Photochemistry of iron in aquatic environments

Tri-layered core-shell structured deferoxamine magnetic particles promote Microcystis aeruginosa growth

This experiment prepared magnetic composite siderophores (DMPs) with strong magnetism, excellent adsorption capacity, and high specific surface area. Exploring the synergistic effect of magnetic nanoparticles and siderophores on Microcystis aeruginosa growth under iron-deficient condition, by utilizing the characteristics of the three-layer core-shell structure of DMPs. This study elucidated the potential mechanism by which DMPs promote the cyanobacterial growth through physiological indicators and transcriptome analysis. On the experiment's final day, cell density in DMPs treatment group at 2, 4, and 8 mg/L were 1.10, 1.14 and 1.16 times higher than those in the control group (Ct), respectively. Similarly, chlorophyll and photosynthetic efficiency results showed improved algae growth with increasing DMPs dosage. The microcystin content in DMPs experimental groups at low, medium, and high concentration were 0.91, 0.86, and 0.83 times that of Ct, indicating alleviation of iron deficiency stress. Additionally, based on extracellular polymers, intracellular and extracellular siderophores, and visualization techniques, DMPs nanoparticles captured free iron sources in the environment, promoting algae growth by entering algal cells and facilitating the uptake and utilization of free iron ions from the solution. During the experiment, the iron uptake and transport genes (feoA and feoB) were significantly upregulated, whereas the algal siderophore synthesis gene (pchF) and the TonB-dependent transport system gene (TonB_C) were significantly downregulated, suggesting heightened activity in intracellular iron uptake and transport. This indicates an abundance of intracellular iron, eliminating the need for secrete siderophores to overcome iron deficiency. Microcystis aeruginosa increased iron bioavailability by using iron transported through DMPs in the environment while internalizing these DMPs. This study explored the mechanism of this synergistic effect to boost algal growth, and provided new ideas for elucidating the mechanism of cyanobacterial bloom outbreaks as well as the innovative application of biotechnology.

TGF-β-driven LIF expression influences neutrophil extracellular traps (NETs) and contributes to peritoneal metastasis in gastric cancer

Gastric cancer (GC), notorious for its poor prognosis, often advances to peritoneal dissemination, a crucial determinant of detrimental outcomes. This study intricately explores the role of the TGFβ-Smad-LIF axis within the tumor microenvironment in propagating peritoneal metastasis, with a specific emphasis on its molecular mechanism in instigating Neutrophil Extracellular Traps (NETs) formation and encouraging GC cellular functions. Through a blend of bioinformatics analyses, utilizing TCGA and GEO databases, and meticulous in vivo and in vitro experiments, LIF was identified as pivotally associated with GC metastasis, notably, enhancing the NETs formation through neutrophil stimulation. Mechanistically, TGF-β was substantiated to elevate LIF expression via the activation of the Smad2/3 complex, culminating in NETs formation and consequently, propelling peritoneal metastasis of GC. This revelation uncovers a novel potential therapeutic target, promising a new avenue in managing GC and mitigating its metastatic propensities.

Mineral-mediated stability of organic carbon in soil and relevant interaction mechanisms

Soil, the largest terrestrial carbon reservoir, is central to climate change and relevant feedback to environmental health. Minerals are the essential components that contribute to over 60% of soil carbon storage. However, how the interactions between minerals and organic carbon shape the carbon transformation and stability remains poorly understood. Herein, we critically review the primary interactions between organic carbon and soil minerals and the relevant mechanisms, including sorption, redox reaction, co-precipitation, dissolution, polymerization, and catalytic reaction. These interactions, highly complex with the combination of multiple processes, greatly affect the stability of organic carbon through the following processes: (1) formation or deconstruction of the mineral-organic carbon association; (2) oxidative transformation of the organic carbon with minerals; (3) catalytic polymerization of organic carbon with minerals; and (4) varying association stability of organic carbon according to the mineral transformation. Several pieces of evidence related to the carbon turnover and stability during the interaction with soil minerals in the real eco-environment are then demonstrated. We also highlight the current research gaps and outline research priorities, which may map future directions for a deeper mechanisms-based understanding of the soil carbon storage capacity considering its interactions with minerals.

