Role Of Microbial Activity Research Articles

Study of the Main Factors Responsible for the Phenomenon of Self-Combustion Observed in Solid Biomass Waste Piles

This article discusses the causes of wood-derived biomass self-combustion. We researched technical and academic literature on chemical processes at SENAI CIMATEC. The factors influencing spontaneous combustion are microbial activity, temperature, humidity, and the concentration of O2 and CO2. Fungus and bacteria play a leading role in microbial activity, degrading organic matter and releasing toxic and flammable gases. The temperature is crucial for monitoring this phenomenon.

Unveiling the power of COD/N on constructed wetlands in a short-term experiment: Exploring microbiota co-occurrence patterns and assembly dynamics

Constructed wetlands (CWs) are a cost-effective and environmentally friendly wastewater treatment technology. The influent chemical oxygen demand (COD)/nitrogen (N) ratio (CNR) plays a crucial role in microbial activity and purification performance. However, the effects of CNR changes on microbial diversity, interactions, and assembly processes in CWs are not well understood. In this study, we conducted comprehensive mechanistic experiments to investigate the response of CWs to changes in influent CNR, focusing on the effluent, rhizosphere, and substrate microbiota. Our goal is to provide new insights into CW management by integrating microbial ecology and environmental engineering perspectives. We constructed two groups of horizontal subsurface flow constructed wetlands (HFCWs) and set up three influent CNRs to analyse the microbial responses and nutrient removal. The results indicated that increasing influent CNR led to a decrease in microbial α-diversity and niche width. Genera involved in nitrogen removal and denitrification, such as Rhodobacter, Desulfovibrio, and Zoogloea, were enriched under medium/high CNR conditions, resulting in higher nitrate (NO3--N) removal (up to 99 %) than that under lower CNR conditions (<60 %). Environmental factors, including water temperature (WT), pH, and phosphorus (P), along with CNR-induced COD and NO3--N play important roles in microbial succession in HFCWs. The genus Nitrospira, which is involved in nitrification, exhibited a significant negative correlation (p < 0.05) with WT, COD, and P. Co-occurrence network analysis revealed that increasing influent CNR reduced the complexity of the network structure and increased microbial competition. Analysis using null models demonstrated that the microbial community assembly in HFCWs was primarily driven by stochastic processes under increasing influent CNR conditions. Furthermore, HFCWs with more stochastic microbial communities exhibited better denitrification performance (NO3--N removal). Overall, this study enhances our understanding of nutrient removal, microbial co-occurrence, and assembly mechanisms in CWs under varying influent CNRs.

Toward Oral Thanatomicrobiology-An Overview of the Forensic Implications of Oral Microflora.

The oral cavity is home to numerous microorganisms including bacteria, fungi, and viruses which together form the oral microflora. It is the second most diverse microbial site in the human body after the gastrointestinal tract. Microbial degradation is a common phenomenon that occurs after death, with the early and advanced stages of decomposition being closely associated with oral microbial activity. This article reviews the current state of knowledge on the role of the oral microflora in postmortem events, and highlights the growing importance of terms such as forensic microbiology and thanatomicrobiome. This article also discusses next-generation sequencing, metagenomic sequencing studies, and RNA sequencing to study the oral thanatomicrobiome and epinecrotic communities in forensic oral genetics. The indigenous microorganisms in the oral cavity are among the first to respond to the process of decomposition. DNA/RNA sequencing is a relatively simple, precise, and cost-effective method to estimate biological diversity during various stages of postmortem decomposition. The field of thanatomicrobiology is rapidly evolving into a key area in forensic research. This article briefly narrates oral microflora and its implications in forensic odontology. The role of microbial activity in postmortem events is gaining importance in forensic research, and further studies are needed to fully understand the potential applications of advanced technology in the study of the oral thanatomicrobiome.

