Photochemical cycling of iron in the surface ocean mediated by microbial iron(III)-binding ligands

Sunlight drives the abiotic formation of nitrous oxide in fresh and marine waters

Nitrous oxide (N2O) is a potent greenhouse gas and the main stratospheric ozone-depleting agent, yet its sources are not well resolved. In this work, we experimentally show a N2O production pathway not previously considered in greenhouse gas budgets, which we name photochemodenitrification. Sunlight induces substantial and consistent N2O production under oxic abiotic conditions in fresh and marine waters. We measured photochemical N2O production rates using isotope tracers and determined that nitrite is the main substrate and that nitrate can also contribute after being photoreduced to nitrite. Additionally, this N2O production was strongly correlated to the radiation dose. Photochemodenitrification exceeded biological N2O production in surface waters. Although previously overlooked, this process may contribute considerably to global N2O emissions through its occurrence in fresh and marine surface waters.

Microbial competition for iron determines its availability to the ferrous wheel

Iron plays a pivotal role in regulating ocean primary productivity. Iron is supplied from diverse sources such as the atmosphere and the geosphere, and hence iron biogeochemical research has focused on identifying and quantifying such sources of "new" iron. However, the recycling of this new iron fuels up to 90% of the productivity in vast oceanic regions. Evidence points to the key role of microbes in mediating this recycling, referred to as the "ferrous wheel", that remobilises iron initially supplied to ocean biota. In the iron-limited subantarctic waters of the Southern Ocean, iron uptake is dominated by microbes smaller than 2 μm and exhibits seasonal and depth-related variations. The microbial community within the <2 μm size fraction comprises heterotrophic bacteria and picophytoplankton, both competing for iron. Here, we dissect the demand component of the ferrous wheel by separately assessing iron uptake by heterotrophic bacteria and photoautotrophic picophytoplankton. To explore the seasonal and depth-related variability in iron uptake, the influence of light on iron uptake in both bacterial and phytoplankton communities was examined. We observed that picoeukaryote phytoplankton demonstrated iron uptake rates 10 times greater than those observed in bacteria when normalized to biomass. Light was shown to stimulate iron uptake by 8- to 16-fold in phytoplankton and by 4- to 8-fold in heterotrophic bacteria. These results highlight the unexpectedly significant role of picoeukaryotic phytoplankton in driving the speed of the ferrous wheel, with implications for iron recycling across diurnal cycles, different oceanic depths, and seasonally.

DFT and TD-DFT studies to elucidate the configurational isomers of ferric aerobactin, ferric petrobactin, and their ferric photoproducts

Iron-chelating siderophores such as aerobactin and petrobactin are produced by marine bacteria to sequester iron under low iron stress. Those that contain a citrate moiety undergo light-catalyzed ligand-to-metal charge transfer, inducing decarboxylation and formation of photoproducts. In this work, we employed density functional theory to obtain the optimized geometries and determine the relative energies and geometric parameters of different configurations of Fe(III)-coordinated aerobactin, petrobactin, and their photoproducts. Time-dependent density functional theory was then used to compute the UV-Vis absorption spectra of these species, and the comparison against experimental spectra further elucidated the structural configurations most likely to be adopted by these compounds. Frequency calculations provided Fe-O force constants on the same order as other siderophores. The relative energies and predicted spectra support the cis-cis C-fac configuration for ferric aerobactin and the cis-trans C-mer configuration for its photoproduct, while only mild support is found for specific configurations of the ferric petrobactin structures (meta-mer and meta-fac for the precursor, cis-cis para-fac for the photoproduct). The predicted ferric petrobactin spectra are found to be fairly insensitive to the configuration of the ferric complex.

Cyanohydrin Equilibria Implicate Non-Aromatic Aldehydes in Photochemical Production of Oceanic Carbon Monoxide

Carbonyls have previously been dismissed as significant precursors for carbon monoxide (CO) photoproduction from natural chromophoric dissolved organic matter (CDOM). Here, we used hydrogen cyanide (HCN), which reacts with carbonyls to form photochemically inert cyanohydrins, as a probe to re-examine the role of carbonyls in CO photoproduction. Adding HCN to low-absorbance euphotic zone seawater decreased CO photoproduction. Modeling [HCN] (∼5 to 364 μM) vs the percent decrease in CO photoproduction (%CO↓) yielded carbonyl-cyanohydrin dissociation equilibrium constants, KD, and maximum %CO↓, %CO↓max values. Four Atlantic and Pacific seawater KDs (66.7 ± 19.6 μM) overlap aqueous aliphatic but not aromatic aldehyde KDs. Phenylacetaldehyde (PA) and other β,γ-unsaturated aldehydes are proposed as prototypical CO precursors. Direct photolysis of ∼10 nM PA can supply the measured daily production of HCN-sensitive CO at an open-ocean site near Bermuda. HCN's %CO↓max was 31 ± 2.5% in North Atlantic seawater vs the 13 ± 2.5% inhibition of CO photoproduction by borohydride, a dilemma since only borohydride affects most ketones. Borohydride also decreased CDOM absorption much more than did HCN. This puzzle probably reflects differing steric and solvation requirements in HCN- and borohydride-CDOM reactions. This study demonstrates cyanophilic aldehydes to be a significant source of open-ocean CO and reveals new clues regarding CDOM photochemistry mechanisms.

Whole-Cell Biosensor for Iron Monitoring as a Potential Tool for Safeguarding Biodiversity in Polar Marine Environments

Iron is a key micronutrient essential for various essential biological processes. As a consequence, alteration in iron concentration in seawater can deeply influence marine biodiversity. In polar marine environments, where environmental conditions are characterized by low temperatures, the role of iron becomes particularly significant. While iron limitation can negatively influence primary production and nutrient cycling, excessive iron concentrations can lead to harmful algal blooms and oxygen depletion. Furthermore, the growth of certain phytoplankton species can be increased in high-iron-content environments, resulting in altered balance in the marine food web and reduced biodiversity. Although many chemical/physical methods are established for inorganic iron quantification, the determination of the bio-available iron in seawater samples is more suitably carried out using marine microorganisms as biosensors. Despite existing challenges, whole-cell biosensors offer other advantages, such as real-time detection, cost-effectiveness, and ease of manipulation, making them promising tools for monitoring environmental iron levels in polar marine ecosystems. In this review, we discuss fundamental biosensor designs and assemblies, arranging host features, transcription factors, reporter proteins, and detection methods. The progress in the genetic manipulation of iron-responsive regulatory and reporter modules is also addressed to the optimization of the biosensor performance, focusing on the improvement of sensitivity and specificity.

The iron(III) coordinating properties of citrate and α-hydroxycarboxylate containing siderophores

The iron(III) binding properties of citrate and rhizoferrin, a citrate containing siderophore, are compared. Citrate forms many oligonuclear complexes, whereas rhizoferrin forms a single mononuclear complex. The α-hydroxycarboxylate functional group, which is present in both citrate, and rhizoferrin, has a high affinity and selectivity for iron(III) under most biological conditions. The nature of the toxic form of iron found in the blood of patients suffering from many haemoglobinopathies and haemochromatosis is identified as a mixture of iron(III)citrate complexes. The significance of the presence of this iron pool to patients suffering from systemic iron overload is discussed. The wide utilisation of the α-hydroxycarboxylate functional group in siderophore structures is described, as is their photo-induced decarboxylation leading to the release of iron(II) ions. The importance of this facile dissociation to algal iron uptake is discussed.

