Geochemical Carbon Cycle Research Articles

Silicon as a key driver of phytolith and phytolith-occluded carbon sequestration for climate change mitigation in rice ecosystems - a review

Anthropogenic activities raise the atmosphere greenhouse gases especially carbon dioxide caveat Global warming and climate change in recent years. Silica is one of the principal elements in the earth crust and consider as an important source for plant growth and development. Silica is absorbed as mono silicic acid (H4SiO4) deposit as opal stone / Phytolith in the cellular spaces and vascular bundles of the plant parts. The highest phytolith content of rice plant was observed in straw and incorporated into the soil during harvesting operation and acts as a resilience material. The Lsi1 and Lsi2 transporters along with major and secondary plant nutrients undergone polymerization with adsorbed Si for the formation of phytolith structure in rice. The rice Phytolith gives structural support, acts as a defence mechanism, imparts biotic and abiotic stresses, and nullifies the toxicity of some of the toxic metals and salinization of soil. Phytolith occluded carbon is formed by occlusion mechanism along with phytolith and other elements. PhytOC involves in geo chemical carbon cycle and climate change mitigation process. During the past sixty years, the annual carbon sequestration varied between 0.81×106 and 3.88×106 Mg-e-CO2 and maximum of 37×108 Mg-e-CO2 within phytoliths of rice crop in China. Archaeology, paleo botany, geology and pale ecological research on Phytoliths are carried out, because silica is a base, non-degradable and preserve as microfossils. Use of silicon rich organic and inorganic sources enhances the Aboveground Net Primary Productivity (ANPP) and Phytolith C sequestration in the rice eco system.

Validating the rhenium proxy for rock organic carbon oxidation using weathering profiles

Chemical weathering over geological timescales acts as a source or sink of atmospheric carbon dioxide (CO2), while influencing long-term redox cycling and atmospheric oxygen (O2) at Earth's surface. There is a growing recognition that the oxidative weathering of rock organic carbon (OCpetro) can release more CO2 than is locally drawn down by silicate weathering, and may vary due to changes in erosion and climate. The element rhenium (Re) has emerged as a proxy to track the oxidative weathering of OCpetro, yet uncertainties in its application remain namely that we lack a systematic assessment of the comparative mobility of Re and OCpetro during sedimentary rock weathering. Here we measure Re and OCpetro loss across gradients in rock weathering at 9 global sites, spanning a range of initial OCpetro values from ∼0.2 % to >10 %. We use titanium to account for volume changes during weathering and assess Re and OCpetro loss alongside major elements that reflect silicate (Na, Mg), carbonate (Ca, Mg) and sulfide (S) weathering. Across the dataset, Re loss is correlated with OCpetro loss but not with loss of any other major element. Across the weathering profiles, the average molar ratio of OCpetro to Re loss was 0.84 ± 0.15, with 8 out of 9 sites having a ratio >0.74. At one site (Marcellus Shale), the average ratio was lower at 0.58 ± 0.11. The excess loss of Re matches expectations that, typically, between ∼0 and 20 % of the Re liberated by sedimentary rock weathering derives from silicate or sulfide phases, while some OCpetro may be physically or chemically protected from weathering. Overall, our measurements provide validation for the Re proxy of OCpetro oxidation and allow future work to further improve our knowledge of regional and global-scale rates of this important source of CO2 in the geochemical carbon cycle.

Characteristics of organic matter driven by Eichhornia crassipes during co-contamination with per(poly)fluoroalkyl substances (PFASs) and microplastics (MPs)