Fate of carbon influenced by the in-situ growth of phototrophic biofilms at the soil-water interface of paddy soil

Phototrophic biofilms (PBs) are commonly found in the sediment/soil-water interface of paddy soils and have a significant impact on carbon cycles. However, the specific carbon fate influenced by the in-situ growth of PBs in paddy soil remains unclear. In this study, we investigated the effect of in situ PBs growth on methane and carbon dioxide emissions, as well as dissolved organic matter (DOM) transformation. Our findings demonstrated a negative correlation between PBs growth and methane and carbon dioxide emissions, while showing a positive correlation with DOM composition. The in-situ growth of PBs reduced methane emissions by approximately 79 % and carbon dioxide emissions by approximately 33 % in the daytime, and also slowed down the degradation rate of dissolved organic matter from over 30.4 % to <16 %. Microsensor measurements revealed that these changes were attributed to the increased concentration and penetration depth of oxygen, as well as variations in pH caused by the growth of in situ PBs. Co-occurrence analysis indicated a robust correlation between DOM transformation and the significantly suppressed methanogenesis by methanogens such as Methanosaeta, Methanomassiliicoccus and Methanosarcina, and also the notably enhanced methane oxidation by methanotrophs including Methylobacterium, Methyloversatilis and Methylomonas, in response to the growth of PBs. These findings shed light on the impact of in situ PBs on methane and carbon dioxide emissions and DOM transformation, providing new insights for understanding carbon cycling in paddy soils.

New Insight into the Natural Detoxification of Cr(VI) in Fe-Rich Surface Soil: Crucial Role of Photogenerated Silicate-Bound Fe(II)

Photoexcitation of natural semiconductor Fe(III) minerals has been proven to generate Fe(II), but the photogeneration of Fe(II) in Fe-rich surface soil as well as its role in the redox biogeochemistry of Cr(VI) remains poorly understood. In this work, we confirmed the generation of Fe(II) in soil by solar irradiation and proposed a new mechanism for the natural reductive detoxification of Cr(VI) to Cr(III) in surface soil. The kinetic results showed that solar irradiation promoted the reduction of Cr(VI) in Fe-rich soils, while a negligible Cr(VI) reduction was observed in the dark. Fe(II), mainly in the form of silicate-bound Fe(II), was generated under solar irradiation and responsible for the reduction of Cr(VI) in soils, which was evidenced by sequential extraction, transmission electron microscopy with electron energy loss spectroscopy, and electron transfer calculation. Photogenerated silicate-bound Fe(II) resulted from the massive clay-iron (hydr)oxide associations, consisting of iron (hydr)oxides (e.g., hematite and goethite) and kaolinite. These associations could generate Fe(II) under solar irradiation either via intrinsic excitation to produce photoelectrons or via the ligand-to-metal charge transfer process after the formation of clay-iron (hydr)oxide-organic matter complexes, which was proven by photoluminescence spectroscopy and X-ray photoelectron spectroscopy. These findings highlight the important role of photogenerated Fe(II) in Cr(VI) reduction in surface soil, which advances a fundamental understanding of the natural detoxification of Cr(VI) as well as the redox biogeochemistry of Cr(VI) in soil.

Rapid Release of Halocarbons from Saline Water by Iron-Based Photochemistry

Methyl halides play important roles in stratospheric ozone depletion, but their formation mechanisms are not well defined. This study demonstrated that iron-based photochemistry significantly enhanced alkyl halide production by promoting the reaction of the representative monomer of lignin with halide ions in saline water under solar light irradiation. The methyl chloride (CH3Cl) emission from the light/Fe(III) process was 2 orders of magnitude higher than dark treatment and in the absence of iron. In addition, bromide and iodide showed better reactivity in the formation of the corresponding methyl bromide (CH3Br) and methyl iodide (CH3I). Alkyl halides identified from seawater, brackish water, and salt pan water under sunlight irradiation were positively correlated with the Fe(III) concentrations, indicating that iron-based photochemistry is ubiquitous. This work suggested that the photoinduced formation of methyl radical and redox cycling of iron triggered by the Fenton-like reaction are responsible for the enhanced release of alkyl halides. This study represents an abiotic formation pathway of alkyl halides, which accounts for a portion of the unidentified sources of halocarbons in the ocean.