New variochelins from soil-isolated Variovorax sp. H002

The soil bacterial genus Variovorax produce distinct photoreactive siderophores that may play a crucial role in the iron cycle within the rhizosphere. This study focused on exploring the natural products of the soil-isolated Variovorax sp. H002, leading to the isolation of variochelins A-E (1-5), a series of lipohexapeptide siderophores. NMR and MS/MS analyses revealed that these siderophores share a common core structure - a linear hexapeptide with β-hydroxyaspartate and hydroxamate functional groups, serving in iron-binding coordination. Three new variochelins C-E (3-5) were characterized by varied fatty acyl groups at their N-termini; notably, 4 and 5 represent the first variochelins with N-terminal unsaturated fatty acyl groups. Furthermore, the variochelin biosynthetic gene cluster was identified through draft genome sequencing and gene knockout experiments. Compounds 1-5 exhibited antimicrobial activities against Gram-negative bacteria, including several soil-isolated plant pathogens.

Impact of atmospheric deposition on the dynamics of ultraphytoplanktonic populations in the Gulf of Gabès during an intense dust event (MERITE-HIPPOCAMPE campaign)

The ultraphytoplankton composition and dynamics were assessed during a Saharan dust event occurring off the southern Tunisian coasts during the MERITE-HIPPOCAMPE Trans-Mediterranean oceanographic cruise. The composition of atmospheric dust was characterized in terms of nutriments and trace metals. Data-assimilative hydrodynamic model revealed no differences in the hydrological features along the sampling track and almost no water transport occurred during the period of atmospheric deposition. Dust deposition increased the growth rates and the productivity of the major phytoplanktonic cytometric groups, resulting in the highest surface biomass along the Mediterranean transect. One group, distinguished by low fluorescence and nanoplanktonic size, reacted to dust deposition within hours, exhibiting the highest growth rate and net productivity. The dust composition showed a substantial enrichment with organic phosphorous representing (56 % of Total phosphorus) and trace metals mainly Fe, Mn and V.

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.

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.

Tistrellabactins A and B Are Photoreactive C-Diazeniumdiolate Siderophores from the Marine-Derived Strain Tistrella mobilis KA081020-065

The C-diazeniumdiolate group in the amino acid graminine is emerging as a new microbially produced Fe(III) coordinating ligand in siderophores, which is photoreactive. While the few siderophores reported from this class have only been isolated from soil-associated microbes, here we report the first C-diazeniumdiolate siderophores tistrellabactins A and B, isolated from the bioactive marine-derived strain Tistrella mobilis KA081020-065. The structural characterization of the tistrellabactins reveals unique biosynthetic features including an NRPS module iteratively loading glutamine residues and a promiscuous adenylation domain yielding either tistrellabactin A with an asparagine residue or tistrellabactin B with an aspartic acid residue at analogous positions. Beyond the function of scavenging Fe(III) for growth, these siderophores are photoreactive upon irradiation with UV light, releasing the equivalent of nitric oxide (NO) and an H atom from the C-diazeniumdiolate group. Fe(III)-tistrellabactin is also photoreactive, with both the C-diazeniumdiolate and the β-hydroxyaspartate residues undergoing photoreactions, resulting in a photoproduct without the ability to chelate Fe(III).

Insight into the effect of natural organic matter on the photooxidation of arsenite induced by colloidal ferric hydroxides in water

Surface complexation of arsenite (As(III)) on colloidal ferric hydroxide (CFH) plays an important role not only in the adsorptive immobilization of As(III) but also in the subsequent oxidation of As(III) to arsenate (As(V)) through light-induced ligand-to-metal charge transfer (LMCT) in water at near-neutral pH. However, the effects of natural organic matter (NOM), especially humic substances (HSs) and low molecular weight carboxylic acids (CAs), on the photochemistry of the CFH-As(III) system have not been sufficiently understood. In this work, the inhibition of photooxidation of As(III) in terms of the observed apparent rate constant (k_obs) by six HSs (below 16 mg L^-1) and seven CAs (below 2.5 mM) has been observed in water containing 66 μM Fe(III) and 5 μM As(III) at pH 7 under simulated solar irradiation consisting of UVA (λ_max 365 nm) and UVB (λ_max 313 nm) lights. Total inhibition factors (T) have been determined from the combined effect of light-screening factor (S) and competitive complexation factor (C), wherein both S and C varied with NOM concentration. S was obtained by determining the absorbance of NOM, and C was obtained by fitting modified Langmuir or Freundlich models to the amount of As(III) desorbed from CFH upon the addition of NOM. Statistical analysis between the experimental T_exp and the calculated one according to T_cal = S × C showed that the Freundlich model (RMSE for HS 0.1609 and for CA 0.1771) was better than the Langmuir model and was statistically robust (Q_LOO²= 0.691 > 0.5). This work provided an estimation method for the effects of NOM on As(III) photooxidation in the presence of CFH as well as a deeper understanding of the transformation of arsenic species in sunlit water.

Isolation of dissolved organic matter from aqueous solution by precipitation with FeCl3: mechanisms and significance in environmental perspectives

Ferric ions can bind strongly with dissolved organic matter (DOM), including humic acids (HA), fulvic acids (FA), and protein-like substances, whereas isolation of Fe-DOM precipitates (Fe-DOM_P) and their biochemical characteristics remain unclear. In this work FeCl₃ was used to isolate DOM components from various sources, including river, lake, soil, cow dung, and standard tryptophan and tyrosine, through precipitation at pH 7.5-8.5. The Fe-DOM_P contribute to total DOM by approximately 38.6-93.8% of FA, 76.2% of HA and 25.0-30.4% of tryptophan and tyrosine, whilst fluorescence spectra allowed to monitor/discriminate the various DOM fractions in the samples. The relative intensity of the main infrared peaks such as 3406‒3383 cm^-1 (aromatic OH), 1689‒1635 cm^-1 (‒COOH), 1523-1504 cm^-1 (amide) and 1176-1033 cm^-1 (‒S=O) show either to decline or disappear in Fe‒DOM_P. These results suggest the occurrence of Fe bonds with various functional groups of DOM, indicating the formation of π-d electron bonding systems of different strengths in Fe‒DOM_P. The novel method used for isolation of Fe-DOM_P shows promising in opening a new frontier both at laboratory and industrial purposes. Furthermore, results obtained may provide a better understanding of metal-organic complexes involved in the regulation of the long-term stabilization/sequestration of DOM in soils and waters.

Benzilbis(2-hydroxyanil) – highly efficient ligand for ferric ion (Fe3+) sensing

Benzilbis(2-hydroxyanil) was found to be an efficient and selective turn-on fluorophore for ferric ion (Fe3+) sensing.

C-Diazeniumdiolate Graminine in the Siderophore Gramibactin Is Photoreactive and Originates from Arginine

Siderophores are synthesized by microbes to facilitate iron acquisition required for growth. Catecholate, hydroxamate, and α-hydroxycarboxylate groups comprise well-established ligands coordinating Fe(III) in siderophores. Recently, a C-type diazeniumdiolate ligand in the newly identified amino acid graminine (Gra) was found in the siderophore gramibactin (Gbt) produced by Paraburkholderia graminis DSM 17151. The N-N bond in the diazeniumdiolate is a distinguishing feature of Gra, yet the origin and reactivity of this C-type diazeniumdiolate group has remained elusive until now. Here, we identify l-arginine as the direct precursor to l-Gra through the isotopic labeling of l-Arg, l-ornithine, and l-citrulline. Furthermore, these isotopic labeling studies establish that the N-N bond in Gra must be formed between the Nδ and Nω of the guanidinium group in l-Arg. We also show the diazeniumdiolate groups in apo-Gbt are photoreactive, with loss of nitric oxide (NO) and H+ from each d-Gra yielding E/Z oxime isomers in the photoproduct. With the loss of Gbt's ability to chelate Fe(III) upon exposure to UV light, our results hint at this siderophore playing a larger ecological role. Not only are NO and oximes important in plant biology for communication and defense, but so too are NO-releasing compounds and oximes attractive in medicinal applications.

