Abstract:

[Objective] Microbial reduction of iron oxides is a core process in the iron biogeochemical cycle under anaerobic conditions. Quinones, as key active components of soil organic matter, play important roles in regulating iron oxide transformation, yet their regulatory mechanisms remain unclear. This study aimed to investigate the concentration-dependent regulatory effects and mechanisms of the typical quinone compound 1,2-dihydroxy-9,10-anthraquinone (AIQ) on iron oxide biotransformation. [Methods] Three AIQ concentration gradients (0.02, 0.05 and 0.10 mmol∙L⁻¹) were established to construct an anaerobic reductive transformation system of iron oxide-AIQ coprecipitates mediated by Shewanella oneidensis MR-1. After 288 h of continuous incubation, combined analyses using X-ray diffraction (XRD), scanning electron microscopy (SEM), high-performance liquid chromatography (HPLC), and kinetic modeling were conducted to systematically investigate the effects of AIQ on mineral-phase evolution, the dynamics of different Fe(Ⅱ) species, and microbial metabolic activity during iron oxide bioreduction, and to elucidate the underlying mechanisms. [Results] XRD and SEM analyses revealed a significant concentration-dependent regulatory effect of AIQ on mineral-phase transformation. In the 0.02 mmol∙L⁻¹ AIQ treatment, lepidocrocite remained stable during the first 24 h; however, at 48 h, the characteristic diffraction peaks of lepidocrocite began to disappear, and new peaks corresponding to magnetite appeared, indicating the onset of reductive transformation. Magnetite content reached 10% at 48 h and increased to 74% at 288 h, accompanied by the progressive dissolution of lepidocrocite and crystal growth of magnetite, with particle sizes of 50-90 nm. In contrast, in the 0.05 and 0.10 mmol∙L⁻¹ AIQ treatments, no characteristic diffraction peaks of magnetite or other secondary iron oxides were observed throughout the 288-h incubation. Instead, lepidocrocite showed a “fluctuating increase” in crystallinity, particularly at low angles (10°-30°), with the 0.10 mmol∙L⁻¹ treatment exhibiting higher crystallinity than the 0.05 mmol∙L⁻¹ treatment. SEM images further confirmed that no secondary minerals formed at medium and high AIQ concentrations and that lepidocrocite grains became larger with increasing AIQ concentration. The total Fe(Ⅱ) concentration, which directly reflected the extent of microbial Fe(Ⅲ) reduction, increased rapidly during the first 24 h and then gradually reached a plateau. At 288 h, total Fe(Ⅱ) concentrations in the 0.02, 0.05, and 0.10 mmol·L⁻¹ AIQ treatments were 2.89, 2.48, and 2.31 mmol∙L⁻¹, respectively, showing a negative correlation with AIQ concentration. Kinetic fitting using pseudo-first-order, pseudo-second-order, and Elovich models showed that the pseudo-second-order model (R² > 0.99) best described the Fe(Ⅱ) reduction process, indicating that the reduction rate was controlled by multiple factors. The half-saturation times for the 0.02, 0.05, and 0.10 mmol·L⁻¹ AIQ treatments were 9.47, 7.37, and 6.38 h, respectively, demonstrating that higher AIQ concentrations led to shorter equilibration times. The Elovich model further revealed that the initial reaction rate constant (α) decreased from 0.505 to 0.36 mmol∙L⁻¹∙h⁻¹ with increasing AIQ concentration, while the rate-decay coefficient (β) also decreased, indicating that a low AIQ concentration promoted a higher initial reduction rate but a faster subsequent decline. Regarding Fe(Ⅱ) speciation, at the low AIQ concentration (0.02 mmol∙L⁻¹), aqueous Fe(Ⅱ) increased rapidly to 0.94 mmol∙L⁻¹ within 24 h and then decreased to 0.44 mmol∙L⁻¹ at 288 h, with its proportion decreasing from 58% to 15%. Structural Fe(Ⅱ) continuously increased to 1.93 mmol·L⁻¹, accounting for 68% of total Fe(Ⅱ), indicating that most biogenic Fe(Ⅱ) was ultimately incorporated into the mineral lattice. In contrast, at medium and high AIQ concentrations (0.05 and 0.10 mmol∙L⁻¹), aqueous Fe(Ⅱ) continuously increased to approximately 1.27 mmol∙L⁻¹, still accounting for 51%-52% of total Fe(Ⅱ) at 288 h, while structural Fe(Ⅱ) reached only approximately 0.78 mmol∙L⁻¹, accounting for 30%-32%. These results suggest that higher AIQ concentrations complexed with more Fe(Ⅱ), retaining it in the aqueous phase and preventing its effective adsorption onto mineral surfaces and incorporation into the crystal lattice. Microbial metabolic activity, as reflected by lactate consumption and acetate production, was significantly inhibited with increasing AIQ concentration. At 288 h, residual lactate concentrations in the 0.02, 0.05, and 0.10 mmol∙L⁻¹ AIQ treatments were 3.77, 4.03, and 4.48 mmol∙L⁻¹, respectively, while acetate production was 1.52, 1.14, and 0.91 mmol∙L⁻¹, respectively. First-order kinetic fitting of lactate consumption showed good fits (R²>0.95) for all treatments, with the rate constant increasing with AIQ concentration, indicating that a high AIQ concentration accelerated initial lactate consumption but led to earlier metabolic equilibrium and a higher residual substrate concentration. For acetate production, the goodness of fit (R²) decreased markedly from 0.94 to 0.44 at 0.10 mmol∙L⁻¹ AIQ, indicating deviation from first-order kinetics. Correlation analysis revealed that both lactate consumption and acetate production were significantly positively correlated with total Fe(Ⅱ) production (R²=0.76 and 0.92; t=5.535 and 12.279; p=9.621×10⁻⁵ and 6.960×10⁻⁵, respectively; both p<0.05), confirming a tight stoichiometric coupling between microbial metabolism and iron reduction. [Conclusion] Based on these results, AIQ regulates iron oxide biotransformation through multiple synergistic mechanisms in a concentration-dependent manner. At a low concentration (0.02 mmol∙L⁻¹), AIQ only slightly interferes with microbial iron reduction. Limited AIQ adsorption on iron oxide surfaces, low intracellular toxicity, and unobstructed electron transfer from lactate to Fe(Ⅲ) allow sufficient Fe(Ⅱ) to be adsorbed onto mineral surfaces and incorporated into the crystal lattice, providing the mineral-bound Fe(Ⅱ) necessary to trigger the phase transformation from lepidocrocite to magnetite. At medium and high concentrations (0.05 or 0.10 mmol∙L⁻¹), AIQ completely inhibits mineral transformation through three synergistic mechanisms: 1) adsorption onto iron oxide surfaces through hydrophobic interactions, hydrogen bonding, and electrostatic forces, occupying active sites and forming a physical barrier that hinders direct microbe-mineral contact and extracellular electron transfer; 2) complexation with biogenic Fe(Ⅱ), retaining more Fe(Ⅱ) in the aqueous phase and reducing effective Fe(Ⅱ) loading on mineral surfaces, thereby inhibiting secondary mineral nucleation; and 3) interference with microbial metabolic pathways, including potential intracellular toxicity, competition for electrons in the respiratory chain, and disruption of key enzymatic steps in acetate production, collectively reducing microbial metabolic activity and iron reduction efficiency. This study reveals the concentration-dependent mechanism of quinone-mediated iron oxide biotransformation, providing a scientific basis for advancing the theory of coupled soil iron cycling and organic matter dynamics and offering important insights into iron mineral transformation and organic carbon preservation in anaerobic soils and sediments.