Microbial iron acquisition is influenced by spatial and temporal conditions in a glacial influenced river and estuary system

In Arctic regions, glaciers are major sources of iron to rivers and streams; however, estuaries are considered iron sinks due to the coagulation and flocculation processes that occur at higher salinities. It is unknown how iron dynamics in a glacial influenced river and estuary environment affect microbial mechanisms for iron acquisition. Microbial taxonomic and functional sequencing was performed on samples taken throughout the year from the Kenai River and the estuary, Alaska. Despite distinct iron, sodium, and other nutrient concentrations, the river and estuary did not have statistically different microbial communities nor was time of sampling significant. However, ferrous iron transport (Feo) system genes were more abundant in river environments, while siderophore genes were more abundant and diverse in estuary environments. Siderophore transport and iron storage genes were found in all samples, but gene abundance and distribution were potentially influenced by physical drivers such as discharge rates and nutrient distributions. Differences in iron metabolism between river and estuary ecosystems indicate environmental conditions drive microbial mechanisms to sequester iron. This could have implications for iron transport as the Arctic continues to warm.

Iron transport mechanisms and their evolution focusing on chloroplasts

Iron (Fe) is an essential element for photosynthetic organisms, required for several vital biological functions. Photosynthesis, which takes place in the chloroplasts of higher plants, is the major Fe consumer. Although the components of the root Fe uptake system in dicotyledonous and monocotyledonous plants have been extensively studied, the Fe transport mechanisms of chloroplasts in these two groups of plants have received little attention. This review focuses on the comparative analysis of Fe transport processes in the evolutionary ancestors of chloroplasts (cyanobacteria) with the processes in embryophytes and green algae (Viridiplantae). The aim is to summarize how chloroplasts are integrated into cellular Fe homeostasis and how Fe transporters and Fe transport mechanisms have been modified by evolution.

Neodymium affects the generation of reactive oxygen species via GSK-3β/Nrf2 signaling in the gill of zebrafish

Rare earth element neodymium (Nd) is widely used in industry and agriculture, which may result in the pollution of aquatic environment. In this study, we exposed zebrafish with 10, 50, and 100 μg/L Nd for four weeks. The results showed that Nd could be accumulated in fish gill and Nd accumulation affected the equilibrium of nutrient elements. Nd decreased the antioxidant enzymes' activity and gene expression level, but enhanced the generation of reactive oxygen species (ROS). Moreover, various concentration of Nd treatments inhibited Nrf2 signaling in gill. To examine the critical role of GSK-3β/Nrf2 signaling on ROS generation under Nd stress, we further interfered gsk-3β gene in zebrafish under 100 μg/L Nd exposure. The result showed that gsk-3β gene interference induced Nrf2 signaling as well as the expression and activity of antioxidant enzymes in fish gill. In all, Nd could be accumulated in fish gill, and the signaling of GSK-3β/Nrf2 was involved in regulating ROS generation under Nd treatments.

Methane formation driven by light and heat prior to the origin of life and beyond

Methane is a potent greenhouse gas, which likely enabled the evolution of life by keeping the early Earth warm. Here, we demonstrate routes towards abiotic methane and ethane formation under early-earth conditions from methylated sulfur and nitrogen compounds with prebiotic origin. These compounds are demethylated in Fenton reactions governed by ferrous iron and reactive oxygen species (ROS) produced by light and heat in aqueous environments. After the emergence of life, this phenomenon would have greatly intensified in the anoxic Archean by providing methylated sulfur and nitrogen substrates. This ROS-driven Fenton chemistry can occur delocalized from serpentinization across Earth's humid realm and thereby substantially differs from previously suggested methane formation routes that are spatially restricted. Here, we report that Fenton reactions driven by light and heat release methane and ethane and might have shaped the chemical evolution of the atmosphere prior to the origin of life and beyond.

Hydroxyl radicals in natural waters: Light/dark mechanisms, changes and scavenging effects

Hydroxyl radicals (•OH) are the most active, aggressive and oxidative reactive oxygen species. In the natural aquatic environment, •OH plays an important role in the biogeochemistry cycle, biotransformation, and pollution removal. This paper reviewed the distribution and formation mechanism of •OH in aquatic environments, including natural waters, colloidal substances, sediments, and organisms. Furthermore, factors affecting the formation and consumption of •OH were thoroughly discussed, and the mechanisms of •OH generation and scavenging were summarized. In particular, the effects of climate change and artificial work on •OH in the largest natural aquatic environment, i.e., marine environment was analyzed with the help of bibliometrics. Moreover, Fenton reactions make the •OH variation more complicated and should not be neglected, especially in those areas with suspended particles and sediments. Regarding the •OH variation in the natural aquatic environment, more attention should be given to global change and human activities.