Phytoplankton Responses to Bacterially Regenerated Iron in a Southern Ocean Eddy

In the Subantarctic sector of the Southern Ocean, vertical entrainment of iron (Fe) triggers the seasonal productivity cycle but diminishing physical supply during the spring to summer transition forces microbial assemblages to rapidly acclimate. Here, we tested how phytoplankton and bacteria within an isolated eddy respond to different dissolved Fe (DFe)/ligand inputs. We used three treatments: one that mimicked the entrainment of new DFe (Fe-NEW), another in which DFe was supplied from bacterial regeneration of particles (Fe-REG), and a control with no addition of DFe (Fe-NO). After 6 days, 3.5 (Fe-NO, Fe-NEW) to 5-fold (Fe-REG) increases in Chlorophyll a were observed. These responses of the phytoplankton community were best explained by the differences between the treatments in the amount of DFe recycled during the incubation (Fe-REG, 15% recycled c.f. 40% Fe-NEW, 60% Fe-NO). This additional recycling was more likely mediated by bacteria. By day 6, bacterial production was comparable between Fe-NO and Fe-NEW but was approximately two-fold higher in Fe-REG. A preferential response of phytoplankton (haptophyte-dominated) relative to high nucleic acid (HNA) bacteria was also found in the Fe-REG treatment while the relative proportion of diatoms increased faster in the Fe-NEW and Fe-NO treatments. Comparisons between light and dark incubations further confirmed the competition between picophytoplankton and HNA for DFe. Overall, our results demonstrate great versatility by microorganisms to use different Fe sources that results in highly efficient Fe recycling within surface waters. This study also encourages future research to further investigate the interactions between functional groups of microbes (e.g. HNA and cyanobacteria) to better constraint modeling in Fe and carbon biogeochemical cycles.

Siderophores as an iron source for picocyanobacteria in deep chlorophyll maximum layers of the oligotrophic ocean

Prochlorococcus and Synechococcus are the most abundant photosynthesizing organisms in the oceans. Gene content variation among picocyanobacterial populations in separate ocean basins often mirrors the selective pressures imposed by the region's distinct biogeochemistry. By pairing genomic datasets with trace metal concentrations from across the global ocean, we show that the genomic capacity for siderophore-mediated iron uptake is widespread in Synechococcus and low-light adapted Prochlorococcus populations from deep chlorophyll maximum layers of iron-depleted regions of the oligotrophic Pacific and S. Atlantic oceans: Prochlorococcus siderophore consumers were absent in the N. Atlantic ocean (higher new iron flux) but constituted up to half of all Prochlorococcus genomes from metagenomes in the N. Pacific (lower new iron flux). Picocyanobacterial siderophore consumers, like many other bacteria with this trait, also lack siderophore biosynthesis genes indicating that they scavenge exogenous siderophores from seawater. Statistical modeling suggests that the capacity for siderophore uptake is endemic to remote ocean regions where atmospheric iron fluxes are the smallest, especially at deep chlorophyll maximum and primary nitrite maximum layers. We argue that abundant siderophore consumers at these two common oceanographic features could be a symptom of wider community iron stress, consistent with prior hypotheses. Our results provide a clear example of iron as a selective force driving the evolution of marine picocyanobacteria.

Effect of Photoreduction of Semiconducting Iron Mineral-Goethite on Microbial Community in the Marine Euphotic Zone

Marine euphotic zone is the pivotal region for interplay of light-mineral-microorganism and elements cycle, in which semiconducting minerals exist widely and iron-bearing goethite is a typical and widespread one. In this work, we have conducted in-depth researches on the effect of ferrous [Fe(II)] ions dissolved by photoreduction of goethite on microbial community structure and diversity. The mineral phase, structure and morphology of synthesized goethite were characterized by Raman, X-ray diffraction (XRD), energy disperse spectroscopy (EDS), environmental scanning electron microscope (ESEM), and atomic force microscope (AFM). Photoelectrochemical measurements tested photoelectric response and redox activity of goethite, having proved its significant property of photoelectric response with 44.11% increment of the average photocurrent density relative to the dark current density. The photoreduction experiments of goethite were conducted under light condition in simulated seawater. It has suggested the photoreduction of goethite could occur and Fe(III) was reduced to Fe(II). The dissolved Fe(II) from the photoreduction of goethite under light condition was nearly 11 times than that group without light after a 10-day reaction. Furthermore, results of microbial community sequencing analysis indicated that dissolved Fe(II) could affect the structure and regulate the decrease of microbial community diversity. The emergence of dominant bacteria associated with iron oxidation and transport protein has suggested their obvious selectivity and adaptability in the environment with adding dissolved Fe(II). This work revealed the photoreduction process of semiconducting goethite was remarkable, giving rise to a non-negligible dissolved Fe(II) and its selective effect on the structure, diversity, as well as the function of microbial community. This light-induced interaction between minerals and microorganisms may also further regulate correlative metabolic pathways of carbon cycle in the marine euphotic zone.

Structural Variation of Precipitates Formed by Fe(II) Oxidation and Impact on the Retention of Phosphate

The oxidation-precipitation process of Fe(II) is ubiquitous in the environment and critically affects the fate of contaminants and nutrients in natural systems where Fe(II) is present. Here, we explored the effect of H₂O₂ concentration on the structure of precipitates formed by Fe(II) oxidation and compared the precipitates to those formed by Fe(III) hydrolysis. Additionally, the phosphate retention under different H₂O₂ concentrations was evaluated. XRD, TEM, PDA, XPS, and UV-visible absorbance spectroscopy were used to characterize the structure of the formed precipitates; UV-visible absorbance spectroscopy was also used to determine the residual phosphate and Fe(II) in solution. It was found that the predominant precipitates in Fe(II) solution changed from planar-shaped crystalline lepidocrocite (γ-FeOOH) to poor short-range order (poorly crystalline) spherical-shaped hydrous ferric oxide (HFO) with increasing H₂O₂ concentrations. Although the HFO precipitates formed from Fe(II) resembled those formed from Fe(III) hydrolysis, the former was larger and had clearer lattice fringes. During the formation of γ-FeOOH, both Fe(II)-Fe(III) complexes and ligand-to-metal charge transfer processes were observed, and it was found that Fe(II) was present in the planar-shaped precipitates. Fe(II) might be present in the interior of precipitates as Fe(OH)₂, which could serve as a nucleus for the epitaxial growth of γ-FeOOH. In addition, the extent of phosphate retention increased with the H₂O₂ concentration, indicating the increased reactivity of formed precipitates with H₂O₂ concentration. More phosphate was retained via coprecipitation with Fe than adsorption on the preformed Fe precipitates due to the incorporation of phosphate within the structure of the formed Fe hydroxyphosphate via coprecipitation.

Petrobactin, a siderophore produced by Alteromonas, mediates community iron acquisition in the global ocean

It is now widely accepted that siderophores play a role in marine iron biogeochemical cycling. However, the mechanisms by which siderophores affect the availability of iron from specific sources and the resulting significance of these processes on iron biogeochemical cycling as a whole have remained largely untested. In this study, we develop a model system for testing the effects of siderophore production on iron bioavailability using the marine copiotroph Alteromonas macleodii ATCC 27126. Through the generation of the knockout cell line ΔasbB::kmr, which lacks siderophore biosynthetic capabilities, we demonstrate that the production of the siderophore petrobactin enables the acquisition of iron from mineral sources and weaker iron-ligand complexes. Notably, the utilization of lithogenic iron, such as that from atmospheric dust, indicates a significant role for siderophores in the incorporation of new iron into marine systems. We have also detected petrobactin, a photoreactive siderophore, directly from seawater in the mid-latitudes of the North Pacific and have identified the biosynthetic pathway for petrobactin in bacterial metagenome-assembled genomes widely distributed across the global ocean. Together, these results improve our mechanistic understanding of the role of siderophore production in iron biogeochemical cycling in the marine environment wherein iron speciation, bioavailability, and residence time can be directly influenced by microbial activities.