Co-contamination with MPs and PFASs has been recorded, particularly in surface-water environments. Floating macrophyte microcosms are an important part of the surface water ecosystem, and dissolved organic matter (DOM) driven by floating macrophytes (FMDDOM) is critical for maintaining material circulation. However, knowledge gaps remain regarding the impact of MPs and PFASs co-pollution on FMDDOM. An greenhouse simulation experiment was conducted in this study to investigate the effects of four PFASs, perfluorooctanoic acid (PFOA), perfluoro-octane-sulfonic acid (PFOS), perfluoro-2-methyl-3-oxahexanoic acid (Gen X), and potassium 9-chlorohexadecafluoro-3-oxanonane-1-sulfonate (F-53B), on FMDDOM sourced from Eichhornia crassipes (E. crassipes), a typical floating macrophyte, in the presence and absence of polystyrene (PS) MPs. Four PFASs increased FMDDOM release from E. crassipes, leading to a 32.52–77.49 % increase in dissolved organic carbon (DOC) levels. PS MPs further increased this, with results ranging from −21.28 % to 26.49 %. Based on the parallel factor analysis (PARAFAC), FMDDOM was classified into three types of fluorescent components: tryptophan-like, humic-like, and tyrosine-like compounds. Contaminants of MPs and PFASs modified the relative abundance of these three components. Protein secondary structure analysis showed that fluorocarbon bonds tended to accumulate on the α-helix of proteins in FMDDOM. The relative abundance of fluorescent and chromophorous FMDDOMs varied from 0.648 ± 0.044 to 0.964 ± 0.173, indicating that the photochemical structures of the FMDDOM were modified. FMDDOM exhibits decreased humification and increased aromaticity when contaminated with MPs and PFASs, which may be detrimental to the geochemical cycling of carbon. This study offers a theoretical basis for assessing the combined ecological risks of MPs and PFASs in floating macrophyte ecosystems.

The Role of Dioxygen in Microbial Bio-Oxygenation: Challenging Biochemistry, Illustrated by a Short History of a Long Misunderstood Enzyme.

A Special Issue of Microorganisms devoted to 'Microbial Biocatalysis and Biodegradation' would be incomplete without some form of acknowledgement of the many important roles that dioxygen-dependent enzymes (principally mono- and dioxygenases) play in relevant aspects of bio-oxygenation. This is reflected by the multiple strategic roles that dioxygen -dependent microbial enzymes play both in generating valuable synthons for chemoenzymatic synthesis and in facilitating reactions that help to drive the global geochemical carbon cycle. A useful insight into this can be gained by reviewing the evolution of the current status of 2,5-diketocamphane 1,2-monooxygenase (EC 1.14.14.108) from (+)-camphor-grown Pseudomonas putida ATCC 17453, the key enzyme that promotes the initial ring cleavage of this natural bicyclic terpene. Over the last sixty years, the perceived nature of this monooxygenase has transmogrified significantly. Commencing in the 1960s, extensive initial studies consistently reported that the enzyme was a monomeric true flavoprotein dependent on both FMNH2 and nonheme iron as bound cofactors. However, over the last decade, all those criteria have changed absolutely, and the enzyme is currently acknowledged to be a metal ion-independent homodimeric flavin-dependent two-component mono-oxygenase deploying FMNH2 as a cosubstrate. That transition is a paradigm of the ever evolving nature of scientific knowledge.

The influence of cold seepage on the grain size and geochemistry of sediments from the Laptev Sea shelf

The East Siberian Arctic shelf is renowned for its active methane-rich fluid seeps, confirmed by numerous gas flares in the water column and direct observations of methane release from the seafloor. These cold-seep sites serve as natural laboratories for studying the impact of methane seepage on early diagenesis, authigenic mineral formation, and the geochemical cycles of carbon, sulfur, and other elements. This study examines the grain size and chemical composition of two sediment types from the Laptev Sea shelf: cold-seep sediments influenced by sulfate-driven anaerobic methane oxidation, and unaffected “seep-free” sediments (i.e., control site). High methane concentrations (up to 649 μM/L) in the surface layer of sediments indicate that the sulfate–methane transition zone is found close to the sediment–water interface approximately 5–6 cm below the seabed. At the cold-seep site, thin layers containing up to 40% sand fraction were discovered among the fine-grained sediments. We infer that upward-migrating fluids may contribute to the remobilization of the fine fraction, resulting in concentration of coarse particles. Cold-seep sediments are characterized by molybdenum (Mo) enrichment compared to the lithological background, although one “seep-free” core exhibits evidence of authigenic Mo deposition. The Mo concentration (up to 16.8 ppm) indicates that enrichment occurs under sulfidic conditions restricted to pore water, which are caused by anaerobic methane oxidation. The high Mo concentration in one “seep-free” core likely implies past cold-seep activity. A general Mo–U covariation indicates that the molybdenum enrichment in cold-seep sediments from the Laptev Sea outer shelf is controlled by a weak particulate shuttle process.

