Zimeng Wang

Fe(II) can be oxidized by mixotrophic nitrate-reducing Fe(II)-oxidizing (NRFeOx) bacteria to Fe(III), leading to mineral precipitation. However, the effects of this process on the metabolism and reproductive ability of Fe(II)-oxidizing bacterial cells have not yet been quantified. In this study, we investigated the effects of Fe(II) on the mixotrophic NRFeOx bacteria Acidovorax sp. BoFeN1 by performing cell plate counts, chemical analyses, scanning electron microscopy (SEM) imaging, live/dead fluorescence staining, and proteomic analysis. The results showed that nitrate-reducing Fe(II)-oxidizing bacteria Acidovorax sp. BoFeN1 undergo a striking physiological state in which respiration persists while cell reproduction collapses after their oxidation of Fe(II). Across 0.1-10 mM Fe(II), colony-forming units dropped by up to 10³-fold, yet acetate consumption remained largely unaffected, and nitrate reduction was even slightly promoted. SEM-EDS revealed extensive Fe(III) encrustation on cells, while live/dead staining showed intact inner membranes where nitrate reduction occurs. Adding an Fe(III) chelator, citrate, restored reproductive capacity without notable changes in Fe(II) oxidation or nitrate reduction, indicating that it was the products of Fe(II) oxidation, the secondary Fe(III) minerals, that impaired cell reproduction. The phenotypes observed in this study were supported by proteomic analyses, which showed that Fe(II) increased the abundance of proteins involved in nitrate reduction and the respiratory electron transport chain. These findings uncover a mineral encrustation-driven uncoupling between respiration and reproduction, suggest how natural organic ligands may buffer NRFeOx bacterial communities in Fe-rich anoxic environments, and point to cellular-level levers for mitigating denitrification failures and N₂O risks in engineered and natural systems.

Iron sulfide minerals are critical mediators of uranium (U) immobilization in anoxic environments, yet the electron transfer mechanisms across Fe- and S-containing phases remain incompletely understood. Here, we demonstrate that redox-driven structural transformations unlock a dual pathway for U(VI) reduction. Comparative experiments using pristine mackinawite (FeS), partially oxidized FeS (O-FeS), and sulfur-enriched FeS (S-FeS) revealed that FeS and O-FeS reduce U(VI) primarily through oxidation of structural S(-II), whereas sulfidation-induced structural alterations in S-FeS activate otherwise inert Fe(II) as a coreductant, as evidenced by Fe(III) formation. This dual electron-transfer pathway shows pH dependence. At pH 6.5, U(VI) reduction to U(V)/U(IV) is nearly complete in S-FeS, while the reduction extent decreases to 34% at pH 8.5, due to the formation of a passivated surface layer rich in Fe(III) and sulfur that inhibits further electron transfer. These findings demonstrate that oxidation- and sulfidation-driven variations in FeS stoichiometry and structure regulate uranium reduction and immobilization pathways, with important implications for predicting the fate of redox-sensitive metal contaminants in dynamic subsurface environments.

Microplastics (MPs) are emerging soil pollutants that can disrupt essential biogeochemical processes, yet their effects on phosphorus (P) cycling remain underexplored. Here, we conducted a 150-day incubation experiment using agricultural soil amended with either polyethylene (PE, conventional) or polylactic acid (PLA, biodegradable) MPs to investigate their impact on microbially mediated P cycling. MPs altered soil P cycling and decreased available phosphorus (AP) by ∼15% after 90 days. Fourier transform infrared spectroscopy revealed weakened AP-associated functional groups (P-O-P, P-O, and P═O), most pronounced under PLA treatment. These shifts were accompanied by reduced abundances of key P-cycling taxa (Bacillus, Paenibacillus, and Sphingomonas) and downregulation of phosphatase gene abundance (phoA/D/X: -65.4% in PE, -59.8% in PLA). Correspondingly, the activities of acid, neutral, and alkaline phosphatases were all suppressed, with alkaline phosphatase in PE-treated soil reduced by 34.1%. Together, these results demonstrate that MPs disturb biotic transformation pathways, leading to subsequent alterations in the chemical speciation of soil P and decreased AP content. Notably, significant disruption was observed for both conventional and biodegradable types. Our findings challenge the prevailing assumption of environmental benignity for biodegradable plastics and underscore the urgent need for mechanistic assessments of their byproducts. Such disruption may hinder microbial P mobilization and decrease fertilizer use efficiency, ultimately threatening soil health and agricultural sustainability.