Biosynthesis Pathways, Transport Mechanisms and Biotechnological Applications of Fungal Siderophores

Iron (Fe) is the fourth most abundant element on earth and represents an essential nutrient for life. As a fundamental mineral element for cell growth and development, iron is available for uptake as ferric ions, which are usually oxidized into complex oxyhydroxide polymers, insoluble under aerobic conditions. In these conditions, the bioavailability of iron is dramatically reduced. As a result, microorganisms face problems of iron acquisition, especially under low concentrations of this element. However, some microbes have evolved mechanisms for obtaining ferric irons from the extracellular medium or environment by forming small molecules often regarded as siderophores. Siderophores are high affinity iron-binding molecules produced by a repertoire of proteins found in the cytoplasm of cyanobacteria, bacteria, fungi, and plants. Common groups of siderophores include hydroxamates, catecholates, carboxylates, and hydroximates. The hydroxamate siderophores are commonly synthesized by fungi. L-ornithine is a biosynthetic precursor of siderophores, which is synthesized from multimodular large enzyme complexes through non-ribosomal peptide synthetases (NRPSs), while siderophore-Fe chelators cell wall mannoproteins (FIT1, FIT2, and FIT3) help the retention of siderophores. S. cerevisiae, for example, can express these proteins in two genetically separate systems (reductive and nonreductive) in the plasma membrane. These proteins can convert Fe (III) into Fe (II) by a ferrous-specific metalloreductase enzyme complex and flavin reductases (FREs). However, regulation of the siderophore through Fur Box protein on the DNA promoter region and its activation or repression depend primarily on the Fe availability in the external medium. Siderophores are essential due to their wide range of applications in biotechnology, medicine, bioremediation of heavy metal polluted environments, biocontrol of plant pathogens, and plant growth enhancement.

Novel manganese cycling at very low ionic strengths in the Columbia River Estuary

Mixing of waters of different ionic strengths induces the geochemical cycling of reactive elements. The most reactive zone is where the gradient in ionic strength is steepest. In oxygenated systems, the redox-active metal manganese cycles between soluble and particulate fractions through three oxidation states, manganese(II), manganese(III) and manganese(IV). This cycling strongly affects the mobility of inorganic and organic chemicals. The most accessible environmental system where waters with different ionic strengths mix are estuaries. During six Eulerian studies in the Columbia River Estuary, each up to 26 h, we measured manganese speciation and concentration across a salinity (S_P) gradient centred around S_P = 0.06-6, equivalent to a seawater ionic strength (I_Sp) of 1.2-120 mM. This zone, representing the region between freshwater and the more intensively studied estuarine turbidity maximum, presents a highly dynamic geochemical environment in which the manganese cycle propagates through four steps as I_Sp increases due to mixing: 1. Before a measurable change in I_Sp, manganese, as particulate manganese(III/IV) oxides (MnO_x), undergoes reduction, independent of photochemical processes, to soluble manganese(III) stabilized in organic complexes (Mn(III)-L) and manganese(II); 2. As I_Sp increases between 5 and 80 mM, Mn(III)-L reduction continues and manganese(II) adsorbs onto particle surfaces; 3. As I_Sp increases further, though remaining below 80 mM (S_P ≈ 4), adsorbed manganese(II) desorbs and/or is oxidized and is released as Mn(III)-L or oxidises further to MnO_x; 4. The breakdown of Mn(III)-L complexes leads to higher manganese(II) and MnO_x, which at Mid-Estuary-Salinities (I_Sp = 320-480 mM) precipitates. This manganese cycling in low I_Sp waters directly affects a system's redox chemistry and provides a window into understanding the extensive, yet hidden, freshwater/saline water interface in aquifers, soils, sediments and estuaries.

Cyanochelins, an Overlooked Class of Widely Distributed Cyanobacterial Siderophores, Discovered by Silent Gene Cluster Awakening

Cyanobacteria require iron for growth and often inhabit iron-limited habitats, yet only a few siderophores are known to be produced by them. We report that cyanobacterial genomes frequently encode polyketide synthase (PKS)/nonribosomal peptide synthetase (NRPS) biosynthetic pathways for synthesis of lipopeptides featuring β-hydroxyaspartate (β-OH-Asp), a residue known to be involved in iron chelation. Iron starvation triggered the synthesis of β-OH-Asp lipopeptides in the cyanobacteria Rivularia sp. strain PCC 7116, Leptolyngbya sp. strain NIES-3755, and Rubidibacter lacunae strain KORDI 51-2. The induced compounds were confirmed to bind iron by mass spectrometry (MS) and were capable of Fe³⁺ to Fe²⁺ photoreduction, accompanied by their cleavage, when exposed to sunlight. The siderophore from Rivularia, named cyanochelin A, was structurally characterized by MS and nuclear magnetic resonance (NMR) and found to contain a hydrophobic tail bound to phenolate and oxazole moieties followed by five amino acids, including two modified aspartate residues for iron chelation. Phylogenomic analysis revealed 26 additional cyanochelin-like gene clusters across a broad range of cyanobacterial lineages. Our data suggest that cyanochelins and related compounds are widespread β-OH-Asp-featuring cyanobacterial siderophores produced by phylogenetically distant species upon iron starvation. Production of photolabile siderophores by phototrophic cyanobacteria raises questions about whether the compounds facilitate iron monopolization by the producer or, rather, provide Fe²⁺ for the whole microbial community via photoreduction. IMPORTANCE All living organisms depend on iron as an essential cofactor for indispensable enzymes. However, the sources of bioavailable iron are often limited. To face this problem, microorganisms synthesize low-molecular-weight metabolites capable of iron scavenging, i.e., the siderophores. Although cyanobacteria inhabit the majority of the Earth's ecosystems, their repertoire of known siderophores is remarkably poor. Their genomes are known to harbor a rich variety of gene clusters with unknown function. Here, we report the awakening of a widely distributed class of silent gene clusters by iron starvation to yield cyanochelins, β-hydroxy aspartate lipopeptides involved in iron acquisition. Our results expand the limited arsenal of known cyanobacterial siderophores and propose products with ecological function for a number of previously orphan gene clusters.

Photogranulation in a Hydrostatic Environment Occurs with Limitation of Iron

Filamentous cyanobacteria are an essential element of oxygenic photogranules for granule-based wastewater treatment with photosynthetic aeration. Currently, mechanisms for the selection of this microbial group and their development in the granular structure are not well understood. Here, we studied the characteristics and fate of iron in photogranulation that proceeds in a hydrostatic environment with an activated sludge (AS) inoculum. We found that the level of Fe in bulk liquids (Fe_BL) sharply increased due to the decay of the inoculum but quickly diminished along with the bloom of microalgae and the advent of the oxic environment. Iron linked with extracellular polymeric substances (Fe_EPS) continued to decline but reached steady low values, which occurred along with the appearance of granular structure. Strong negative correlations were found between Fe_EPS and the pigments specific for cyanobacteria. Spectroscopies revealed the presence of amorphous ferric oxides in pellet biomass, which seemed to remain unaltered during the photogranulation process. These results suggest that the availability of Fe_EPS in AS inoculums-after algal bloom-selects cyanobacteria, and the limitation of this Fe pool becomes an important driver for cyanobacteria to granulate in a hydrostatic environment. We therefore propose that the availability of iron has a strong influence on the photogranulation process.