Photodegradation of organic micropollutants in aquatic environment: Importance, factors and processes

Photochemical reactions widely occur in the aquatic environment and play fundamental roles in aquatic ecosystems. In particular, solar-induced photodegradation is efficient for many organic micropollutants (OMPs), especially those that cannot undergo hydrolysis or biodegradation, and thus can mitigate chemical pollution. Recent reports indicate that photodegradation may play a more important role than biodegradation in many OMP transformations in the aquatic environment. Photodegradation can be influenced by the water matrix such as pH, inorganic ions, and dissolved organic matter (DOM). The effect of the water matrix such as DOM on photodegradation is complex, and new insights concerning the disparate effects of DOM have recently been reported. In addition, the photodegradation process is also influenced by physical factors such as latitude, water depth, and temporal variations in sunlight as these factors determine the light conditions. However, it remains challenging to gain an overview of the importance of photodegradation in the aquatic environment because the reactions involved are diverse and complex. Therefore, this review provides a concise summary of the importance of photodegradation and the major processes related to the photodegradation of OMPs, with particular attention given to recent progress on the major reactions of DOM. In addition, major knowledge gaps in this field of environmental photochemistry are highlighted.

Sunlight-Induced Interfacial Electron Transfer of Ferrihydrite under Oxic Conditions: Mineral Transformation and Redox Active Species Production

Fe(II)-catalyzed ferrihydrite transformation under anoxic conditions has been intensively studied, while such mechanisms are insufficient to be applied in oxic environments with depleted Fe(II). Here, we investigated expanded pathways of sunlight-driven ferrihydrite transformation in the presence of dissolved oxygen, without initial addition of dissolved Fe(II). We found that sunlight significantly facilitated the transformation of ferrihydrite to goethite compared to that under dark conditions. Redox active species (hole-electron pairs, reactive radicals, and Fe(II)) were produced from the ferrihydrite interface via the photoinduced electron transfer processes. Experiments with systematically varied wet chemistry conditions probed the relative contributions of three pathways for the production of hydroxyl radicals: (1) oxidation of water (5.0%); (2) reduction of dissolved oxygen (40.9%); and (3) photolysis of Fe(III)-hydroxyl complexes (54.1%). Results also showed superoxide radicals as the main oxidant for Fe(II) reoxidation under acidic conditions, thus promoting the ferrihydrite transformation. The presence of inorganic ions (chloride, sulfate, and nitrate) did not only affect the hydrolysis and precipitation of Fe(III) but also the generation of radicals via photoinduced charge transfer reactions. The involvement of redox active species and the accompanying mineral transformations would exert a profound effect on the fate of multivalent elements and organic contaminants in aquatic environments.

Light- and H2O2-Mediated Redox Transformation of Thallium in Acidic Solutions Containing Iron: Kinetics and Mechanistic Insights

The redox transformation between the oxidation states of thallium (Tl(I) and Tl(III)) is the key to influencing its toxicity, reactivity, and mobility. Dissolved iron (Fe) is widely distributed in the environment and coexists at a high level with Tl in acidic mine drainages (AMDs). While ultraviolet (UV) light and H₂O₂ can directly (by inducing Tl(III) reduction) and indirectly (by inducing Fe(III) to form reactive intermediates) impact the redox cycles of Tl in Fe(III)-containing solutions, the kinetics and mechanism remain largely unclear. This study is the first to investigate the UV light- and H₂O₂-mediated Tl redox kinetics in acidic Fe(III) solutions. The results demonstrate that UV light and H₂O₂ could directly reduce Tl(III) to Tl(I), with the extent of reduction dependent on the presence of Fe(III) and the solution pH. At pH 3.0, Tl(I) was completely oxidized to Tl(III), which can be ascribed to the generation of hydroxyl radicals (^•OH) from the Fe(III) photoreduction or Fe(III) reaction with H₂O₂. The kinetics of Tl(I) oxidation were strongly affected by the Fe(III) concentration, pH, light source, and water matrix. Kinetic models incorporating Tl redox kinetics with Fe redox kinetics were developed and satisfactorily interpreted Tl(III) reduction and Tl(I) oxidation under the examined conditions. These findings emphasize the roles of the UV light- and H₂O₂-driven Fe cycles in influencing the redox state of Tl, with implications for determining its mobility and fate in the environment.

