Iron Isotope Fractionation during Fe(II) Oxidation Mediated by the Oxygen-Producing Marine Cyanobacterium Synechococcus PCC 7002

Precise measurement of Fe isotopes in marine and biological samples by pseudo-high-resolution multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS)

This paper introduces an enhanced technique for analyzing iron isotopes in complex marine and biological samples. A dedicated iron purification method for biological marine matrices, utilizing three ion exchange columns, is validated. The MC-ICPMS in pseudo-high-resolution mode determines precise iron isotopic ratios, with sensitivity improved through the DSN-100 desolvating nebulizer system and Apex-IR. Only 2 µg of iron on DSN versus 1 µg on Apex is needed for six replicates (30-60 times improvement) while 10 to 20 µg is required for a single measurement on a wet system considering the resolution power (Rp) is maintained at 11,000-13,000. The Ni-doping method with a Fe/Ni ratio of 1 yields more accurate isotopic ratios than standard-sample bracketing alone. Measurement reproducibility of triplicate samples from marine biological experiments on MC-ICPMS is ± 0.03‰ (2SD) for δ56Fe and ± 0.07‰ for δ57Fe (2SD). This study introduces a novel iron purification process specifically designed for marine and biological samples, enhancing sensitivity and enabling more reliable measurements with smaller sample sizes and reduced uncertainties. It proposes iron isotopic compositions for biological reference materials, offering a valuable reference dataset in diverse scientific disciplines.

Contrasted redox-dependent structural control on Fe isotope fractionation during its adsorption onto and assimilation by heterotrophic soil bacteria

Despite the importance of structural control on metal stable isotope fractionation in inorganic and abiotic systems, the link between metal structural changes and related isotopic fractionation during reactions with organic surfaces and live cells remains poorly established. We conducted reversible adsorption of Fe(II) and Fe(III) on the surface of exopolysaccharide (EPS)-rich and EPS-poor Pseudomonas aureofaciens, and we allowed Fe intracellular uptake by growing cells. We analyzed the Fe isotopic composition of the remaining fluid and cell biomass, and compared the isotopic fractionation during adsorption and assimilation reaction with relative changes in Fe structural status between aqueous solution and bacterial cells, based on available and newly collected X-ray absorption spectroscopy (XAS) observations. Iron(III) adsorption onto P. aureofaciens at 2.8 ≤ pH ≤ 6.0 produced an enrichment of the cell surface in heavier isotopes with Δ57Fecell-solution ranging from +0.7 to +2.1‰, without a link to pH in EPS-rich cultures. In contrast, the magnitude of isotopic fractionation increased with pH in EPS-poor cultures. Iron(II) adsorption produced an even larger enrichment of the cell surface in heavier isotopes, by up to 3.2‰, tentatively linked to Fe(III) hydroxide precipitation. Intracellular assimilation of Fe(II) favored heavier isotopes and led to Δ57Fecell-solution of +0.8‰. In addition, Fe(III) cellular uptake produced an enrichment of the bacterial biomass in lighter isotopes with Δ57Fecell-solution of -1‰. The XAS analyses demonstrated the dominance of Fe(III)-phosphate complexes both at the cell surface and in the cell interior. We suggest that heavier isotope enrichment of the cell surface relative to the aqueous solution is due to strong Fe(III)-phosphoryl surface complexes and Fe complexation to ligands responsible for metal transfer from the surface to the inner cell. In case of Fe(II) adsorption or assimilation, its partial oxidation within the cell compartments may lead to cell enrichment in heavier isotopes. In contrast, loss of symmetry of assimilated Fe(III) relative to the aqueous Fe3+ ion and longer bonds of intracellular ions relative to aqueous Fe(III)-citrate or hydroxo-complexes could produce an enrichment of cells in lighter isotopes. The versatile nature of Fe(II) and Fe(III) fractionation without a distinct effect of pH and surface exopolysaccharide coverage suggests that, in natural soil and sedimentary environments, Fe isotope fractionation during interaction with heterotrophic bacteria will be primarily governed by Fe complexation with DOM and Fe redox status in the soil pore water.