Probing the Bioavailability of Dissolved Iron to Marine Eukaryotic Phytoplankton Using In Situ Single Cell Iron Quotas

We present a new approach for quantifying the bioavailability of dissolved iron (dFe) to oceanic phytoplankton. Bioavailability is defined using an uptake rate constant (k_in-app) computed by combining data on: (a) Fe content of individual in situ phytoplankton cells; (b) concurrently determined seawater dFe concentrations; and (c) growth rates estimated from the PISCES model. We examined 930 phytoplankton cells, collected between 2002 and 2016 from 45 surface stations during 11 research cruises. This approach is only valid for cells that have upregulated their high-affinity Fe uptake system, so data were screened, yielding 560 single cell k _in-app values from 31 low-Fe stations. We normalized k _in-app to cell surface area (S.A.) to account for cell-size differences. The resulting bioavailability proxy (k _in-app/S.A.) varies among cells, but all values are within bioavailability limits predicted from defined Fe complexes. In situ dFe bioavailability is higher than model Fe-siderophore complexes and often approaches that of highly available inorganic Fe'. Station averaged k _in-app/S.A. are also variable but show no systematic changes across location, temperature, dFe, and phytoplankton taxa. Given the relative consistency of k _in-app/S.A. among stations (ca. five-fold variation), we computed a grand-averaged dFe availability, which upon normalization to cell carbon (C) yields k _in-app/C of 42,200 ± 11,000 L mol C^-1 d^-1. We utilize k _in-app/C to calculate dFe uptake rates and residence times in low Fe oceanic regions. Finally, we demonstrate the applicability of k _in-app/C for constraining Fe uptake rates in earth system models, such as those predicting climate mediated changes in net primary production in the Fe-limited Equatorial Pacific.

Northern High-Latitude Organic Soils As a Vital Source of River-Borne Dissolved Iron to the Ocean

Organic soils in the Arctic-boreal region produce small aquatic humic ligands (SAHLs), a category of naturally occurring complexing agents for iron. Every year, large amounts of SAHLs-loaded with iron mobilized in river basins-reach the oceans via river runoff. Recent studies have shown that a fraction of SAHLs belong to the group of strong iron-binding ligands in the ocean. That means, their Fe(III) complexes withstand dissociation even under the conditions of extremely high dilution in the open ocean. Fe(III)-loaded SAHLs are prone to UV-photoinduced ligand-to-metal charge-transfer which leads to disintegration of the complex and, as a consequence, to enhanced concentrations of bioavailable dissolved Fe(II) in sunlit upper water layers. On the other hand, in water depths below the penetration depth of UV, the Fe(III)-loaded SAHLs are fairly resistant to degradation which makes them ideally suited as long-lived molecular transport vehicles for river-derived iron in ocean currents. At locations where SAHLs are present in excess, they can bind to iron originating from various sources. For example, SAHLs were proposed to contribute substantially to the stabilization of hydrothermal iron in deep North Atlantic waters. Recent discoveries have shown that SAHLs, supplied by the Arctic Great Rivers, greatly improve dissolved iron concentrations in the Arctic Ocean and the North Atlantic Ocean. In these regions, SAHLs play a critical role in relieving iron limitation of phytoplankton, thereby supporting the oceanic sink for anthropogenic CO₂. The present Critical Review describes the most recent findings and highlights future research directions.

Iron transport in cyanobacteria - from molecules to communities

Iron is an essential micronutrient for the ecologically important photoautotrophic cyanobacteria which are found across diverse aquatic environments. Low concentrations and poor bioavailability of certain iron species exert a strong control on cyanobacterial growth, affecting ecosystem structure and biogeochemical cycling. Here, we review the iron-acquisition pathways cyanobacteria utilize for overcoming these challenges. As the molecular details of cyanobacterial iron transport are being uncovered, an overall scheme of how cyanobacteria handle and exploit this scarce and redox-active micronutrient is emerging. Importantly, the range of biological solutions used by cyanobacteria to increase iron fluxes goes beyond transport and includes behavioral traits of colonial cyanobacteria and intricate cyanobacteria-bacteria interactions.

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.

Photoactive siderophores: Structure, function and biology

It is well known that bacteria and fungi have evolved sophisticated systems for acquiring the abundant but biologically inaccessible trace element iron. These systems are based on high affinity Fe(III)-specific binding compounds called siderophores which function to acquire, transport, and process this essential metal ion. Many hundreds of siderophores are now known and their numbers continue to grow. Extensive studies of their isolation, structure, transport, and molecular genetics have been undertaken in the last three decades and have been comprehensively reviewed many times. In this review we focus on a unique subset of siderophores that has only been recognized in the last 20 years, namely those whose iron complexes display photoactivity. This photoactivity, which typically results in the photooxidation of the siderophore ligand with concomitant reduction of Fe(III) to Fe(II), seemingly upsets the siderophore paradigm of forming and transporting only extremely stable Fe(III) complexes into microbial cells. Here we review their structure, synthesis, photochemistry, photoproduct coordination chemistry and explore the potential biological and ecological consequences of this photoactivity.

Screening for Microbial Metal-Chelating Siderophores for the Removal of Metal Ions from Solutions

To guarantee the supply of critical elements in the future, the development of new technologies is essential. Siderophores have high potential in the recovery and recycling of valuable metals due to their metal-chelating properties. Using the Chrome azurol S assay, 75 bacterial strains were screened to obtain a high-yield siderophore with the ability to complex valuable critical metal ions. The siderophore production of the four selected strains Nocardioides simplex 3E, Pseudomonas chlororaphis DSM 50083, Variovorax paradoxus EPS, and Rhodococcus erythropolis B7g was optimized, resulting in significantly increased siderophore production of N. simplex and R. erythropolis. Produced siderophore amounts and velocities were highly dependent on the carbon source. The genomes of N. simplex and P. chlororaphis were sequenced. Bioinformatical analyses revealed the occurrence of an achromobactin and a pyoverdine gene cluster in P. chlororaphis, a heterobactin and a requichelin gene cluster in R. erythropolis, and a desferrioxamine gene cluster in N. simplex. Finally, the results of the previous metal-binding screening were validated by a proof-of-concept development for the recovery of metal ions from aqueous solutions utilizing C₁₈ columns functionalized with siderophores. We demonstrated the recovery of the critical metal ions V(III), Ga(III), and In(III) from mixed metal solutions with immobilized siderophores of N. simplex and R. erythropolis.

Photosynthesis of a Dihydroimidazopyridine Chelate Shines Light on the Reactions of a Photoactivated Iron(III) Complex with O2

The high-spin (S = ⁵/₂) meridional diastereoisomer of [Fe^III(tpena)]²⁺ (tpena = N,N,N'-tris(2-pyridylmethyl)ethylendiamine-N'-acetate), mer-[Fe(tpena)]²⁺, undergoes photolytic CO₂ release to produce an iron(II) intermediate of a radical dihydroimidazopyridine ligand (L^•). The structure of this unprecedented transient iron(II)(L^•) complex is supported by UV-vis and Mössbauer spectroscopies, DFT calculations, as well as the X-ray structural characterization of an μ-oxo iron(III) complex of the oxidized derivative of L^•, namely, [Fe^III₂O(Cl)₂(L⁺)₂](ClO₄)₄(MeCN)₂ (L⁺ = 2-(2-(bis(pyridin-2-ylmethyl)amino)ethyl)-2,3-dihydro-1H-imidazo[1,5-a]pyridin-4-ium). [Fe^III₂O(Cl)₂(L⁺)₂]⁴⁺ is obtained only in the absence of O₂. Under aerobic conditions, O₂ will intercept the iron(II)(L^•) complex to form a putative Fe(III)-alkylperoxide complex which cascades to an iron(II) complex of SBPy3 (SBPy3 = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine). Thus, through different oxidative pathways, the unknown ligand L⁺ or SBPy3 forms by loss of a one-carbon-atom or a two-carbon-atom unit, respectively, from the glycyl arm of tpena. Acceleration of the photodecarboxylation step is achieved by addition of thiocyanate because of transient formation of a more photoreactive NCS^- adduct of [Fe(tpena)]²⁺. This has allowed for kinetic observation of the reaction of [Fe^II(L^•)]²⁺ with O₂ which is, unexpectedly, promoted also by light. We propose that this corresponds to the energy needed for the conversion of the ring-closed radical ligand L^• to a ring-opened tautomer to allow for O₂ insertion between the C and Fe atoms of the iron(II) complex.

