Facilitating extracellular respiration of Methanosarcina barkeri with magnetite nanoparticle: Formation of conductive magnetite nanoparticle-membrane complex.

Extracellular electron transfer (EET) by type I Methanosarcina is relevant to bioenergy recovery, greenhouse gas emissions, environmental remediation and metal corrosion. However, its extracellular respiration is limited by low methanophenazine (MP) levels in the cell membrane, resulting in poor conductivity and restricted electron flux. Here, we grew Methanosarcina barkeri, the most studied species in type I Methanosarcina, with methanol as a sole electron donor and anthraquinone-2,6-disulfonate (AQDS) as a sole electron acceptor. Nearly all AQDS was reduced in the presence of magnetite nanoparticles, whereas less than 50% was reduced in their absence. The increase in concentration of cell proteins and number of gene copies of mcrA was accompanied by the increase in concentration of magnetite nanoparticles. Cell morphology showed that magnetite nanoparticles highly aggregated to form a single sheet and covered on cell surface, and did not penetrate the cells. When magnetite nanoparticles were replaced by semiconductive α-hematite nanoparticles, the AQDS-dependent extracellular respiration was also facilitated, but the increase was significantly weaker. Conducting atomic force microscopy (CAFM) and scanning electrochemical microscope (SECM) showed that magnetite nanoparticles formed conductive magnetite nanoparticle-membrane complexes on the cell surface, thereby enhancing extracellular electron transfer. Transcriptomics showed that expression of most of genes for membrane-bound MP-dependent hydrogenase (Vht), F420H2 dehydrogenase (Fpo) and heterodisulfide reductase (HdrDE) was significantly upregulated. These results suggest a novel model for AQDS-dependent extracellular respiration in M. barkeri induced by magnetite nanoparticles, improving mechanistic understanding of conductive nanoparticle-mediated extracellular electron transfer in this archaeon.