Bioinduced precipitation of smectites and carbonates in photosynthetic diatom-rich microbial mats: Effect of culture medium

The association of MgII clays with dolomite is common in ancient and modern lacustrine environments, although, the role of microbial activity in their precipitation remains a challenge. In this study, microbial culture experiments provide the first evidence of saponite and dolomite precipitation on diatom-bearing photosynthetic microbial mats. Microbial strains were isolated from photosynthetic microbial mats collected from Caballo Alba lake in Segovia, Spain. The microbial communities´ growth and mineral precipitates varied function of the type of media used, semi-solid or liquid, respectively. The prokaryotic composition identified from cultures was dominated by Cyanobacteria and Proteobacteria. Bacteroidetes were the most abundant heterotrophic bacteria. Saponite, aragonite, and dolomite crystals were closely attached to the phytoplanktonic communities grown in liquid cultures. Diatoms are replaced by saponite and dolomite in liquid cultures where diatoms are unprovided of protection of extracellular polymeric substances (EPS). Saponite grew around the diatom frustules, as dissolution of the biogenic silica occurs in a medium depleted in silica, replacing diatoms by saponite pseudomorphs. To a lesser extent, cyanobacterial cell walls were also covered by saponite crystals. In addition, amorphous silica of the frustules also provided a suitable template for the nucleation of dolomite crystals. Alternatively, MgII-rich aragonite crystals with fibrous morphologies precipitated within saponite crystal aggregates, suggesting that clay minerals enhanced the nucleation and precipitation of dolomite. In the semi-solid medium, where calcite precipitated in association with Cyanobacteria, diatoms were capable of developing a sheet of EPS which protected their frustules from dissolution. Here, we suggest that, in alkaline lakes phytoplanktonic diatoms might have contributed to the precipitation of clay minerals and carbonates since the Jurassic.

Mineralogical and Genomic Constraints on the Origin of Microbial Mn Oxide Formation in Complexed Microbial Community at the Terrestrial Hot Spring.

Manganese (Mn) oxides are widespread on the surface environments of the modern Earth. The role of microbial activities in the formation of Mn oxides has been discussed for several decades. However, the mechanisms of microbial Mn oxidation, and its role in complex microbial communities in natural environments, remain uncertain. Here, we report the geochemical, mineralogical, and metagenomic evidence for biogenic Mn oxides, found in Japanese hot spring sinters. The low crystallinity of Mn oxides, and their spatial associations with organic matter, support the biogenic origin of Mn oxides. Specific multicopper oxidases (MCOs), which are considered Mn-oxidizing enzymes, were identified using metagenomic analyses. Nanoscale nuggets of copper sulfides were, also, discovered in the organic matter in Mn-rich sinters. A part of these copper sulfides most likely represents traces of MCOs, and this is the first report of traces of Mn-oxidizing enzyme in geological samples. Metagenomic analyses, surprisingly, indicated a close association of Mn oxides, not only in aerobic but also in anaerobic microbial communities. These new findings offer the unique and unified positions of Mn oxides, with roles that have not been ignored, to sustain anaerobic microbial communities in hot spring environments.

Analytical Modeling for Water Chemistry Changes in River Bank Filtration Systems

Riverbank filtration system is considered one of the economic and sustainable solutions to river water pollution especially in tropical countries such as Malaysia. In this work, an analytical model is developed to simulate the contaminant attenuation in riverbank filtration systems by using the separation of variables method. The basic aim of the model is to understand the role of microbial activity that occurs in riverbed sediments on reducing the concentration of the contaminant in the aquifer and changing the water characteristics. Graphically, it is found that the model can simulate the infiltration process of polluted river water effectively. Also, the analytical model results, as well as experimental data, show that nitrate (18.6 and 34.1 mg-NO3/L) and sulphate (20.9 – 22.1 mg-SO4/L) can be consumed by bacteria in the first 0.5 m of the aquifer, and reduced by more than 95% for both compounds. The model is applied for the first riverbank filtration system in Malaysia. Sensitivity analysis results highlight the importance of dissolved organic matter (DOM) concentration (ranged from 1.0 to 12.4 mg/L) for RBF efficacy in which a higher concentration of DOM leads to faster consumption of pollutants.

