Iron plaque formation and its effect on key elements cycling in constructed wetlands: Functions and outlooks

Controlling factors of iron plaque formation and its adsorption of cadmium and arsenic throughout the entire life cycle of rice plants

Iron (Fe) plaque, which forms on the surface of rice roots, plays a crucial role in immobilizing heavy metal(loids), thus reducing their accumulation in rice plants. However, the principal factors influencing Fe plaque formation and its adsorption capacity for heavy metal(loid)s throughout the rice plant's lifecycle remain poorly understood. Thus, this study investigated the dynamics of Fe plaque formation and its ability to adsorb cadmium (Cd) and arsenic (As) across different growth stages, aiming to identify the key drivers behind these processes. The findings reveal that the rate of radial oxygen loss (ROL) and the abundance of plaque-associated microbes are the primary drivers of Fe plaque formation, with their relative importance ranging from 1.4% to 81%. Similarly, the adsorption of As by Fe plaque is principally determined by the rate of ROL and the quantity of Fe plaque, with subsequent effects from the total Fe in rhizospheric soil, arsenate-reducing bacteria, and organic matter-degrading bacteria. The relative importance of these factors ranges from 6.0% to 11.7%. By contrast, the adsorption of Cd onto Fe plaque is primarily affected by competition for adsorption sites with ammonium in soils and the presence of organic matter-degrading bacteria, contributing 25.5% and 23.5% to the adsorption process, respectively. These findings provide significant insights into the development of Fe plaque and its absorption of heavy metal(loid)s throughout the lifecycle of rice plants.

DNA-stable isotope probing and metagenomics reveal Fe(II) oxidation by core microflora in microoxic rhizospheric habitats to mitigate the accumulation of cadmium and phenanthrene in rice

Rice rhizosphere soil-porewater microdomains exist within an iron (Fe)-rich microoxic habitat during paddy soil flooding. However, the response mechanisms of core microflora in this habitat to Fe(II)-oxidation-mediated cadmium (Cd) and phenanthrene (Phen) remain unclear. Using gel-stabilized gradient systems to replicate the microoxic conditions in the rice rhizosphere porewater, we found that microaerophilic rhizobacteria drove Fe(II) oxidation to yield iron oxides, thereby reducing the Cd and Phen contents in the rhizosphere porewater and rice (Cd and Phen decreased by 15.9%-78.0% and 10.1%-37.4%, respectively). However, co-exposure to Cd and Phen resulted in a greater reduction in the Cd uptake and a greater increase in the Phen uptake in rice as compared to those in the Cd or Phen treatments, possibly attributing to the cation-π interactions between Cd and Phen, as well as competition between the adsorption sites on the roots. The elevation of Cd-tolerant genes and Phen-degradation genes in biogenic cell-mineral aggregates unveiled the survival strategies of rhizobacteria with respect to Cd and Phen in the microoxic habitat. Potential Cd-tolerant rhizobacteria (e.g., Pandoraea and Comamonas) and Phen-degradation rhizobacteria (e.g., Pseudoxanthobacter) were identified through the DNA-SIP and 16S rRNA gene amplicon sequencing. Metagenomic analysis further confirmed that these core microbes harbor Cd-tolerant, Phen-degradation, and Fe(II) oxidation genes, supporting their metabolic potential for Cd and/or Phen in the microoxic habitat of the rice rhizosphere. These findings suggest the potential mechanism and ecological significance of core rhizospheric microbial-driven Fe(II) oxidation in mitigating the bioavailability of Cd and Phen in paddy soil during flooding.

Unveiling the microplastic perturbation on surface flow constructed wetlands with macrophytes of different life forms: Responses of nitrogen removal and sensory quality

Microplastics (MPs) discharging into constructed wetlands pose risks to these ecosystems. Nevertheless, the perturbation of MPs to different types of macrophytes, which play important roles in purifying pollutants of wetlands, has not been fully elucidated. In this study, polystyrene MPs (PS-MPs) perturbation on nitrogen removal and sensory quality of surface flow constructed wetlands planted with emergent and submerged macrophytes were investigated. PS-MPs enhanced N removal efficiencies temporarily, whereas the N removal rate constants were declined as exposure time was prolonged. The NH4+-N removal rate constants declined by 25.78 % and 34.03 % in E and S groups respectively. The NO3--N removal rate constants declined by 22.13 % in the S groups. Denitrifiers including Thiobacillus, Rhodobacter, and Sulfuritalea were stressed. The sensory quality deteriorated after PS-MPs exposure, which was significantly related to changes in Chlorophyll a, particle size distribution, and colored dissolved organic matter. Turbidity in E groups and chroma in S groups were greatly affected by PS-MPs. Overall, under MPs exposure, macrophytes in E groups were more suitable for nitrogen removal, and macrophytes in S groups better purified the turbidity. The study could provide the basis for better allocation of macrophytes in CWs to reduce the purifying risk by PS-MPs disturbance.