Biological Oxidation of Manganese Mediated by the Fungus Neoroussoella solani MnF107

Manganese oxides are highly reactive minerals and influence the geochemical cycling of carbon, nutrients, and numerous metals in natural environments. Natural Mn oxides are believed to be dominantly formed by biotic processes. A marine Mn-oxidizing fungus Neoroussoella solani MnF107 was isolated and characterized in this study. SEM observations show that the Mn oxides are formed on the fungal hyphal surfaces and parts of the hypha are enveloped by Mn oxides. TEM observations show that the Mn oxides have a filamentous morphology and are formed in a matrix of EPS enveloping the fungal cell wall. Mineral phase analysis of the fungal Mn oxides by XRD indicates that it is poorly crystalline. Chemical oxidation state analysis of the fungal Mn oxides confirms that it is predominantly composed of Mn(IV), indicating that Mn(II) has been oxidized to Mn (IV) by the fungus.

Carbon sequestration during Fe(III)-Fulvic acid coprecipitation and kinetics of hydrated Cu(II)

The dynamic interactions among iron (Fe) oxides and organic matter (OM) play vital roles in controlling the geochemical cycling of carbon (C) and toxic trace metals. Although the effects of OM on the Fe oxides transformation, and dynamic with toxic trace metals have been studied previously, it is still poor to understand the mechanistic and quantitative on the C sequestration during the Fe(III) mineralization process, and kinetics with hydrated Cu(II) in the acid drainage. In this study, Mössbauer spectroscopy, X-ray absorption spectroscopy (XAS), high resolution transmission electron microscopy (HRTEM), and density functional theory (DFT) were used to elucidate the underlying mechanism for the fine structure of iron minerals, fulvic acid (FA) sequestration and hydrated Cu(II) coordination on Fe oxides. FA dosages played an essential role in the formation of Fe oxides, which can inhibit the formation of Fe-O octahedrons and benefit to the Fe-O tetrahedron. FA molecules were mainly sequestrated on the surfaces of Fe oxides and van der Waals forces were the main interaction between FA and Fe oxides. The hydrated Cu(II) oxides mainly distributed on the surface of iron oxides as divalent oxidation states, and feasible coordinated with phenolic hydroxyl group of FA and Fe oxides. The knowledge of the dynamic coupling between Fe oxide transformation under the effect of FA and the fate of hydrated Cu(II) on Fe oxides provided the basis for predicting the geochemical process of Fe, C and toxic trace metals.

Overlooked Formation of Carbonate Radical Anions in the Oxidation of Iron(II) by Oxygen in the Presence of Bicarbonate

AbstractIron(II), (Fe(H2O)62+, (FeII) participates in many reactions of natural and biological importance. It is critically important to understand the rates and the mechanism of FeII oxidation by dissolved molecular oxygen, O2, under environmental conditions containing bicarbonate (HCO3−), which exists up to millimolar concentrations. In the absence and presence of HCO3−, the formation of reactive oxygen species (O2⋅−, H2O2, and HO⋅) in FeII oxidation by O2 has been suggested. In contrast, our study demonstrates for the first time the rapid generation of carbonate radical anions (CO3⋅−) in the oxidation of FeII by O2 in the presence of bicarbonate, HCO3−. The rate of the formation of CO3⋅− may be expressed as d[CO3⋅−]/dt=[FeII[[O2][HCO3−]2. The formation of reactive species was investigated using 1H nuclear magnetic resonance (1H NMR) and gas chromatographic techniques. The study presented herein provides new insights into the reaction mechanism of FeII oxidation by O2 in the presence of bicarbonate and highlights the importance of considering the formation of CO3⋅− in the geochemical cycling of iron and carbon.

Overlooked Formation of Carbonate Radical Anions in the Oxidation of Iron(II) by Oxygen in the Presence of Bicarbonate.

Iron(II), (Fe(H2 O)6 2+ , (FeII ) participates in many reactions of natural and biological importance. It is critically important to understand the rates and the mechanism of FeII oxidation by dissolved molecular oxygen, O2 , under environmental conditions containing bicarbonate (HCO3 - ), which exists up to millimolar concentrations. In the absence and presence of HCO3 - , the formation of reactive oxygen species (O2 ⋅- , H2 O2 , and HO⋅) in FeII oxidation by O2 has been suggested. In contrast, our study demonstrates for the first time the rapid generation of carbonate radical anions (CO3 ⋅- ) in the oxidation of FeII by O2 in the presence of bicarbonate, HCO3 - . The rate of the formation of CO3 ⋅- may be expressed as d[CO3 ⋅- ]/dt=[FeII [[O2 ][HCO3 - ]2 . The formation of reactive species was investigated using 1 H nuclear magnetic resonance (1 H NMR) and gas chromatographic techniques. The study presented herein provides new insights into the reaction mechanism of FeII oxidation by O2 in the presence of bicarbonate and highlights the importance of considering the formation of CO3 ⋅- in the geochemical cycling of iron and carbon.