Interactions between the nitrogen-fixing cyanobacterium Trichodesmium and siderophore-producing cyanobacterium Synechococcus under iron limitation

As diazotrophic cyanobacteria of tremendous biomass, Trichodesmium continuously provide a nitrogen source for carbon-fixing cyanobacteria and drive the generation of primary productivity in marine environments. However, ocean iron deficiencies limit growth and metabolism of Trichodesmium. Recent studies have shown the co-occurrence of Trichodesmium and siderophore-producing Synechococcus in iron-deficient oceans, but whether siderophores secreted by Synechococcus can be used by Trichodesmium to adapt to iron deficiency is not clear. We constructed a mutant Synechococcus strain unable to produce siderophores to explore this issue. Synechococcus filtrates with or without siderophores were added into a Trichodesmium microbial consortium consisting of Trichodesmium erythraeum IMS 101 as the dominant microbe with chronic iron deficiency. By analyzing the physiological phenotype, metagenome, and metatranscriptome, we investigated the interactions between the nitrogen-fixing cyanobacterium Tricodesmium and siderophore-producing cyanobacterium Synechococcus under conditions of iron deficiency. The results indicated that siderophores secreted by Synechococcus are likely to chelate with free iron in the culture medium of the Trichodesmium consortium, reducing the concentration of bioavailable iron and posing greater challenges to the absorption of iron by Trichodesmium. These findings revealed the characteristics of iron-competitive utilization between diazotrophic cyanobacteria and siderophore-producing cyanobacteria, as well as potential interactions, and provide a scientific basis for understanding the regulatory effects of nutrient limitation on marine primary productivity.

Microbial biosignatures in ancient deep-sea hydrothermal sulfides

Deep-sea hydrothermal systems provide ideal conditions for prebiotic reactions and ancient metabolic pathways and, therefore, might have played a pivotal role in the emergence of life. To understand this role better, it is paramount to examine fundamental interactions between hydrothermal processes, non-living matter, and microbial life in deep time. However, the distribution and diversity of microbial communities in ancient deep-sea hydrothermal systems are still poorly constrained, so evolutionary, and ecological relationships remain unclear. One important reason is an insufficient understanding of the formation of diagnostic microbial biosignatures in such settings and their preservation through geological time. This contribution centers around microbial biosignatures in Precambrian deep-sea hydrothermal sulfide deposits. Intending to provide a valuable resource for scientists from across the natural sciences whose research is concerned with the origins of life, we first introduce different types of biosignatures that can be preserved over geological timescales (rock fabrics and textures, microfossils, mineral precipitates, carbonaceous matter, trace metal, and isotope geochemical signatures). We then review selected reports of biosignatures from Precambrian deep-sea hydrothermal sulfide deposits and discuss their geobiological significance. Our survey highlights that Precambrian hydrothermal sulfide deposits potentially encode valuable information on environmental conditions, the presence and nature of microbial life, and the complex interactions between fluids, micro-organisms, and minerals. It further emphasizes that the geobiological interpretation of these records is challenging and requires the concerted application of analytical and experimental methods from various fields, including geology, mineralogy, geochemistry, and microbiology. Well-orchestrated multidisciplinary studies allow us to understand the formation and preservation of microbial biosignatures in deep-sea hydrothermal sulfide systems and thus help unravel the fundamental geobiology of ancient settings. This, in turn, is critical for reconstructing life's emergence and early evolution on Earth and the search for life elsewhere in the universe.

Iron isotopic fractionation driven by low-temperature biogeochemical processes

Iron is geologically important and biochemically crucial for all microorganisms, plants and animals due to its redox exchange, the involvement in electron transport and metabolic processes. Despite the abundance of iron in the earth crust, its bioavailability is very limited in nature due to its occurrence as ferrihydrite, goethite, and hematite where they are thermodynamically stable with low dissolution kinetics in neutral or alkaline environments. Organisms such as bacteria, fungi, and plants have evolved iron acquisition mechanisms to increase its bioavailability in such environments, thereby, contributing largely to the iron cycle in the environment. Biogeochemical cycling of metals including Fe in natural systems usually results in stable isotope fractionation; the extent of fractionation depends on processes involved. Our review suggests that significant fractionation of iron isotopes occurs in low-temperature environments, where the extent of fractionation is greatly governed by several biogeochemical processes such as redox reaction, alteration, complexation, adsorption, oxidation and reduction, with or without the influence of microorganisms. This paper includes relevant data sets on the theoretical calculations, experimental prediction, as well as laboratory studies on stable iron isotopes fractionation induced by different biogeochemical processes.