Weakening of sulfate removal by aquatic plants in iron-based constructed wetlands: The rhizosphere is a sink or source of sulfur?

The fate of sulfur (S) was controlled by a complex interaction of abiotic and microbial reactions in constructed wetlands (CWs). Although zero-valent iron (ZVI) was generally considered to promote nitrogen (N) and S cycle by providing electrons, but its binding effect on sulfate (SO42--S) removal with the rhizosphere oscillating redox conditions had not been determined. This study found that the presence of plants increased SO42-_S removal in Con-CW, while decreased it by 3.93 % in ZVI-CW accompanied by the decrease of S content in the rhizosphere substrates. The enrichment of S oxidation genes (soxA/Y and yedZ), organic S decomposition genes (aslA) and plants radial oxygen loss (ROL) accelerated the transformation of solid-phase S to SO42--S, resulting in ZVI-CW turn from S sink to S source. Overall, the source-sink transformation provided a theoretical guidance for comprehending S cycling in CWs.

Mechanism for Fe(III) to decrease cadmium uptake of wheat plant: Rhizosphere passivation, competitive absorption and physiological regulation

The presence of dissolved Fe(III) and Fe(III)-containing minerals has been found to alleviate cadmium (Cd) accumulation in wheat plants grown in Cd-contaminated soils, but the specific mechanism remains elusive. In this work, hydroponic experiments were conducted to dissect the mechanism for dissolved Fe(III) (0-2000 μmol L-1) to decrease Cd uptake of wheat plants and study the influence of Fe(III) concentration and Cd(II) pollution level (0-20 μmol L-1) on the Cd uptake process. The results indicated that dissolved Fe(III) significantly decreased Cd uptake through rhizosphere passivation, competitive absorption, and physiological regulation. The formation of poorly crystalline Fe(III) oxides facilitated the adsorption and immobilization of Cd(II) on the rhizoplane (over 80.4 %). In wheat rhizosphere, the content of CaCl2-extractable Cd decreased by 52.7 % when Fe(III) concentration was controlled at 2000 μmol L-1, and the presence of Fe(III) may reduce the formation of Cd(II)-organic acid complexes (including malic acid and succinic acid secreted by wheat roots), which could be attributed to competitive reactions. Down-regulation of Cd uptake genes (TaNramp5-a and TaNramp5-b) and transport genes (TaHMA3-a, TaHMA3-b and TaHMA2), along with up-regulation of the Cd efflux gene TaPDR8-4A7A, contributed much to the reduction of Cd accumulation in wheat plants in the presence of Fe(III). The inhibitory effect of Fe(III) on Cd uptake and transport in wheat plants declined with increasing Cd(II) concentration, particularly at 20 μmol L-1. This work provides important implications for remediating Cd-contaminated farmland soil and ensuring the safe production of wheat by using dissolved Fe(III) and Fe(III)-containing minerals.

Biochemical mechanisms underlying iron plaque-mediated phosphorus accumulation and uptake in rice roots

The iron oxyhydroxides of iron plaque on the surface of rice root are crucial for the uptake of nutrition elements, especially phosphorus (P), but the effects of iron oxyhydroxides of iron plaque on the accumulation and uptake of P remain largely unknown. In this study, we investigated the regulatory mechanism of iron plaque on P uptake in rice via hydroponics of whole plant and simulation of iron oxyhydroxides-coated suspension cells in rice. The hydroponic experiment results showed that the presence of iron plaque increased the P content in rice shoots. The simulation experiment results further confirmed that after iron plaque coating, the P contents in the whole cell and on the cell wall were significantly increased from 5.16 mg/g and 2.73 mg/g to 8.85 mg/g and 5.27 mg/g, respectively. In addition, our data also showed that iron plaque coating led to an increase in cell surface potentials from -380 ± 40 mV to -200 ± 30 mV, thus promoting the adsorption of more P. Taken together, this study demonstrated that the iron plaque coating increased the surface potential of the cells, thus enhancing cellular P enrichment, eventually promoting P efficient adsorption in rice. Deciphering these regulatory mechanisms provide an insight into P biogeochemical cycling at the soil-plant interface and offer theoretical basis and practical references for the improvement of P bioavailability in rice production.

