Project description:Nodule-forming bacteria play crucial roles in plant health and nutrition by fixing atmospheric nitrogen. Despite the importance of this relationship, how nodule-forming bacteria are affected by plant exudates and soil minerals is not fully characterized. Of particular interest are the effects of plant-derived methanol and lanthanide metals on the growth of nitrogen-fixing Rhizobiales. Prior work has demonstrated that select Bradyrhizobium are able to assimilate methanol only in the presence of lanthanide metals; however, the pathway enabling assimilation remains unknown. Here we characterize Bradyrhizobium sp. USDA 3456 to determine the pathways involved in methanol metabolism. Based on genomic analyses, we hypothesized that methanol assimilation in these organisms occurs via the lanthanide-dependent methanol dehydrogenase XoxF, followed by oxidation of formaldehyde via the glutathione-linked oxidation pathway, subsequent oxidation of formate via formate dehydrogenases, and finally assimilation of CO2 via the Calvin Benson Bassham (CBB) pathway. Transcriptomics revealed upregulation of the aforementioned pathways in Bradyrhizobium sp. USDA 3456 during growth on methanol. Assays demonstrated increased activity of the glutathione-linked oxidation pathway and formate dehydrogenases during growth on methanol compared to succinate. 13C-labeling studies demonstrate the presence of CBB intermediates and label incorporation during growth on methanol. Our findings provide multiple lines of evidence supporting the proposed XoxF-CBB pathway and, combined with genomic analyses, suggest that this metabolism is widespread among Bradyrhizobium and Sinorhizobium species.
Project description:Microbial extracellular electron uptake (EEU) is central to bioelectrochemical processes and biocorrosion, yet its molecular mechanisms remain incompletely understood. Here, we investigate how excess Fe2+ modulates EEU in Desulfovibrio ferrophilus IS5, a strain that causes severe anaerobic iron corrosion via outer-membrane cytochromes (OMCs)-mediated electron uptake. We show that IS5 grown with elevated Fe2+ exhibits substantially enhanced EEU. This enhancement arises through two complementary mechanisms: (i) increased abundance of functional OMCs via upregulation of a cytochrome assembly protein, and (ii) an additional electron transfer route mediated by FeS nanoparticles precipitated on the IS5 outer membrane. Remarkably, IS5 with low OMCs expression but biosynthesized FeS can rapidly shift to EEU before OMCs induction. These findings suggest that during iron corrosion, when IS5 cells are embedded within thick corrosion crusts and biofilms and face both high Fe2+ concentrations and organic limitation, they exploit OMCs and FeS nanoparticles in parallel to sustain high-rate EEU from iron. This study advances the mechanistic understanding of EEU-driven iron corrosion and highlights a potential avenue for manipulating bioelectrochemical systems.