Iron Isotopes in Acid Mine Drainage: Extreme and Divergent Fractionation between Solid (Schwertmannite, Jarosite, and Ferric Arsenate) and Aqueous Species

Examination of stable Fe isotopes is a powerful tool to explore Fe cycling in a range of environments. However, the isotopic fractionation of Fe in acid mine drainage (AMD) has received little attention and is poorly understood. Here, we analyze Fe isotopes in waters and Fe(III)-rich solids along an AMD flow-path. Aqueous Fe spanned a concentration and δ⁵⁶Fe range of ∼420 mg L^-1 and + 0.04‰ at the AMD source to ∼100 mg L^-1 and -0.81‰ at ∼450 m downstream. Aqueous As (up to ∼33 mg L^-1) and SO₄^2- (up to ∼2000 mg L^-1), like aqueous Fe, decreased in concentration down the flow-path. X-ray absorption spectroscopy indicated that downstream attenuation in aqueous Fe, As, and SO₄^2- was due to the precipitation of amorphous ferric arsenate (AFA), schwertmannite, and jarosite. The Fe(III) in these solids displayed extreme variability in δ⁵⁶Fe, spanning +3.95‰ in AFA near the AMD source to -1.34‰ in schwertmannite at ∼450 m downstream. Similarly, the isotopic contrast between solid Fe(III) precipitates and aqueous Fe (Δ⁵⁶Fe_ppt-aq) dropped along the flow-path from about +4.1 to -1.1‰. The shift from positive to negative Δ⁵⁶Fe_ppt-aq reflects divergence between competing equilibrium versus kinetic fractionation processes.

Extracellular polymeric substance induces biogenesis of vivianite under inorganic phosphate-free conditions

Vivianite is often found in reducing environments rich in iron and phosphorus from organic debris degradation or phosphorus mineral dissolution. The formation of vivianite is essential to the geochemical cycling of phosphorus and iron elements in natural environments. In this study, extracellular polymeric substances (EPS) were selected as the source of phosphorus. Microcosm experiments were conducted to test the evolution of mineralogy during the reduction of polyferric sulfate flocs (PFS) by Shewanella oneidensis MR-1 (S. oneidensis MR-1) at EPS concentrations of 0, 0.03, and 0.3 g/L. Vivianite was found to be the secondary mineral in EPS treatment when there was no phosphate in the media. The EPS DNA served as the phosphorus source and DNA-supplied phosphate could induce the formation of vivianite. EPS impedes PFS aggregation, contains redox proteins and stores electron shuttle, and thus greatly promotes the formation of minerals and enhances the reduction of Fe(III). At EPS concentration of 0, 0.03, and 0.3 g/L, the produced HCl-extractable Fe(II) was 107.9, 111.0, and 115.2 mg/L, respectively. However, when the microcosms remained unstirred, vivianite can be formed without the addition of EPS. In unstirred systems, the EPS secreted by S. oneidensis MR-1 could agglomerate at some areas, resulting in the formation of vivianite in the proximity of microbial cells. It was found that vivianite can be generated biogenetically by S. oneidensis MR-1 strain and EPS may play a key role in iron reduction and concentrating phosphorus in the oligotrophic ecosystems where quiescent conditions prevail.

