Metabolic trade-offs in <i>Desulfosporosus meridiei</i> : dose-dependent FeS nanoparticles modulate extracellular electron transfer and gene expression for mineral transformation

Biogenic ferrous sulfide nanoparticles (FeS NPs) regulate sulfate (SO₄^2-)-reducing bacteria (SRB)-driven iron/sulfur cycling in SO₄^2--rich anaerobic environments, yet their dose-dependent impacts on SRB metabolism remain unclear. This study revealed how FeS NPs dose modulates <i>Desulfosporosus meridiei</i> (a model SRB) in reducing schwertmannite (Sch). SRB preferentially reduced Fe(III) over SO₄^2- in Sch <i>via</i> FeS NPs-mediated extracellular electron transfer (EET). At low FeS doses (0-6 mM), the <i>dsr</i> gene expression (sulfur metabolism) associated with mineral transformation increased despite a decline in SRB abundance, accompanied by a significant enhancement in Fe(III) reduction rate, yielding siderite and pyrite as dominant products. This enhancement was attributed to FeS NPs acting as electron conduits, as evidenced by a 4-9-fold surge in bio-current intensity. However, at high FeS doses (≥6 mM), nanoparticle aggregation formed a relatively thick mineral encrustation on cell surfaces, blocking EET pathways and leaving goethite as a residual phase. Strikingly, SRB exhibited a metabolic trade-off, suppressing population growth to amplify <i>dsr</i>-driven electron flux under FeS stress. This adaptive strategy underscored SRB's resilience in FeS-rich environments while highlighting dose-dependent bifurcations in mineral transformation pathways. This study provided a new insight into manipulating SRB-dominated biogeochemical processes by controlling FeS NPs dose.