophic bacterial community as an anodic catalyst in a microbial fuel cell (MFC) setup with ferricyanide catholyte in 78 days light–dark (16–8 h) cycle-based operation was investigated. The biofilm developed with Spirulina inducted a recalcitrant bacterial community comprising Halomonas, Alcanivorax, Pelagibacterium, and Rhizobiales as the major genera. In an extended dark phase (9 days) within the cyclic operation, a sequential shift of the metabolism from photosynthesis to fermentative states and an increased heterotrophic population were observed. Under direct contact with the graphite anode, the biofilm initiated oscillating open-circuit potentials in the MFC in response to the light–dark circadian trend. The MFC delivered maxima of 587 μW m^–2 and 418 μW m^–2 (at 10 kΩ) under the corresponding circadian and extended dark phases, respectively. The anodic potential shifted to a more negative value, reaching −415.5 mV in the dark starvation period. Analyses of electrode reaction rates (extracted from Tafel plots), corrosion potential, corrosion current, polarization resistance, and residual redox charges (extracted from cyclic voltammograms) were performed to understand the redox processes. Two redox peaks corresponding to 0.6 V (irreversible, extracellular) and 0.26 V (reversible, cell-surface attached) were attributed to redox mediation in this process. Additionally, catholyte-diffused ferricyanide interacts with the biofilm, getting trapped in the matrix polymeric structures, thus preventing the sudden cytotoxic elimination of cells and promoting oxidative charge accumulation over its surface, improving the anodic potential. Rapid respiratory oxygen consumption, the biofilm’s structural reorganization, and ferricyanide’s chemical speciation inside the biofilm were the primary factors that govern the anodic performance of the biofuel cell during the prolonged dark phase operations. The critical findings unveiled through this study advance our understanding of the resilience of phototroph-based multispecies anodic catalysts for developing biophotovoltaic devices for long-term operations.