Influences of lithium on soil properties and enzyme activities

Lithium is an emerging environmental contaminant under the current sustainable energy strategy, but little is known about its contamination characteristic in soil. In this study, soil properties and enzyme activities in soils treated with 10–1280 mg kg−1 lithium were measured. The results showed that the content of ammonium nitrogen, total nitrogen, and exchangeable potassium significantly increased by 64.39%–217.73%, 23.06%–131.86%, and 4.76%–16.10%, while electric conductivity and available phosphorus content in lithium treated soils was respectively as 1.10-fold–13.44-fold and 1.27-fold–6.66-fold comparing to CK value. Soil pH and cation exchange capacity slightly declined and increased, respectively, and there was no significant variation in total organic carbon. However, nitrate nitrogen and sulfate content significantly decreased under higher lithium stress. On the other hand, lower lithium treatment level of 10, 20, 40, or 80 mg kg−1 selectively promoted the activities of sucrase, urease, aryl sulfatase, and peroxidase, while the protease, neutral phosphatase, phytase, and lipase were significantly inhibited under all lithium levels, indicating a weaken geochemical cycling of carbon, nitrogen, phosphorus, and sulfur. Then, lithium's 10% and 50% ecological dose (ED10 and ED50) was respectively fitted as 21.18 and 1408.67 mg kg−1 basing on Geometric Mean Index. The influences of lithium on soil were adverse. This study provided important insights into understanding the characteristics of lithium contamination, informing risk assessment and guiding remediation.

Seasonal variations of n-damo bacterial community in the subtropical Mai Po mangrove wetland of Hong Kong

Mangrove wetland is one of the most important ecosystems along the coast and often suffers from global climate change as well as the anthropogenic activities. Nitrite-dependent anaerobic methane oxidization (n-damo) bacteria play an important role in linking the microbial geochemical cycles of carbon and nitrogen in the wetlands, but the pattern of their diversity and abundance to a changing mangrove wetland is poorly understood. In the present work, we investigated the seasonal variations of n-damo bacteria in vegetated and non-vegetated sediments of a subtropical mangrove wetland by amplifying 16 S rDNA and pmoA gene sequences. Results indicated that n-damo bacterial communities in the surface sediments maintained freshwater lineage characteristics as the majority after the long rainy season, while the abundance of marine lineages dominated in all the subsurface sediments. Shannon-Wiener and Chao1 indices based on 16 S rDNA gene sequences decreased largely in reed sediments by the end of rainy season, but increased in both surface and subsurface sediments of mangrove and mudflat zones, especially in the non-vegetated surface sediments. The majority of n-damo bacterial pmoA gene sequences shifted from Cluster SCS-2 to Cluster SCS-1 by the end of the dry season. Results suggested that Clusters SCS-1 and SCS-3 would be more likely detected in relatively pristine marine sediments, while Cluster SCS-2 tended to be abundant in coastal habitats with a strong terrestrial/anthropogenic influence. Vegetation had either direct or indirect influence on the decrease of alpha diversity of n-damo pmoA gene when the abundance showed an increase by the end of the wet season. The abundances of n-damo bacteria increased by two orders of magnitude to 5.1–9.4 × 107 copies/g dry sediment after the rainy season. Results suggested that NH4+ and NH4+/NO2− were significant factors governing the diversity of n-damo community in mangrove wetland sediments. The findings of this present work provided an in-depth understanding on the variations of n-damo bacterial community to seasonal changes in the subtropical mangrove wetland.

Spatial variation of the C: N ratio and its relationship with the carbon cycle in the middle reaches of the Yellow River during summer

ABSTRACT Rivers are the critical link between soil, the atmosphere, and the ocean’s extensive carbon reservoirs, but the discharge of sewage into rivers has not only increased the content of organic matter but also significantly altered the geochemical cycles of carbon and nitrogen in the land, water, and atmosphere. However, few studies have been conducted to establish the relationship between the changes and distribution of river pollutants and the river carbon cycle. In this study, twenty water samples were collected from sixteen rivers in the middle reaches of the Yellow River (YR). The results showed that the concentrations of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in the mainstream decreased from north to south, while DIC and DOC in tributaries showed the opposite trend in the middle reaches of YR during summer. The distribution of pCO2 and the C: N ratio had prominent regional distribution patterns in this study area, in which pCO2 tended to decrease from south to north, and the C: N ratio varied increasingly from south to north. The source or sink state of CO2 maybe can be judged by the variation of the C: N ratio in this study area during summer.

