Alkane degradation coupled to Fe(III) reduction mediated by Gram-positive bacteria

Petroleum hydrocarbon contamination, such as n-alkanes, poses a significant global threat to ecosystems and human health. Microbial remediation emerges as a promising strategy for addressing this issue through both aerobic and anaerobic processes. Notably, the majority of anaerobic hydrocarbon degraders identified to date are Gram-negative bacteria. In this study, two electroactive Gram-positive strains, Lysinibacillus spp. strains SL-6A and SL-12A, were isolated from oil-contaminated soils in the Shengli Oilfield, China. Our experiments demonstrated that these strains effectively degraded n-hexadecane (n-C₁₆) through extracellular Fe(III) reduction. When ferric citrate was used as the electron acceptor, strains SL-6A and SL-12A degraded 94.2 % and 87.4 % of n-C₁₆, respectively, within 72 hours. This process was further confirmed using Fe(III)-containing minerals. Surface-enhanced Raman spectroscopy, UV-vis spectroscopy, and cyclic voltammetry data collectively indicated that surface-associated c-type cytochromes (c-Cyts) were crucial for extracellular electron transfer (EET), facilitating Fe(III) reduction. In addition, our strains were capable of producing flavin mononucleotide (FMN), a well-known redox-active organic molecule involved in EET processes, particularly in the presence of Fe(III). Whole-genome sequencing confirmed the pathways for n-alkane degradation and the synthesis of c-Cyts and FMN in our strains. This research highlights the potential of electroactive Gram-positive bacteria in hydrocarbon degradation in contaminated soils.