The biological uptake of dissolved iron in the changing Daya Bay, South China Sea: Effect of pH and DO

The oceanic acidification and coastal hypoxia have potential to enhance biological uptake of dissolved iron (Fe) by phytoplankton. In this study, the Fe uptake rate (FeUR) in Daya Bay was significantly negatively correlated with pH and dissolved oxygen (DO) (r = -0.81 and -0.73, respectively, p < 0.001). In addition, binary regression (FeUR = -1.45 × pH - 0.10 × DO + 13.64) also indicated that both pH and DO played key roles in FeUR variations. As pH and DO decreased, Fe uptake by phytoplankton was promoted, and the contribution of nano-phytoplankton to Fe uptake increased significantly, while that of pico-FeUR decreased. These will result in the phytoplankton community to be miniaturized and Fe requirement of phytoplankton goes higher, thereby leading changes of phytoplankton composition and coastal ecosystem. This study helps to understand how Fe could affect the coastal ecosystem under the increasing anthropogenic influences.

Bacterial-type ferroxidase tunes iron-dependent phosphate sensing during Arabidopsis root development

Access to inorganic phosphate (Pi), a principal intermediate of energy and nucleotide metabolism, profoundly affects cellular activities and plant performance. In most soils, antagonistic Pi-metal interactions restrict Pi bioavailability, which guides local root development to maximize Pi interception. Growing root tips scout the essential but immobile mineral nutrient; however, the mechanisms monitoring external Pi status are unknown. Here, we show that Arabidopsis LOW PHOSPHATE ROOT 1 (LPR1), one key determinant of Fe-dependent Pi sensing in root meristems, encodes a novel ferroxidase of high substrate specificity and affinity (apparent K_M ∼ 2 μM Fe²⁺). LPR1 typifies an ancient, Fe-oxidizing multicopper protein family that evolved early upon bacterial land colonization. The ancestor of streptophyte algae and embryophytes (land plants) acquired LPR1-type ferroxidase from soil bacteria via horizontal gene transfer, a hypothesis supported by phylogenomics, homology modeling, and biochemistry. Our molecular and kinetic data on LPR1 regulation indicate that Pi-dependent Fe substrate availability determines LPR1 activity and function. Guided by the metabolic lifestyle of extant sister bacterial genera, we propose that Arabidopsis LPR1 monitors subtle concentration differentials of external Fe availability as a Pi-dependent cue to adjust root meristem maintenance via Fe redox signaling and cell wall modification. We further hypothesize that the acquisition of bacterial LPR1-type ferroxidase by embryophyte progenitors facilitated the evolution of local Pi sensing and acquisition during plant terrestrialization.

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Arabidopsis thaliana LPR1 multicopper oxidase typifies a novel ferroxidase cohort

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Fe availability tunes LPR1-dependent root responses to phosphate (Pi) limitation

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LPR1 specificity links Fe-Pi interactions to root Pi sensing via redox cycling

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Streptophyte ancestors acquired LPR1-type ferroxidase from soil bacteria by HGT

Influence of Fe(III) source, light quality, photon flux and presence of oxygen on photoreduction of Fe(III)-organic complexes - Implications for light-influenced coastal freshwater and marine sediments