Light/Dark and Temperature Cycling Modulate Metabolic Electron Flow in Pseudomonas aeruginosa Biofilms

Sunlight drives phototrophic metabolism, which affects redox conditions and produces substrates for nonphototrophs. These environmental parameters fluctuate daily due to Earth’s rotation, and nonphototrophic organisms can therefore benefit from the ability to respond to, or even anticipate, such changes. Circadian rhythms, such as daily changes in body temperature, in host organisms can also affect local conditions for colonizing bacteria. Here, we investigated the effects of light/dark and temperature cycling on biofilms of the opportunistic pathogen Pseudomonas aeruginosa PA14. We grew biofilms in the presence of a respiratory indicator dye and found that enhanced dye reduction occurred in biofilm zones that formed during dark intervals and at lower temperatures. This pattern formation occurred with cycling of blue, red, or far-red light, and a screen of mutants representing potential sensory proteins identified two with defects in pattern formation, specifically under red light cycling. We also found that the physiological states of biofilm subzones formed under specific light and temperature conditions were retained during subsequent condition cycling. Light/dark and temperature cycling affected expression of genes involved in primary metabolic pathways and redox homeostasis, including those encoding electron transport chain components. Consistent with this, we found that cbb₃-type oxidases contribute to dye reduction under light/dark cycling conditions. Together, our results indicate that cyclic changes in light exposure and temperature have lasting effects on redox metabolism in biofilms formed by a nonphototrophic, pathogenic bacterium.

Dissolved iron and isotopic geochemical characteristics in a typical tropical river across the floodplain: The potential environmental implication

Iron (Fe) is an essential element for bio-physiological functioning terrestrial organisms, in particular of aquatic organisms. It is therefore crucial to understand the aquatic iron cycle and geochemical characteristics, which is also significant to obtain the key information on earth-surface evolution. The stable iron isotopic composition (δ⁵⁶Fe) of the dissolved fraction is determined in the Mun River (main tributary of Mekong River), northeast Thailand to distinguish the human and nature influenced riverine iron geochemical behavior. The results show that dissolved Fe concentration ranges from 8.04 to 135.27 μg/L, and the δ⁵⁶Fe ranges from -1.34‰ to 0.48‰, with an average of 0.23‰, 0.14‰ and -0.15‰ in the upper, middle and lower reaches, respectively. The δ⁵⁶Fe values of river water are close to that of the bulk continental crust and other tropical rivers. The correlations between δ⁵⁶Fe and Fe, Al, and physicochemical parameters show mixing processes of different Fe end-members, including the rock weathering end-member (low Fe/Al ratio and high δ⁵⁶Fe), the urban activities end-member (high Fe/Al ratio and moderate δ⁵⁶Fe), and a third end-member with probable sources from the Chi River and reservoir. For the most river water samples, the primary contribution is attributed to rock weathering, and the second is urban activities (only a few samples are from the upper and middle reaches). Thus, Fe isotopes could be employed as a proxy to identify and quantify the natural and anthropogenic contributions, respectively. These findings also provide data support for the scientific management of water resources in the Mun River catchment and other large tropical rivers.

Iron availability is a key factor for freshwater cyanobacterial survival against saline stress

Estuaries are among the most productive ecosystems and dynamic environments on Earth. Varying salinity is the most important challenge for phytoplankton survival in estuaries. In order to investigate the role of iron nutrition on phytoplankton survival under salinity stress, a freshwater cyanobacterial strain was cultivated in media added with different proportions of seawater (measured with siderophore activities), and supplied with gel-immobilized ferrihydrite as iron source. Results showed that the strain grew well in media with 0% seawater supplied with ferrihydrite as iron source. Surprisingly, the biomasses in media with 50% seawater, with more newly excreted siderophore, were similar to those with 0% seawater, but better than those with 6.25%, 12.5% and 25% seawater. Smaller iron isotopic discriminations between the cyanobacterial cells associated iron and dissolved iron were observed in media with 0% and 50% seawater suggested that higher fractions of iron uptake from aqueous dissolved iron reservoir by these comparatively larger biomasses. In summary, this study proved that iron availability plays a key role in cyanobacterial survival under varying salinity stress, and suggested that siderophores introduced by seawater may accelerate iron dissolution, increase iron availability, and make cyanobacterial cells overcome the adverse effects of high-salinity, and indicated that siderophore excretion is a kind of survival strategy for phytoplankton in face of salinity stress.