Biomimetic poly(γ-glutamic acid) hydrogels based on iron (III) ligand coordination for cartilage tissue engineering

For the problems in the research on differentiation of mesenchymal stem cells (BMSCs), such as poor differentiation tendency and low differentiation efficiency, a novel photo-crosslinked extracellular matrix (ECM) inspired double network hydrogel that composed of poly(γ-glutamic acid) (γ-PGA) hydrogel and Fe³⁺ ligand coordination was designed and manufactured. Compared with those traditional γ-PGA based hydrogels, the introduction of Fe³⁺ significantly enhanced the mechanical properties of the hydrogel and accelerated the chondrogenesis efficiency of BMSCs chondrogenesis. The experimental results confirmed that the mechanical properties of hydrogel enhanced by the introduction of metal ions Fe³⁺ could promote BMSCs proliferation, induce cartilage-specific gene expression, and increase secretion of hydroxyproline (HYP) and glycosaminoglycan (GAG). As a result, this method could promote chondrogenic differentiation of BMSCs, accelerate the regeneration of cartilage, and was prospective to be conducive to the research work of cartilage defect repair. Thus, the mechanically enhanced γ-PGA hydrogel scaffold by Fe³⁺ could mediate BMSCs differentiation and provide a scientific and theoretical basis for research and development of biomedical materials on cartilage tissue engineering field.

Iron Uptake Mechanisms in Marine Phytoplankton

Oceanic phytoplankton species have highly efficient mechanisms of iron acquisition, as they can take up iron from environments in which it is present at subnanomolar concentrations. In eukaryotes, three main models were proposed for iron transport into the cells by first studying the kinetics of iron uptake in different algal species and then, more recently, by using modern biological techniques on the model diatom Phaeodactylum tricornutum. In the first model, the rate of uptake is dependent on the concentration of unchelated Fe species, and is thus limited thermodynamically. Iron is transported by endocytosis after carbonate-dependent binding of Fe(III)' (inorganic soluble ferric species) to phytotransferrin at the cell surface. In this strategy the cells are able to take up iron from very low iron concentration. In an alternative model, kinetically limited for iron acquisition, the extracellular reduction of all iron species (including Fe') is a prerequisite for iron acquisition. This strategy allows the cells to take up iron from a great variety of ferric species. In a third model, hydroxamate siderophores can be transported by endocytosis (dependent on ISIP1) after binding to the FBP1 protein, and iron is released from the siderophores by FRE2-dependent reduction. In prokaryotes, one mechanism of iron uptake is based on the use of siderophores excreted by the cells. Iron-loaded siderophores are transported across the cell outer membrane via a TonB-dependent transporter (TBDT), and are then transported into the cells by an ABC transporter. Open ocean cyanobacteria do not excrete siderophores but can probably use siderophores produced by other organisms. In an alternative model, inorganic ferric species are transported through the outer membrane by TBDT or by porins, and are taken up by the ABC transporter system FutABC. Alternatively, ferric iron of the periplasmic space can be reduced by the alternative respiratory terminal oxidase (ARTO) and the ferrous ions can be transported by divalent metal transporters (FeoB or ZIP). After reoxidation, iron can be taken up by the high-affinity permease Ftr1.

Quantitative high-throughput approach to chalkophore screening in freshwaters

There is an increasing need to study the effects of trace metal micronutrients on microorganisms in natural waters. For Fe, small Fe-binding ligands called siderophores, which are secreted from cells and bind Fe with high affinity, have been demonstrated to modulate bioavailability of this critical nutrient. Relatively little is known about secretion of strong Cu-binding ligands (chalkophores) that may help organisms navigate the divide between Cu nutrition and toxicity. A barrier to environmental chalkophore research is a lack of literature on chalkophore analysis. Here we report the development of a quantitative, high-throughput approach to chalkophore screening based on a popular competitive-ligand binding assay for siderophores wherein ligands compete for metal in a chromogenic ternary complex of chrome azurol sulfonate-metal-surfactant. We developed the assay for high-throughput analysis using a microplate reader. The method performance is slightly better than that of comparable screening approaches for siderophores. We find that levels of other metals in natural samples may be capable of causing matrix interferences (a neglected source of analytical uncertainty in siderophore screening) and that for our method this can be overcome by standard additions. In this respect the high-throughput nature of the technique is a distinct advantage. To demonstrate practical use, we tested samples from field mesocosm studies that were set up with and without Cu and Fe amendments; we find trends in results that are logical in the environmental context of our application. This approach will be useful in areas such as risk assessment for a rapid survey of metal speciation and bioavailability; investigators who perform structural studies might also benefit from this approach to rapidly screen and select samples with high Fe/Cu binding capacity for further study.

Insights into the bioavailability of oceanic dissolved Fe from phytoplankton uptake kinetics

Phytoplankton growth in large parts of the world ocean is limited by low availability of dissolved iron (dFe), restricting oceanic uptake of atmospheric CO2. The bioavailability of dFe in seawater is however difficult to appraise since it is bound by a variety of poorly characterized organic ligands. Here, we propose a new approach for evaluating seawater dFe bioavailability based on its uptake rate constant by Fe-limited cultured phytoplankton. We utilized seven phytoplankton species of diverse classes, sizes, and provenances to probe for dFe bioavailability in 12 seawater samples from several ocean basins and depths. All tested phytoplankton acquired organically bound Fe in any given sample at similar rates (after normalizing to cellular surface area), confirming that multiple, Fe-limited phytoplankton species can be used to probe dFe bioavailability in seawater. These phytoplankton-based uptake rate constants allowed us to compare water types, and obtain a grand average estimate of seawater dFe bioavailability. Among water types, dFe bioavailability varied by approximately four-fold, and did not clearly correlate with Fe concentrations or any of the measured Fe speciation parameters. Compared with well-studied Fe complexes, seawater dFe is more available than model siderophore Fe, but less available than inorganic Fe. Exposure of seawater to sunlight, however, significantly enhanced dFe bioavailability. The rate constants established in this work, not only facilitate comparison between water types, but also allow calculation of Fe uptake rates by phytoplankton in the ocean based on measured dFe concentrations. The approach established and verified in this study, opens a new way for determining dFe bioavailability in samples across the ocean, and enables modeling of in situ Fe uptake rates by phytoplankton using dFe concentrations from GEOTRACES datasets.

Lesson from Ecotoxicity: Revisiting the Microbial Lipopeptides for the Management of Emerging Diseases for Crop Protection

Microorganisms area treasure in terms of theproduction of various bioactive compounds which are being explored in different arenas of applied sciences. In agriculture, microbes and their bioactive compounds are being utilized in growth promotion and health promotion withnutrient fortification and its acquisition. Exhaustive explorations are unraveling the vast diversity of microbialcompounds with their potential usage in solving multiferous problems incrop production. Lipopeptides are one of such microbial compounds which havestrong antimicrobial properties against different plant pathogens. These compounds are reported to be produced by bacteria, cyanobacteria, fungi, and few other microorganisms; however, genus Bacillus alone produces a majority of diverse lipopeptides. Lipopeptides are low molecular weight compounds which havemultiple industrial roles apart from being usedas biosurfactants and antimicrobials. In plant protection, lipopeptides have wide prospects owing totheirpore-forming ability in pathogens, siderophore activity, biofilm inhibition, and dislodging activity, preventing colonization bypathogens, antiviral activity, etc. Microbes with lipopeptides that haveall these actions are good biocontrol agents. Exploring these antimicrobial compounds could widen the vistasof biological pest control for existing and emerging plant pathogens. The broader diversity and strong antimicrobial behavior of lipopeptides could be a boon for dealing withcomplex pathosystems and controlling diseases of greater economic importance. Understanding which and how these compounds modulate the synthesis and production of defense-related biomolecules in the plants is a key question—the answer of whichneeds in-depth investigation. The present reviewprovides a comprehensive picture of important lipopeptides produced by plant microbiome, their isolation, characterization, mechanisms of disease control, behavior against phytopathogens to understand different aspects of antagonism, and potential prospects for future explorations as antimicrobial agents. Understanding and exploring the antimicrobial lipopeptides from bacteria and fungi could also open upan entire new arena of biopesticides for effective control of devastating plant diseases.