Iron(III) photoreduction is an important source of Fe(II) in illuminated aquatic and sedimentary environments. Under oxic conditions, the Fe(II) can be re-oxidized by oxygen (O₂) forming reactive O-species such as hydrogen peroxide (H₂O₂) which further react with Fe(II) thus enhancing Fe(II) oxidation rates. However, it is unknown by aquatic sediments how the parameters wavelength of radiation, photon flux, origin of Fe(III) source and presence or absence of O₂ influence the extent of Fe(II) and H₂O₂ turnover. We studied this using batch experiments with different Fe(III)-organic complexes mimicking sedimentary conditions. We found that wavelengths <500 nm are necessary to initiate Fe(III) photoreduction and that the photon flux, wavelength and identity of Fe(III)-complexing organic acids control the kinetics of Fe(III) photoreduction. The formation of photo-susceptible Fe(III)-organic complexes did not depend on whether the Fe(III) source was biogenically produced, poorly-crystalline Fe(III) oxyhydroxides or chemically synthesized ferrihydrite. Oxic conditions caused chemical re-oxidation of Fe(II) and accumulation of H₂O₂. The photon flux, wavelength and availability of Fe(III)-complexing organic molecules are critical for the balance between concurrent Fe(III) photoreduction and abiotic Fe(II) oxidation and may even lead to a steady-state concentration of Fe(II) in the micromolar range. These results help understand and predict Fe(III) photoreduction dynamics and in-situ formation of Fe(II) in oxic or anoxic, illuminated and organic-rich environments.

Photo-Fenton Oxidation of Methyl Orange Dye Using South African Ilmenite Sands as a Catalyst

In this study, the viability of South African ilmenite sands as a catalyst in the photo-Fenton-like degradation of methyl orange (MO) dye was investigated. The mineralogy and other properties of the material were characterized. Complete decolorization occurred under acidic conditions (pH < 4) in the presence of ilmenite and H2O2. Light irradiation accelerated the rate of reaction. Parameter optimization revealed that a pH of 2.5, UVB irradiation, 2 g/L catalyst loading, and a hydrogen peroxide concentration of 1.0 mM were required. Under these conditions, complete decolorization was observed after 45 min. Degradation kinetics were best described by the pseudo-first order (PFO) model. Rate constants of 0.095 and 0.034 min−1 were obtained for 5 and 20 mg/L MO concentrations, respectively. A 37% total organic carbon removal was observed after 60 min. This suggests a stepwise MO degradation pathway with intermediate formation rather than complete mineralization. Although iron leaching was detected, the mineralogy of the catalyst recovered after the reaction was similar to the fresh catalyst.

Application of UV radiation for in-situ Cr(VI) reduction from contaminated soil with electrokinetic remediation

Restoring hexavalent chromium (Cr(VI)) to trivalent chromium (Cr(III)) from contaminated soil is a cost-effective alternative for attenuating Cr(VI) toxicity to the ecosystem. A new electrokinetic remediation (EKR) system with UV light was explored to overcome an energy barrier to catalyze Cr(VI) reduction from the surface soil near the anodic reservoir. Natural organic matters and minerals from the contaminated soil acted as electron donors and catalysts for Cr(VI) photo-reduction and no additional chemical reagent. There was almost no residual Cr(VI) in anolyte after UV/EKR compared with the conventional EKR. The reduction improved the efficiency of EKR in the soil near the anodic reservoir by dropped the Cr(VI) negative mass flux caused by electroosmosis advection and concentration diffusion. The pathways of Cr(VI) photo-reduction are possibly dominated by ligand-to-metal charge transfer, i.e., photocatalytic cyclic reduction by Fe(III)/Fe(II) complexes on the surface of the minerals and in soil pore fluid and the photo-induced decomposition of chromate ester. It is concluded that UV/EKR is a clean, efficient, and low-cost method for remediation of Cr(VI)-contaminated soil.

Bioinspired Photoredox Benzylation of Quinones

3-Benzylmenadiones were obtained in good yield by using a blue-light-induced photoredox process in the presence of Fe(III), oxygen, and γ-terpinene acting as a hydrogen-atom transfer agent. This methodology is compatible with a wide variety of diversely substituted 1,4-naphthoquinones as well as various cheap, readily available benzyl bromides with excellent functional group tolerance. The benzylation mechanism was investigated and supports a three-step radical cascade with the key involvement of the photogenerated superoxide anion radical.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Heavy metal and organic load in Haripur creek of Gopalpur along the Bay of Bengal, east coast of India