Key words: iron reduction, mineral-phase transformation, 1,2-dihydroxy-9,10-anthraquinone, Shewanella oneidensis MR-1, kinetic model

摘要：

【目的】醌类化合物对铁氧化物微生物还原转化的调控机制尚不明确。本研究旨在探究典型醌类化合物1,2-二羟基-9,10-蒽醌（AIQ）对铁氧化物生物转化的浓度调控效应及作用机制。【方法】设置0.02、0.05、0.10 mmol∙L⁻¹等3个AIQ浓度，构建希瓦氏菌MR-1介导的铁氧化物-AIQ共沉淀厌氧还原体系，连续培养288 h，分析AIQ对矿物演变、不同形态Fe(Ⅱ)动态变化及微生物代谢的影响。【结果】仅0.02 mmol∙L⁻¹ AIQ促使纤铁矿在48 h还原转化为磁铁矿，其288 h时磁铁矿含量达74%，0.05和0.10 mmol∙L⁻¹则完全抑制该转化。总Fe(II)累积量随AIQ浓度升高而下降，从2.89 mmol∙L⁻¹下降至2.31 mmol∙L⁻¹；低浓度下Fe(II)主要赋存于矿物晶格，结构态占比68%；中高浓度下AIQ络合Fe(II)使其滞留溶液相，溶液态占比51%－52%，结构态Fe(II)降至30%－32%。随AIQ浓度升高，微生物对乳酸钠消耗和乙酸钠生成受显著抑制，0.10 mmol∙L⁻¹时乙酸钠生成偏离一级动力学，R²仅为0.44。乳酸钠消耗、乙酸钠生成与总Fe(II)呈显著正相关（R²=0.76、0.92，p<0.05），证实微生物代谢与铁还原紧密耦合。【结论】高浓度AIQ通过络合Fe(Ⅱ)、与微生物竞争吸附位点、阻碍次生矿物成核，干扰微生物铁还原代谢通路，从而抑制铁氧化物的矿物转化；低浓度AIQ则减轻抑制程度。

关键词: 铁还原, 晶相转化, 1,2-二羟基-9,10-蒽醌, 希瓦氏菌MR-1, 动力学模型