Rhamnolipid Modulates Microbial Interspecies Electron Transfer for Synchronous Sulfidogenesis and Acidogenesis from Mariculture Solid Wastes

Synchronous sulfidogenesis and acidogenesis (SSA) are critical for pollutant removal and resource recovery. However, inefficient electron transfer and metabolic imbalance between acidogenic bacteria and sulfidogens limit SSA performance, especially from mariculture solid wastes (MSW) containing high-strength sulfate. This work unveiled the neglected role and mechanism of rhamnolipid (RL) in modulating microbial interspecies electron transfer for SSA during MSW anaerobic fermentation. RL, at environmentally relevant levels of 20-200 mg/g suspended solids, simultaneously improved sulfide (40.1-87.9%) and short-chain fatty acids (8.0-19.3-fold) yield. Extracellular polymeric substances (EPSs) exhibited higher capacitance and electroactivity to store or transfer electrons in the presence of RL. Proper RL facilitated pili-like filament formation and redox mediator secretion. The flavins and cytochrome c combination was promoted by RL to mediate one-electron transfer with a higher transfer rate via the flavin semiquinone intermediate. RL increased the dipole moment of the α-helix peptide and spontaneously interacted with the C═O of amide groups, enabling efficient electron hopping in EPSs. RL also activated key components in the intracellular electron transfer system, delivering more electron flow to sulfate reductase. Metagenomic and metatranscriptomic analyses verified the differential enrichment of microorganisms and key gene upregulation related to SSA, EPS secretion, quorum sensing, ATP, type IV pili, and electron shuttle synthesis. These findings provide new insight into the roles and interactive mechanisms of biosurfactants in modulating microbial electron transfer.