Accumulation of heavy metals in zooplanktons is used as an essential tool for the evaluation of health of an ecosystem. Such data are used to set further monitoring strategies especially in (coastal) water bodies. In the present study, seasonal bioaccumulation of heavy metals such as Cu, Zn, Pb, Cd and Hg, organic loads such as nitrite and phosphate, contents loads and physicochemical parameters in Haripur creek along the Bay of Bengal in east coast of India is measured in zooplanktons. Coastal organisms are considered an integral part of marine ecosystems and their frequent massive die-off events along the coast of the Bay of Bengal are correlated with the obtained pollutant data. The levels of heavy metals were also correlated with several extrinsic environmental factors such as water transparency, dissolved oxygen, salinity, pH and temperature. Discriminant function analyses and standardised coefficients for canonical variables for the obtained data indicate that the studied environmental factors and organic loads varied as a function of season. Bioaccumulation of the studied heavy metals in zooplanktons was seasonal and infrequently noticed above the standard limit. The computed bioaccumulation factor of the five metals showed that accumulation of Zn was higher in zooplanktons as compared to that of the other studied metals. So, the entry of heavy metals through the food chains or via direct exposure (to organic loads also) to the other inhabiting organisms including fish may be assumed. This could be one of the possible reasons for the observed frequent (fish) die-off events at Gopalpur Coast along the Bay of Bengal and may be considered a clue for future investigations.

Enhancement of Iron-Based Photo-Driven Processes by the Presence of Catechol Moieties

Photo-induced Advanced Oxidation Processes (AOPs) using H2O2 or S2O82− as radical precursors were assessed for the abatement of six different contaminants of emerging concern (CECs). In order to increase the efficiency of these AOPs at a wider pH range, the catechol organic functional compound was studied as a potential assistant in photo-driven iron-based processes. Different salinity regimes were also studied (in terms of Cl− concentration), namely low salt water (1 g·L−1) or a salt–water (30 g·L−1) matrix. Results obtained revealed that the presence of catechol could efficiently assist the photo-Fenton system and partly promote the photo-induced S2O82− system, which was highly dependent on salinity. Regarding the behavior of individual CECs, the photo-Fenton reaction was able to enhance the degradation of all six CECs, meanwhile the S2O82−-based process showed a moderate enhancement for acetaminophen, amoxicillin or clofibric acid. Finally, a response-surface methodology was employed to determine the effect of pH and catechol concentration on the different photo-driven processes. Catechol was removed during the degradation process. According to the results obtained, the presence of catechol in organic macromolecules can bring some advantages in water treatment for either freshwater (wastewater) or seawater (maritime or aquaculture industry).

An evolving view on biogeochemical cycling of iron

Biogeochemical cycling of iron is crucial to many environmental processes, such as ocean productivity, carbon storage, greenhouse gas emissions and the fate of nutrients, toxic metals and metalloids. Knowledge of the underlying processes involved in iron cycling has accelerated in recent years along with appreciation of the complex network of biotic and abiotic reactions dictating the speciation, mobility and reactivity of iron in the environment. Recent studies have provided insights into novel processes in the biogeochemical iron cycle such as microbial ammonium oxidation and methane oxidation coupled to Fe(III) reduction. They have also revealed that processes in the biogeochemical iron cycle spatially overlap and may compete with each other, and that oxidation and reduction of iron occur cyclically or simultaneously in many environments. This Review discusses these advances with particular focus on their environmental consequences, including the formation of greenhouse gases and the fate of nutrients and contaminants.

From the Ocean to the Lab-Assessing Iron Limitation in Cyanobacteria: An Interface Paper

Iron is an essential, yet scarce, nutrient in marine environments. Phytoplankton, and especially cyanobacteria, have developed a wide range of mechanisms to acquire iron and maintain their iron-rich photosynthetic machinery. Iron limitation studies often utilize either oceanographic methods to understand large scale processes, or laboratory-based, molecular experiments to identify underlying molecular mechanisms on a cellular level. Here, we aim to highlight the benefits of both approaches to encourage interdisciplinary understanding of the effects of iron limitation on cyanobacteria with a focus on avoiding pitfalls in the initial phases of collaboration. In particular, we discuss the use of trace metal clean methods in combination with sterile techniques, and the challenges faced when a new collaboration is set up to combine interdisciplinary techniques. Methods necessary for producing reliable data, such as High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS), Flow Injection Analysis Chemiluminescence (FIA-CL), and 77K fluorescence emission spectroscopy are discussed and evaluated and a technical manual, including the preparation of the artificial seawater medium Aquil, cleaning procedures, and a sampling scheme for an iron limitation experiment is included. This paper provides a reference point for researchers to implement different techniques into interdisciplinary iron studies that span cyanobacteria physiology, molecular biology, and biogeochemistry.