The extreme halophilic archaeon Haloferax volcanii reduced Fe(III) to conserve energy from extracellular electron transfer

ABSTRACT Iron availability affects oceanic primary productivity and the strength of the biological carbon pump. However, the mechanism of iron solubility in hypersaline environments is poorly understood. In this work, we investigated the extremely halophilic archaeon Haloferax volcanii to reduce ferrihydrite under 25% salinity, aiming to elucidate its energy metabolism associated with extracellular electron transfer (EET). Ferrihydrite reduction by H. volcanii reached a plateau around the eighth day, with an Fe(III) conversion of approximately 41%. The redox-active substances on the cell surface play a central role via EET. Proteomic analysis revealed that the relative abundance of membrane-bound quinoline b -type cytochrome oxidases (A0A841HB82, A0A841HBL7) increased by 73.8% and 120%, respectively, when H. volcanii used ferrihydrite rather than O 2 as the terminal electron acceptor. Moreover, H. volcanii was found to secrete riboflavin, which can function as an electron shuttle mediating indirect EET. Our findings provide insights into the survival and evolutionary strategies of early life in anaerobic environments on Earth, and the EET capability of H. volcanii highlights its potential applicability in the treatment of pollutants in hypersaline wastewater. IMPORTANCE As global warming intensifies, increased evaporation and seawater intrusion are leading to the expansion of high-salinity environments. Dissolved iron in these environments plays a crucial role in regulating the biogeochemical carbon cycle, thereby influencing global carbon dynamics. Microbial dissimilatory Fe(III) reduction may be the primary process driving the biogeochemical cycling of iron in such hypersaline environments. This study demonstrates that the extreme halophile Haloferax volcanii reduces ferrihydrite under high-salinity, anaerobic conditions. This reduction occurs primarily via membrane-associated cytochromes and secreted riboflavin. Consequently, investigating the extracellular electron transfer (EET) capabilities of halophilic archaea may provide new insights into the origin and maintenance of bioavailable iron under high-salinity stress.