Molecular evidence for the adaptive evolution of Geobacter sulfurreducens to perform dissimilatory iron reduction in natural environments

Abstract

The electrically conductive pili (e-pili) of Geobacter species enable extracellular electron transfer to insoluble metallic minerals, electrodes and other microbial species, which confers biogeochemical significance and global prevalence on Geobacter in diverse anaerobic environments. E-pili are constructed by truncated PilA which is considered to have evolved from full-length pilin by gene fission under positive evolutionary selection. However, this hypothesis is based on phylogenetic analysis and has not yet been experimentally confirmed. Here, we reconstructed an ancestral strain of G. sulfurreducens (designated COMB) carrying full-length PilA by combining genes GSU1496 and GSU1497. The results demonstrated that strain COMB expressed and assembled the full-length fused PilA and exhibited an outer membrane c-type cytochrome profile similar to the wild-type strain. Surprisingly, the generated COMB-pili were also conductive, indicating the evolution of truncated PilA did not occur for conductivity. Moreover, strain COMB minimally reduced Fe(III) oxides but maintained its ability to respire electrodes, demonstrating the truncation of pilin enables iron respiration. This study provides the first experimental evidence that the truncation of pilin in Geobacter species confers adaption to Fe(III)-mineral-mediated selective pressures, and suggests an evolutionary event during which the separation of the GSU1497 gene helped Geobacter survive and thrive in natural environments. K E Y W O R D S adaptive evolution, dissimilatory iron reduction, e-pili, Geobacter sulfurreducens, truncated pilin S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section. How to cite this article: Liu X, Ye Y, Xiao K, Rensing C, Zhou S. Molecular evidence for the adaptive evolution of Geobacter sulfurreducens to perform dissimilatory iron reduction in natural environments. Mol Microbiol. 2020;113:783-793.

Cytochrome OmcS is not essential for long-range electron transport inGeobacter sulfurreducensstrain KN400

“…Also, construction of strains that produce an abundance of OmcS, but express synthetic pilins that yield poorly conductive pili, are defective in long-range electron transfer(18,(25)(26)29). The findings reported here, and in another recent study(30), show that deleting the gene for OmcS has no impact on the conductivity of the filaments harvested from G. sulfurreducens, further demonstrating that OmcS filaments cannot be the primary conductive protein nanowires expressed by G. sulfurreducens. Thus, the actual role of OmcS in G. sulfurreducens strain DL-1 warrants further study.Materials and MethodsBacterial strains and growth conditionsGeobacter sulfurreducens strain KN400 (20), G. metallireducens strain GS15, and the omcS strain of G. sulfurreducens DL-1 (6) were obtained from our laboratory culture collection.…”

Metagenomic insights into the metabolism of microbial communities that mediate iron and methane cycling in Lake Kinneret iron-rich methanic sediments

“…4, 36-52 % in transmembrane MHCs and 29-35 % in secreted MHCs). This is not surprising, as deltaproteobacterial lineages are known to reduce particulate metals through extracellular electron transfer (EET), and to conduct DIET (Leang et al, 2003;Reguera et al, 2005;Lovley, 2011;Adhikari et al, 2016). Other MHC sequences were associated with Nitrospirae, Chloroflexi, and Acidobacteria (both secreted and trans-membranal) and with Actinobacteria (secreted MHCs, Fig.…”

“…Our metagenomic results suggest that Lake Kinneret microbiota may transfer electrons directly to extracellular minerals, either through multiheme c-type cytochromes or microbial nanowires, according to mechanisms described in previous reviews (Lovley, 2011;Shi et al, 2016). We investigated the trans-membranal MHCs that are anchored in either the bacterial membrane or archaeal S layer, as well as secreted MHCs.…”

“…The vast majority of classified deltaproteobacterial OmcS hits were assigned as Desulfuromondales at the order level (representing 36-55 % overall and 66-100 % out of Deltaproteobacteria). The evidence for the presence of bacteria that transfer electrons via nanowires not only implies DIET but also indicates the potential of sediment microbiota to conduct EET using the particulate metals (Lovley, 2011;Liu et al, 2019b), and thus mediate iron and manganese reduction.…”

Impact of Fe(III) (Oxyhydr)oxides Mineralogy on Iron Solubilization and Associated Microbial Communities

“…Leang et al (2010) showed that OmcS, a cytochrome that is required for Fe(III) reduction by G. sulfurreducens, was localized along the pili (Leang et al, 2010). The electrically conductive pili play a major role in the adaptation of Geobacter to perform DIR in natural environments (Liu et al, 2019). These differences in the Fe(III)-reducing mechanisms between the two species might explain their difference of affinity for Fe(III) (Liu et al, 2001;Esteve-Núñez et al, 2005) and the relative increase of Geobacter abundance, observed here in presence of goethite and hematite, with the enrichment culture that was the most efficient for Fe-oxide solubilization, i.e., culture D3.…”