Estimation of Terrestrial Net Primary Productivity in the Yellow River Basin of China Using Light Use Efficiency Model

The net primary productivity (NPP) of vegetation is an essential factor of ecosystem functions, including the biological geochemical carbon cycle, which is often impacted by climate change and human activities. It plays a significant role in comprehending the nature of carbon balance in an ecosystem and demonstrates the global and regional carbon cycle dynamics. The present study used an upgraded CASA model to calculate the NPP in the Yellow River Basin (YRB), China. The model’s simulation ability was improved by changing the model parameters. Further, the CASA model was validated by comparing with MODIS-NPP and in situ observed NPP, wherein the accuracy of the CASA model estimation was found satisfactory to estimate NPP changes in the study area. The simulated results of the improved CASA model showed that the mean annual NPP value of vegetation in the YRB was 283.4 gC m–2 a–1 from 2001 to 2020, with a declining trend in spatial distribution from south to north. In contrast, the NPP appeared as an increasing trend in the YRB temporally from 212 gC m–2 a–1 in 2001 to 342 gC m–2 a–1 in 2020, with a mean annual growth rate of 4.6 gC m–2 a–1. The total NPP in the YRB increased by 40,088.3 GgC between 2001 and 2020, from 226.06 TgC to 266.15 TgC. This rise can be attributed to the increase in forests. The average grassland area has reduced by 4651 km2 during the last two decades, significantly impacting the total NPP of grasslands. Although the increase in NPP in wetlands was minimal, accounting for 815.53 GgC, the highest change percentage of 79.78%, could be observed among the six vegetation types due to the anthropogenic influences and climate change. The conditions favorable for vegetation growth and a sustained environment were enhanced by the increased precipitation and temperature and the reinforced ecological protection by the government.

Composition of microorganisms in carbonated mineral waters of the Mukhen deposit (Khabarovsk territory)

The chemical composition, distribution, structure, number of physiological groups of cultivated bacteria and their biodiversity in the cold carbonic mineral waters of Mukhen and in microbial mats were studied. It is shown that the mineral waters are cold, hydrocarbonate-calcium-magnesium, enriched with iron, manganese, barium. Carbon dioxide predominates in the gaseous composition of waters. Microbiological studies have shown that no sanitary-indicative microflora was found in mineral waters, which indicates the purity of underground waters. Carbonic waters were characterized by a low number of physiological groups of autochthonous bacteria. Among the studied microorganisms, chemolithotrophic thionic bacteria predominated, which indicates the predominance of oxidation processes of reduced sulfur compounds with the participation of bacteria in groundwater. In the microbial mats, various chemolithotrophic and heterotrophic microorganisms were identified, participating in the geochemical cycles of carbon, nitrogen, sulfur, iron, manganese, and silicon. The number of physiological groups of bacteria was higher than in mineral waters, along with this saprophytic bacteria predominated significantly. A sufficiently high rate of protein and cellulose decomposition by microorganisms of microbial mats was shown. A low diversity of cultured heterotrophic bacteria with the dominance of microorganisms of the genus Bacillus was found in mineral waters and in microbial mats. By using the methods of X-ray phase analysis, the important role of microorganisms of microbial mats in the precipitation of silicate minerals and the formation of calcium carbonates was shown.

Linking molecular composition to proton and copper binding ability of fulvic acid: A theoretical modeling approach based on FT-ICR-MS analysis