Fe(III) Photoreduction Producing Feaq2+ in Oxic Freshwater Sediment

Iron(III) (Fe(III)) photoreduction plays an important role in Fe cycling and Fe(II) bioavailability in the upper ocean. Although well described for water columns, it is currently unknown to what extent light impacts the production of dissolved Fe(II) (Fe²⁺) in aquatic sediments. We performed high-resolution voltammetric microsensor measurements and demonstrated light-induced Fe²⁺ release in freshwater sediments from Lake Constance. Fe²⁺ concentrations increased up to 40 μM in the top 3 mm of the sediment during illumination in the visible range of light (400-700 nm), even in the presence of oxygen (100-280 μM). The Fe²⁺ release was strongly dependent on the organic matter content of the sediment. A lack of photoreduced Fe²⁺ was measured in sediments with concentrations of organic carbon <6 mg L^-1. The simultaneous presence of sedimentary Fe(III) photoreduction besides microbial and abiotic Fe²⁺ oxidation by oxygen suggests an active Fe redox cycling in the oxic and photic zone of aquatic sediments. Here, we provide evidence for a relevant contribution of Fe(III) photoreduction to the bio-geochemical Fe redox cycle in aquatic freshwater sediments.

Photochemistry of iron in aquatic environments

Light energy is a driver for many biogeochemical element cycles in aquatic systems. The sunlight-induced photochemical reduction of ferric iron (Fe(iii) photoreduction) to ferrous iron (Fe(ii)) by either direct ligand-to-metal charge transfer or by photochemically produced radicals can be an important source of dissolved Fe_aq²⁺ in aqueous and sedimentary environments. Reactive oxygen species (ROS) are formed by a variety of light-dependent reactions. Those ROS can oxidize Fe(ii) or reduce Fe(iii), and due to their high reactivity they are key oxidants in aquatic systems where they influence many other biogeochemical cycles. In oxic waters with circumneutral pH, the produced Fe(ii) reaches nanomolar concentrations and serves as a nutrient, whereas in acidic waters, freshwater and marine sediments, which are rich in Fe(ii), the photochemically formed Fe(ii) can reach concentrations of up to 100 micromolar and be used as additional electron donor for acidophilic aerobic, microaerophilic, phototrophic and, if nitrate is present, for nitrate-reducing Fe(ii)-oxidizing bacteria. Therefore, Fe(iii) photoreduction may not only control the primary productivity in the oceans but has a tremendous impact on Fe cycling in the littoral zone of freshwater and marine environments. In this review, we summarize photochemical reactions involving Fe, discuss the role of ROS in Fe cycling, and highlight the importance of photoreductive processes in the environment.

Biological Control of Chromium Redox and Stable Isotope Composition in the Surface Ocean

While chromium stable isotopes (δ53Cr) have received significant attention for their utility as a tracer of oxygen availability in the distant geological past, a mechanistic understanding of modern oceanic controls on Cr and δ53Cr is still lacking. Here we present total dissolved δ53Cr, concentrations of Cr (III) and total dissolved Cr, and net community productivity (NCP) from the North Pacific. Chromium concentrations show surface depletions in waters with elevated NCP, but not in lower productivity waters. Observed Cr deficits correspond well with calculated Cr export derived from NCP and Cr:C ratios of natural phytoplankton and marine particulates. Chromium (III) concentrations are stable over the diel cycle yet correlate with NCP, with maxima found in highly productive surface waters but not in lower productivity waters, indicating biological control on Cr (III). The relationship between Cr (III) and δ53Cr suggests that δ53Cr distributions may be controlled by the removal of isotopically light Cr (III) at an isotopic enrichment factor (∆53Cr) of -1.08‰ ± 0.25 relative to total dissolved δ53Cr, in agreement with the global δ53Cr-Cr fractionation factor (-0.82‰ ± 0.05). No perturbation to δ53Cr, Cr, or Cr (III) is observed in oxygen-depleted waters (~10 μmol/kg), suggesting no strong control by O2 availability, in agreement with other recent studies. Therefore, we propose that biological productivity is the primary control on Cr and δ53Cr in the modern ocean. Consequently, δ53Cr records in marine sediments may not faithfully record oxygen availability in the Late Quaternary. Instead, our data demonstrate that δ53Cr records may be a useful tracer for biological productivity.

Enhancement of iron-mediated activation of persulfate using catechin: From generation of reactive species to atenolol degradation in water

Persulfate (PS) activation reaction, which forms sulfate radical (SO₄^-), can be used to degrade organic pollutants in water. However, a drawback of this reaction is that the regeneration of ferrous ions requires a high concentration of hydrogen peroxide (Fenton-like reaction) or use of UV radiation. Catechin (CAT), a non-toxic antioxidant of natural origin from tea, is used in this work to improve the sulfate radical-mediated degradation of atenolol (ATL, a model pollutant) in water using relatively low concentrations of reactive chemical species (less than 100 μM). To the best of the author's knowledge, the direct effect of CAT on the oxidation state of iron, which is promoted by the reduction of ferric into ferrous ions with the enhancement of SO₄^- formation in the presence of PS, is demonstrated for the first time. The enhancement versus inhibition effect of CAT and the chemical mechanism of the iron-based activation process are explained. Results show that UVA radiation, which is representative of solar light, accelerates the initial degradation of ATL by more than 30% through ferric iron photolysis. Finally, a reaction mechanism leading to the formation of hydroxyl radicals (HO) and SO₄^- is proposed considering the implication of different activation/reaction chemical steps.

Is Superoxide-Mediated Fe(III) Reduction Important in Sunlit Surface Waters?

Two major pathways are reported to account for photochemical reduction of Fe(III) in sunlit surface waters, namely, ligand-to-metal charge transfer (LMCT) and superoxide-mediated iron reduction (SMIR). In this study, we investigate the impact of Fe(III) speciation (organically complexed (Fe(III)L versus iron oxyhydroxide (AFO)) on Fe(III) reducibility by photogenerated superoxide (O₂^•-) and LMCT. To simulate conditions typical of fresh, estuarine, and coastal waters, we have used Suwannee River Fulvic Acid (SRFA) as a representative of the natural organic matter likely to associate with Fe(III). Our results show that the photolabile Fe(III)SRFA complex is reduced rapidly by LMCT, while O₂^•- does not play a role in reduction of these entities. In contrast, the relatively less photolabile AFO is reduced by both O₂^•- and LMCT. The reduction of AFO by O₂^•- occurs following the dissolution of AFO, and hence, the contribution of O₂^•- to reductive dissolution of AFO is dependent on conditions such as the age of the AFO and initial AFO concentration affecting the rate of dissolution of AFO. Our results further show that while colloidal Fe(III) (in this work, particles >0.025 μm) is reduced by O₂^•-, there is no involvement of O₂^•- in dissolved Fe(III) reduction. Overall, our results show that superoxide-mediated iron reduction will be important only in natural waters containing limited concentrations of Fe binding ligands.

