Gallionellaceae pangenomic analysis reveals insight into phylogeny, metabolic flexibility, and iron oxidation mechanisms

Abstract

The iron-oxidizing Gallionellaceae drive a wide variety of biogeochemical cycles through their metabolisms and biominerals. To better understand the environmental impacts of Gallionellaceae, we need to improve our knowledge of their diversity and metabolisms, especially any novel iron oxidation mechanisms. Here, we used a pangenomic analysis of 103 genomes to resolve Gallionellaceae phylogeny and explore their genomic potential. Using a concatenated ribosomal protein tree and key gene patterns, we determined Gallionellaceae has four genera, divided into two groups: iron-oxidizing bacteria (FeOB) Gallionella , Sideroxydans , and Ferriphaselus with iron oxidation genes ( cyc2 , mtoA ) and nitrite-oxidizing bacteria (NOB) Candidatus Nitrotoga with the nitrite oxidase gene nxr . The FeOB and NOB have similar electron transport chains, including genes for reverse electron transport and carbon fixation. Auxiliary energy metabolisms, including S oxidation, denitrification, and organotrophy, were scattered throughout the FeOB. Within FeOB, we found genes that may represent adaptations for iron oxidation, including a variety of extracellular electron uptake mechanisms. FeOB genomes encoded more predicted c -type cytochromes than NOB genomes, notably more multiheme c -type cytochromes (MHCs) with >10 CXXCH motifs. These include homologs of several predicted outer membrane porin-MHC complexes, including MtoAB and Uet. MHCs efficiently conduct electrons across longer distances and function across a wide range of redox potentials that overlap with mineral redox potentials, which can expand the range of usable iron substrates. Overall, the results of pangenome analyses suggest that the Gallionellaceae genera Gallionella , Sideroxydans , and Ferriphaselus have acquired a range of adaptations to succeed in various environments but are primarily iron oxidizers. IMPORTANCE: Neutrophilic iron-oxidizing bacteria (FeOB) produce copious iron (oxyhydr)oxides that can profoundly influence biogeochemical cycles, notably the fate of carbon and many metals. To fully understand environmental microbial iron oxidation, we need a thorough accounting of iron oxidation mechanisms. In this study, we show the Gallionellaceae FeOB genomes encode both characterized iron oxidases as well as uncharacterized multiheme cytochromes (MHCs). MHCs are predicted to transfer electrons from extracellular substrates and likely confer metabolic capabilities that help Gallionellaceae occupy a range of different iron- and mineral-rich niches. Gallionellaceae appear to specialize in iron oxidation, so it would be advantageous for them to have multiple mechanisms to oxidize various forms of iron, given the many iron minerals on Earth, as well as the physiological and kinetic challenges faced by FeOB. The multiple iron/mineral oxidation mechanisms may help drive the widespread ecological success of Gallionellaceae.