Dissolved organic matter (DOM) is one of the most important ligands governing the geochemical cycling of metals in the environment, but recent studies with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) have shown enormous complexity and diversity of DOM composition. How the diverse molecular composition of DOM affects the reactivity of DOM with metals is still largely unknown, which precludes us from developing accurate geochemical models for the fate of metals in the environment. In this study, we combined FT-ICR-MS analysis and theoretical modeling approaches and specifically elucidated the link between molecular composition and the proton and Cu binding ability of DOM, using the Suwannee River fulvic acid (FA) as a model humic substance. Batch adsorption experiments were conducted to generate different extents of molecular fractionation of FA samples by ferrihydrite. FT-ICR-MS analyses were employed to investigate the changes of molecular composition while Cu titration and the Windermere Humic Aqueous Model (WHAM) were used to quantify the variations on the Cu binding capacities of FA samples. We developed a general theoretical modeling approach, which integrated a suite of theoretical modeling methods, including the Vienna Soil-Organic-Matter-Modeler (VSOMM), SPARC Performs Automated Reasoning in Chemistry (SPARC), and the linear free energy relationships (LFER), for molecular modeling based on FT-ICR-MS data. Based on the FT-ICR-MS results, we found that, despite of the complex molecular composition of FA, FA molecules can be divided into three representative groups and each group of molecules had distinct chemical properties. Interestingly, molecules within the same group had similar distributions of molecular properties. Based on the chemical properties of the three groups of FA molecules, we successfully constructed three molecular models of FA using VSOMM, and quantified the distributions of proton and Cu binding constants with SPARC and LFER. Those independently determined binding constants were comparable to the WHAM default proton and Cu binding constants, supporting the validity of our modeling approach. Our modeling results suggested that the molecular complexity of DOM may be simplified with representative groups of molecules based on their binding ability with metals in theoretical modeling. Our modeling approach based on FT-ICR-MS data shed light on developing mechanistical models for metal reactions with DOM based on the molecular data, which is helpful for predicting the geochemical cycling of carbon and metals in the environment.

Iron and Carbon Isotope Constraints on the Formation Pathway of Iron-Rich Carbonates within the Dagushan Iron Formation, North China Craton

Banded iron formations (BIFs) are enigmatic chemical sedimentary rocks that chronicle the geochemical and microbial cycling of iron and carbon in the Precambrian. However, the formation pathways of Fe carbonate, namely siderite, remain disputed. Here, we provide photomicrographs, Fe, C and O isotope of siderite, and organic C isotope of the whole rock from the ~2.52 Ga Dagushan BIF in the Anshan area, China, to discuss the origin of siderite. There are small magnetite grains that occur as inclusions within siderite, suggesting a diagenetic origin of the siderite. Moreover, the siderites have a wide range of iron isotope compositions (δ56FeSd) from −0.180‰ to +0.463‰, and a relatively negative C isotope composition (δ13CSd = −6.20‰ to −1.57‰). These results are compatible with the reduction of an Fe(III)-oxyhydroxide precursor to dissolved Fe(II) through microbial dissimilatory iron reduction (DIR) during early diagenesis. Partial reduction of the precursor and possible mixing with seawater Fe(II) could explain the presence of siderite with negative δ56Fe, while sustained reaction of residual Fe(III)-oxyhydroxide could have produced siderite with positive δ56Fe values. Bicarbonate derived from both DIR and seawater may have provided a C source for siderite formation. Our results suggest that microbial respiration played an important role in the formation of siderite in the late Archean Dagushan BIF.

Garnet sand reveals rock recycling processes in the youngest exhumed high- and ultrahigh-pressure terrane on Earth

Rock recycling within the forearcs of subduction zones involves subduction of sediments and hydrated lithosphere into the upper mantle, exhumation of rocks to the surface, and erosion to form new sediment. The compositions of, and inclusions within detrital minerals revealed by electron microprobe analysis and Raman spectroscopy preserve petrogenetic clues that can be related to transit through the rock cycle. We report the discovery of the ultrahigh-pressure (UHP) indicator mineral coesite as inclusions in detrital garnet from a modern placer deposit in the actively exhuming Late Miocene-Recent high- and ultrahigh-pressure ((U)HP) metamorphic terrane of eastern Papua New Guinea. Garnet compositions indicate the coesite-bearing detrital garnets are sourced from felsic protoliths. Carbonate, graphite, and CO2 inclusions also provide observational constraints for geochemical cycling of carbon and volatiles during subduction. Additional discoveries include polyphase inclusions of metastable polymorphs of SiO2 (cristobalite) and K-feldspar (kokchetavite) that we interpret as rapidly cooled former melt inclusions. Application of elastic thermobarometry on coexisting quartz and zircon inclusions in six detrital garnets indicates elastic equilibration during exhumation at granulite and amphibolite facies conditions. The garnet placer deposit preserves a record of the complete rock cycle, operative on <10-My geologic timescales, including subduction of sedimentary protoliths to UHP conditions, rapid exhumation, surface uplift, and erosion. Detrital garnet geochemistry and inclusion suites from both modern sediments and stratigraphic sections can be used to decipher the petrologic evolution of plate boundary zones and reveal recycling processes throughout Earth's history.