Iron Plaque: A Shield against Soil Contamination and Key to Sustainable Agriculture

Soils play a dominant role in supporting the survival and growth of crops and they are also extremely important for human health and food safety. At present, the contamination of soil by heavy metals remains a globally concerning environmental issue that needs to be resolved. In the environment, iron plaque, naturally occurring on the root surface of wetland plants, is found to be equipped with an excellent ability at blocking the migration of heavy metals from soils to plants, which can be further developed as an environmentally friendly strategy for soil remediation to ensure food security. Because of its large surface-to-volume porous structure, iron plaque exhibits high binding affinity to heavy metals. Moreover, iron plaque can be seen as a reservoir to store nutrients to support the growth of plants. In this review, the formation process of iron plaque, the ecological role that iron plaque plays in the environment and the interaction between iron plaque, plants and microbes, are summarized.

Root-mediated acidification, phosphatase activity and the phosphorus-cycling microbial community enhance phosphorus mobilization in the rhizosphere of wetland plants

Rhizoremediation of wetland plants is an environmentally friendly strategy for sediment phosphorous (P) removal, the basic underlying principle of which is the complex interactions between roots and microorganisms. This study investigated the immobilization and mobilization mechanisms of P in the rhizosphere of wetland plants using high-resolution spatial visualization techniques and metagenomic sequencing. Two-dimensional visualization of the spatial distribution of P, iron (Fe) and manganese (Mn) indicated that the sequestration of Fe-oxides rather than Mn-oxides caused the depletion of labile P, resulting in an increase in the Fe-adsorbed P fraction. Plants altered the rhizospheric environments and P-cycling microbial community to mobilize low-availability P from sediments. Mineral P solubilization and organic P mineralization were enhanced by local acidification and increased phosphatase activity, respectively. Microbial P mobilization also increased with increasing relative abundances of P solubilization and mineralization genes (gcd and phnW) and decreasing P transportation genes (ugpA, ugpB, and pit) genes in the rhizosphere. These processes led to the remobilization of 10.04 % of inorganic P, and 15.23 % of organic P, in the rhizosphere during the incubation period. However, the resupply of P via the above processes did not compensate for the depletion of rhizospheric P via root uptake and mineral sequestration. Our results provide novel insights into the mechanisms of rhizospheric P cycling, which will help to inform future phytoremediation strategies.

The introduction of influent sulfamethoxazole loads induces changes in the removal pathways of sulfamethoxazole in vertical flow constructed wetlands featuring hematite substrate

High frequent detection of sulfamethoxazole (SMX) in wastewater cannot be effectively removed by constructed wetlands (CWs) with a traditional river sand substrate. The role of emerging substrate of hematite in promoting SMX removal and the effect of influent SMX loads remain unclear. The removal efficiency of SMX in hematite CWs was significantly higher than that in river sand CWs by 12.7-13.8% by improving substrate adsorption capacity, plant uptake and microbial degradation. With increasing influent SMX load, the removal efficiency of SMX in hematite CWs slightly increased, and the removal pathways varied significantly. The contribution of plant uptake was relatively small (< 0.1%) under different influent SMX loads. Substrate adsorption (37.8%) primarily contributed to SMX removal in hematite CWs treated with low-influent SMX. Higher influent SMX loads decreased the contribution of substrate adsorption, and microbial degradation (67.0%) became the main removal pathway. Metagenomic analyses revealed that the rising influent load increased the abundance of SMX-degrading relative bacteria and the activity of key enzymes. Moreover, the abundance of high-risk ARGs and sulfonamide resistance genes in hematite CWs did not increase with the increasing influent load. This study elucidates the potential improvements in CWs with hematite introduction under different influent SMX loads.