Microbial nitrification and acidification of lacustrine sediments deduced from the nature of a sedimentary kaolin deposit in central Japan

Kaolin deposits in the Seto-Tono district, central Japan, were formed by intense kaolinization of lacustrine arkose sediments deposited in small and shallow inland lakes in the late Miocene. Based on mineralogical and stable isotopic (Fe, C, N) studies of Motoyama kaolin deposit in the Seto area, we concluded that it was formed by microbial nitrification and acidification of lacustrine sediments underneath an inland lake. Small amounts of Fe-Ti oxides and Fe-hydroxide in the kaolin clay indicated that iron was oxidized and leached during the kaolinization. The field occurrences indicate that leached ferric iron precipitated on the bottom of the kaolin deposit as limonite crusts, and their significantly fractionated Fe isotope compositions suggest the involvement of microbial activity. The C/N ratios of most of the kaolin clay are distinctly higher than those of modern lacustrine sediment. Although, the possibility of a low-temperature hydrothermal origin of the kaolin deposit cannot be completely ruled out, it is more likely that acidification by dilute nitric acid formed from plant-derived ammonia could have caused the kaolinization, Fe oxidation and leaching. The nitrate-dependent microbial Fe oxidation is consistent with dilute nitric acid being the predominant oxidant.

More than redox, biological organic ligands control iron isotope fractionation in the riparian wetland

Although redox reactions are recognized to fractionate iron (Fe) isotopes, the dominant mechanisms controlling the Fe isotope fractionation and notably the role of organic matter (OM) are still debated. Here, we demonstrate how binding to organic ligands governs Fe isotope fractionation beyond that arising from redox reactions. The reductive biodissolution of soil Fe(III) enriched the solution in light Fe isotopes, whereas, with the extended reduction, the preferential binding of heavy Fe isotopes to large biological organic ligands enriched the solution in heavy Fe isotopes. Under oxic conditions, the aggregation/sedimentation of Fe(III) nano-oxides with OM resulted in an initial enrichment of the solution in light Fe isotopes. However, heavy Fe isotopes progressively dominate the solution composition in response to their binding with large biologically-derived organic ligands. Confronted with field data, these results demonstrate that Fe isotope systematics in wetlands are controlled by the OM flux, masking Fe isotope fractionation arising from redox reactions. This work sheds light on an overseen aspect of Fe isotopic fractionation and calls for a reevaluation of the parameters controlling the Fe isotopes fractionation to clarify the interpretation of the Fe isotopic signature.

Geochemical and Stable Fe Isotopic Analysis of Dissimilatory Microbial Iron Reduction in Chocolate Pots Hot Spring, Yellowstone National Park

Chocolate Pots hot spring (CP) is an Fe-rich, circumneutral-pH geothermal spring in Yellowstone National Park. Relic hydrothermal systems have been identified on Mars, and modern hydrothermal environments such as CP are useful for gaining insight into potential pathways for generation of biosignatures of ancient microbial life on Earth and Mars. Fe isotope fractionation is recognized as a signature of dissimilatory microbial iron oxide reduction (DIR) in both the rock record and modern sedimentary environments. Previous studies in CP have demonstrated the presence of DIR in vent pool deposits and show aqueous-/solid-phase Fe isotope variations along the hot spring flow path that may be linked to this process. In this study, we examined the geochemistry and stable Fe isotopic composition of spring water and sediment core samples collected from the vent pool and along the flow path, with the goal of evaluating whether Fe isotopes can serve as a signature of past or present DIR activity. Bulk sediment Fe redox speciation confirmed that DIR is active within the hot spring vent pool sediments (but not in more distal deposits), and the observed Fe isotope fractionation between Fe(II) and Fe(III) is consistent with previous studies of DIR-driven Fe isotope fractionation. However, modeling of sediment Fe isotope distributions indicates that DIR does not produce a unique Fe isotopic signature of DIR in the vent pool environment. Because of rapid chemical and isotopic communication between the vent pool fluid and sediment, sorption of Fe(II) to Fe(III) oxides would produce an isotopic signature similar to DIR despite DIR-driven generation of large quantities of isotopically light solid-associated Fe(II). The possibility exists, however, for preservation of specific DIR-derived Fe(II) minerals such as siderite (which is present in the vent pool deposits), whose isotopic composition could serve as a long-term signature of DIR in relic hot spring environments.

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.