Functional Groups of Bacteria and Their Involvement in Geochemical Processes in Microbial Communities of Mineral Sources in Sakhalin Island (Far East, Russia)

The chemical composition of mineral waters as well as the diversity, quantity and distribution peculiarities of different functional groups of bacteria in microbial mats in Sakhalin mineral springs was studied. The prevalence of sodium cations in the composition of bicarbonate anions and chloride ions was noticed. The studied waters were substantially enriched with silicone, iodine, boron, iron and strontium. It was established that the main morphogenic bacteria in the mats of cold springs were the colorless sulfur bacteria Thiotrix sp. The microbial mats formed at the outlets of Sakhalin mineral springs contain a high quantity of different functional bacteria groups that play an important role in the geochemical cycles of carbon, nitrogen, sulfur, iron and manganese. Prevalence of bacteria of sulfur and carbon cycles is a peculiarity of microbial mats in Antonovsky and Nevelsky mineral waters. The important role of sulfur cycle bacteria is confirmed by high quantities of thionic, sulfate-reducing bacteria and colorless sulfur bacteria. The mat of Alexandrovsky thermal spring contains a high amount of bacteria of carbon and nitrogen geochemical cycles; an important role of these microorganisms in mat’s functioning is shown. Pure bacterial cultures were isolated; they mostly include gram-positive, spore-forming motile rods Bacillus sp.

Heterotrophic Bacteria Exhibit a Wide Range of Rates of Extracellular Production and Decay of Hydrogen Peroxide

Bacteria have been implicated as both a source and sink of hydrogen peroxide (H2O2), a reactive oxygen species which can both impact microbial growth and participate in the geochemical cycling of trace metals and carbon in natural waters. In this study, simultaneous H2O2 production and decay by twelve species of heterotrophic bacteria were evaluated in both batch and flow-through incubations. While wide species-to-species variability of cell-normalized H2O2 decay rate coefficients [2x10-8 to 5x10-6 hr-1 (cell mL-1)-1] was observed, these rate coefficients were relatively consistent for a given bacterial species. By contrast, observed production rates (below detection limit to 3x102 amol cell-1 hr-1) were more variable even for the same species. Variations based on incubation conditions in some bacterial strains suggest that external conditions may impact extracellular H2O2 levels either through increased extracellular production or leakage of intracellular H2O2. Comparison of H2O2 production rates to previously determined O2- production rates suggests that O2- and H2O2 production are not necessarily linked. Rates measured in this study indicate that bacteria could account for a majority of H2O2 decay observed in aqueous systems but likely only make a modest contribution to dark H2O2 production.

Kinetics of As(V) and carbon sequestration during Fe(II)-induced transformation of ferrihydrite-As(V)-fulvic acid coprecipitates

The dynamic interactions among iron (Fe) oxides, organic matter (OM) and heavy metals/metalloids play crucial roles in controlling the geochemical cycling of carbon (C) and heavy metals/metalloids in natural environments. Although the inhibitory effects of arsenate (As(V)) or OM on the ferrihydrite transformation process have been studied previously, there is still a lack of mechanistic and quantitative understanding on the kinetics of As(V) and C sequestration during the Fe(II)-induced ferrihydrite transformation. In this study, we employed a suite of techniques to elucidate the underlying mechanisms accounting for the temporal changes of As(V) and C distributions and speciation on Fe oxides during the Fe(II)-induced ferrihydrite transformation process. Characterizations with X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and the spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) at different times indicated that the presence of As and/or FA resulted in the formation of more lepidocrocite than goethite. Besides surface adsorption, a portion of As(V) may be incorporated into the lattice structures of newly formed crystalline Fe oxides, and the amount of As(V) sequestration within Fe oxides were correlated with the formation of crystalline Fe oxides. In comparison, FA molecules were either adsorbed on the surfaces of goethite or diffused into the defects or nano pore spaces of lepidocrocite. Our results shed the light on different nanoscale mechanisms accounting for the sequestration of C and As(V) on Fe oxides. The defects or nano pore spaces formed in the structures of lepidocrocite may provide an effective way to sequester C through physical isolation, while the crystal structures of Fe oxides may sequester As(V) through isomorphic substitution. The knowledge on the dynamic coupling between Fe oxide transformation and C/As(V) sequestration provided the basis for accurately predicting the geochemical cycling of trace elements and OM.