Unraveling the mechanism of interaction: accelerated phenanthrene degradation and rhizosphere biofilm/iron plaque formation influenced by phenolic root exudates

Phenolic root exudates (PREs) secreted by wetland plants facilitate the accumulation of iron in the rhizosphere, potentially providing the essential active iron required for the generation of enzymes that degrade polycyclic aromatic hydrocarbon, thereby enhancing their biodegradation. However, the underlying mechanisms involved are yet to be elucidated. This study focuses on phenanthrene (PHE), a typical polycyclic aromatic hydrocarbon pollutant, utilizing representative PREs from wetland plants, including p-hydroxybenzoic, p-coumaric, caffeic, and ferulic acids. Using hydroponic experiments, 16S rRNA sequencing, and multiple characterization techniques, we aimed to elucidate the interaction mechanism between the accelerated degradation of PHE and the formation of rhizosphere biofilm/iron plaque influenced by PREs. Although all four types of PREs altered the biofilm composition and promoted the formation of iron plaque on the root surface, only caffeic acid, possessing a similar structure to the intermediate metabolite of PHE (catechol), could accelerate the PHE degradation rate. Caffeic acid, notable for its catechol structure, plays a significant role in enhancing PHE degradation through two main mechanisms: (a) it directly boosts PHE co-metabolism by fostering the growth of PHE-degrading bacteria, specifically Burkholderiaceae, and by facilitating the production of the key metabolic enzyme catechol 1,2-dioxygenase (C12O) and (b) it indirectly supports PHE biodegradation by promoting iron plaque formation on root surfaces, thereby enriching free iron for efficient microbial synthesis of C12O, a crucial factor in PHE decomposition.

Flooding regimes alleviate lead toxicity and enhance phytostabilization of salix: Evidence from physiological responses and iron-plaque formation

Aggravated metal pollution in wetland and riparian zones has become a global environmental issue, necessitating the identification of sustainable remediation approaches. Salix exhibits great potential as a viable candidate for metal(loid) remediation. However, the underlying mechanisms for its effectiveness in different flooding regimes with Pb pollution have not been extensively studied. In this study, fast-growing Salix×jiangsuensis 'J172' was selected and planted in different Pb polluted soils (control, 400 and 800 mg ∙ kg-1) under non-flooded and flooded (CF: continuous flooding and IF: intermittent flooding) conditions for 60 days. This study aimed to explore the effects of flooding on Salix growth performance, physiological traits, and the relationship between Pb uptake/translocation and root Fe plaques. Salix×jiangsuensis 'J172' exhibited excellent tolerance and adaptation to Pb pollution with a tolerance index (TI) exceeding 0.6, even at the highest Pb levels. Moreover, the TIs under flooded conditions were higher than that under non-flooded conditions, suggesting that flooding could alleviate Pb toxicity under co-exposure to Pb and flooding. Leaf malondialdehyde (MDA) exhibited a dose-dependent response to Pb exposure; however, CF or IF mitigated the oxidative damage induced by Pb toxicity with decreased MDA content (2.2-11.9%). The superoxide dismutase and peroxidase activities were generally enhanced by flooding, but combined stress (flooding and Pb) significantly decreased catalase activity. Pb was predominantly accumulated in Salix roots, and flooding markedly increased root Pb accumulation by 19.2-173.0% compared to non-flooded condition. Additionally, a significant positive correlation was observed between the iron (Fe) content of the root plaque and root Pb accumulation, indicating that the formation of Fe plaque on the root surface could enhance the phytostabilization of Pb in Salix. The current findings highlight that fast-growing woody plants are suitable for phyto-management of metal-polluted wetlands and can potentially minimize the risk of metal mobility in soils.

Iron plaque formation and its influences on the properties of polyethylene plastic surfaces in coastal wetlands: Abiotic factors and bacterial community

Iron (Fe) plaques in coastal wetlands are widely recognized because of their strong adsorption affinity for natural particles, but their interaction behaviors and mechanisms with plastics remain unknown. Through laboratory incubation experiments, paired with multiple characterization methods and microbial analysis, this work focused on the characteristics of Fe plaques on low-density polyethylene plastic surfaces and their relationship with environmental factors in coastal wetlands (Mangrove and Spartina alterniflora soil). The results showed that iron plaques increased the adhesive force of the plastic surface from 65.25 to 300 nN and promoted the oxidation of the plastic surface. Fe plaque formation was stimulated by salinity, anaerobic conditions, natural organic matter, and a weak alkaline scenario (pH 8.0-8.3). The Fe content showed a stable positive correlation with heavy metals loading (i.e., As, Mn, Co, Cr, Pb, and Zn). Furthermore, we revealed that Fe plaque was positively regulated by Nitrospirae through 16S rRNA high-throughput sequencing analysis. Meanwhile, Verrucomicrobia and Kiritimatiellaeota. may act as depressants by consuming salt. This work illustrated that iron plaques could enhance the role of plastics in contaminant migration by altering their adsorption performance, providing new insights into plastic interface behavior and potential ecological effects in coastal wetlands.