Iron Isotope Fractionation during Bio- and Photodegradation of Organoferric Colloids in Boreal Humic Waters

Biodegradation and photolysis of dissolved organic matter (DOM) in boreal high-latitude waters are the two main factors controlling not only the aquatic fluxes and residence time of carbon but also metal nutrients associated with DOM such as Fe. The DOM is usually present in the form of organic and organomineral colloids, which also account for the majority of dissolved Fe. Here, we use the stable Fe isotope approach to unravel the processes controlling Fe behavior during bio- and photodegradation of colloids in boreal Fe- and DOM-rich humic waters (a stream and a fen). The adsorption of Fe colloids onto heterotrophic bacteria Pseudomonas aureofaciens produced enrichment in +0.4‰ (δ⁵⁷Fe) in the heavier isotopes of the cell surface relative to the remaining solution. In contrast, long-term assimilation of Fe by live cells yielded preferential incorporation of lighter isotopes into the cells (-0.7‰ relative to aqueous solution). The sunlight-induced oxidation of Fe(II) in fen water led to the removal of heavier Fe isotopes (+1.5 to +2.5‰) from solution, consistent with Fe(III) hydroxide precipitation from Fe(II)-bearing solution. Altogether, bio- and photodegradation of organoferric colloids, occurring within a few days of exposure time, can produce several per mil isotopic excursions in shallow lentic and lothic inland waters of high-latitude boreal regions. Considerable daily scale variations of Fe isotopic composition should therefore be taken into account during the interpretation of the riverine flux of Fe isotopes to the ocean or tracing weathering processes using Fe isotopes in surface waters at high latitudes.

Genomic analysis of siderophore β-hydroxylases reveals divergent stereocontrol and expands the condensation domain family

Genome mining of biosynthetic pathways streamlines discovery of secondary metabolites but can leave ambiguities in the predicted structures, which must be rectified experimentally. Through coupling the reactivity predicted by biosynthetic gene clusters with verified structures, the origin of the β-hydroxyaspartic acid diastereomers in siderophores is reported herein. Two functional subtypes of nonheme Fe(II)/α-ketoglutarate-dependent aspartyl β-hydroxylases are identified in siderophore biosynthetic gene clusters, which differ in genomic organization-existing either as fused domains (IβH_Asp) at the carboxyl terminus of a nonribosomal peptide synthetase (NRPS) or as stand-alone enzymes (TβH_Asp)-and each directs opposite stereoselectivity of Asp β-hydroxylation. The predictive power of this subtype delineation is confirmed by the stereochemical characterization of β-OHAsp residues in pyoverdine GB-1, delftibactin, histicorrugatin, and cupriachelin. The l-threo (2S, 3S) β-OHAsp residues of alterobactin arise from hydroxylation by the β-hydroxylase domain integrated into NRPS AltH, while l-erythro (2S, 3R) β-OHAsp in delftibactin arises from the stand-alone β-hydroxylase DelD. Cupriachelin contains both l-threo and l-erythro β-OHAsp, consistent with the presence of both types of β-hydroxylases in the biosynthetic gene cluster. A third subtype of nonheme Fe(II)/α-ketoglutarate-dependent enzymes (IβH_His) hydroxylates histidyl residues with l-threo stereospecificity. A previously undescribed, noncanonical member of the NRPS condensation domain superfamily is identified, named the interface domain, which is proposed to position the β-hydroxylase and the NRPS-bound amino acid prior to hydroxylation. Through mapping characterized β-OHAsp diastereomers to the phylogenetic tree of siderophore β-hydroxylases, methods to predict β-OHAsp stereochemistry in silico are realized.

Facet-Specific Photocatalytic Degradation of Organics by Heterogeneous Fenton Chemistry on Hematite Nanoparticles

Hematite nanoparticles are abundant in the photic zone of aquatic environments, where they play a prominent role in photocatalytic transformations of bound organics. Here, we examine the photocatalytic degradation of rhodamine B by visible light using two different structurally well-defined hematite nanoparticle morphologies. In addition to detailed solid characterization and aqueous kinetics measurements, we also exploit species-selective scavengers in electron paramagnetic resonance spectroscopy to sequester specific reaction channels and thereby assess their impact. The photodegradation rates for nanoplates dominated by {001} facets and nanocubes dominated by {012} facets were 0.13 and 0.7 h^-1, respectively, and the turnover frequencies for the active sites on {001} and {012} were 7.89 × 10^-3 and 3.07× 10^-3 s^-1, yielding apparent activation energies of 17.13 and 24.94 kcal/mol within the energetic span model, respectively. Facet-specific differences appear to be directly not linked with the simple aerial cation site density but instead with their extent of undercoordination. By establishing this linkage, the findings lay a foundation for predicting the photocatalytic degradation efficiency for the myriad of possible hematite nanoparticle morphologies and more broadly help unveil key reactions at the interface that may govern photocatalytic organic transformations in natural and engineered aquatic environments.

Cr isotopic insights into ca. 1.9 Ga oxidative weathering of the continents using the Beaverlodge Lake paleosol, Northwest Territories, Canada

The ca. 1.9 Ga Beaverlodge Lake paleosol was studied using redox-sensitive Cr isotopes in order to determine the isotopic response to paleoweathering of a rhyodacite parent rock 500 million years after the Great Oxidation Event. Redox reactions occurring in modern weathering environments produce Cr(VI) that is enriched in heavy Cr isotopes compared to the igneous inventory. Cr(VI) species are soluble and easily leached from soils into streams and rivers, thus, leaving particle-reactive and isotopically light Cr(III) species to build up in soils. The Beaverlodge Lake paleosol and two other published weathering profiles of similar age, the Flin Flon and Schreiber Beach paleosols, are not as isotopically light as modern soils, indicating that rivers were not as isotopically heavy at that time. Considering that the global average δ⁵³ Cr value for the oxidative weathering flux of Cr to the oceans today is just 0.27 ± 0.30‰ (1σ) based on a steady-state analysis of the modern ocean Cr cycle, the oxidative weathering flux of Cr to the oceans at ca. 1.9 Ga would have likely been shifted to lower δ⁵³ Cr values, and possibly lower than the igneous inventory (-0.12 ± 0.10‰, 2σ). Mn oxides are the main oxidant of Cr(III) in modern soils, but there is no evidence that they formed in the studied paleosols. Cr(VI) may have formed by direct oxidation of Cr(III) using molecular oxygen or H₂ O₂ , but neither pathway is as efficient as Mn oxides for producing Cr(VI). The picture that emerges from this and other studies of Cr isotope variation in ca. 1.9 Ga paleosols is of atmospheric oxygen concentrations that are high enough to oxidize iron, but too low to oxidize Mn, resulting in low Cr(VI) inventories in Earth surface environments.

Towards the development of a new generation of whole-cell bioreporters to sense iron bioavailability in oceanic systems-learning from the case of Synechococcus sp. PCC7002 iron bioreporter

Whole-cell bioreporters are genetically modified micro-organisms designed to sense bioavailable forms of nutrients or toxic compounds in aquatic systems. As they represent the most promising cost-efficient tools available for such purpose, engineering and use of bioreporters is rapidly growing in association with wide applicability. Bioreporters are urgently needed to determine phytoplankton iron (Fe) limitation, which has been reported in up to 30% of the ocean, with consequences affecting Earth's global carbon cycle and climate. This study presents a critical evaluation and optimization of the only Cyanobacteria bioreporter available to sense Fe limitation in marine systems (Synechococcus sp. PCC7002). The nonmonotonic biphasic dose-response curve between the bioreporters' signal and Fe bioavailability impairs an appropriate data interpretation, highlighting the need for new carefully designed bioreporters. Here, limitations under low Fe concentrations were related to cellular energy stress, nonlinear expression of the targeted promoter and siderophore expression. Furthermore, we provide critical standard criteria for the development of new Fe bioreporters. Finally, based on gene expression data under a range of marine Fe concentrations, we propose novel sensor genes for the development of new Cyanobacteria Fe bioreporters for distinct marine regions.