Sulfur and CO2 gases emitted during weathering of sulfides: Role of microbial activity and implications to exploration through cover

• Relative abundance of gas produced by weathering processes is mineral dependent. • CS 2 is the dominant sulfur gas produced during sulfide weathering. • Presence of microbes impacts sulfur gas production. • Understanding microbes’ role is key to use of soil gases in mineral exploration. In mineral exploration, a thick cover is challenging and can hinder the discovery of new world class mineral deposits. Traditional surface geochemistry techniques are virtually “blind” in areas where the cover exceeds a few meters. Therefore, alternative surface geochemistry approaches such as the detection of soil gas anomalies over buried mineral deposits have been investigated and developed. This study investigates the source and potential of soil gases for mineral exploration by comparing equilibrium thermodynamic models with gases detected during weathering of “pure” sulfides and sulfide ores in the laboratory. Weathering experiments were conducted under moist settings, both for sterile and non-sterile conditions, and under water-saturated settings for non-sterile environments. Gaseous hydrocarbons were below detection limits, suggesting that any hydrocarbons produced during sulfide oxidation would only reach low abundances. Thermodynamic models undertaken in both moist and water-saturated conditions predict CO 2 as the dominant gas produced. However, CS 2 is the dominant sulfur gas produced in laboratory experiments, contradicting thermodynamic predictions. This suggests that experiments did not reach equilibrium. Sulfide weathering under non-sterile conditions produce greater relative abundance of gases than sterile experiments. This suggests that microbes play an important role in the weathering of sulfide to produce gases. Thus, the mechanisms involved in the production of gases from microbes-sulfide interactions must be considered in soil gas geochemical studies applied to mineral exploration.

Microbial activity effect on aggregate stability after residue addition in a Mollisol and a Vertisol in the Pampas, Argentina

Crop residue addition to the soil promotes an increase in microbial activity and, as a consequence, may improve soil aggregate stability. However, this effect may be different in soils with contrasting aggregation agents. The aim of this study was to evaluate, in an incubation experiment, the role of microbial activity on soil aggregate stability after crop residue addition in two soils with contrasting clay mineralogy. Soybean and maize crop residues were added with three different frequencies on a Mollisol and a Vertisol, which were incubated during a 120-days period. It was studied the dynamics of soil aggregate stability after application of a fast wetting (MWDfw), slow wetting (MWDsw) and stirring after prewetting treatment (MWDst) which evaluates three different breakdown mechanism, i.e., slacking, microcracking and mechanical breakdown, respectively. Microbial activity dynamic was evaluated through measurements of soil respiration rate, microbial biomass-C (MBC) and hot-water extractable carbohydrate-C (HWEC). The Vertisol showed higher resistance to slaking (59% higher) but lower resistance to mechanical breakdown (92% lower) and microcracking (102% lower) than the Mollisol, with a scarce effect of residue quality and addition frequency. The discrete changes recorded in MWDfw throughout the experiment, were positively associated with changes in cumulative respiration (P < 0.001), MBC (P < 0.05) y HWEC (P < 0.05) in the Mollisol, and only with changes in MBC in the Vertisol (P < 0.05). Also, the small changes in MWDst were weakly and positively associated (P < 0.05) with changes in cumulative respiration in the Mollisol but not in the Vertisol. However, changes in MWDsw were closely associated with changes in cumulative soil respiration rate (P < 0.0001) and MBC (P < 0.001) in both soils, and with HWEC only in the Mollisol (P < 0.0001), with a higher effect on these variables than the other aggregate stability test. However, while in the Vertisol the increase in MWDsw as a function of cumulative soil respiration was lineal, in the Mollisol it was detected an evident increase in MWDsw up to a threshold of 500 μg C-CO2 g soil−1 of cumulative soil respiration. Similarly, changes in MWDsw associated with changes in MBC were higher in the Mollisol than in the Vertisol (i.e. 0.018 v. 0.004 mm of MWDsw per unit of MBC). Thus, this research added new evidence about the contrasting role of transitory aggregation agents that are provided by microbial activity on aggregate stability in two contrasting soils such as a Mollisol and a Vertisol.

Microbial colonization of metal sulfide minerals at a diffuse-flow deep-sea hydrothermal vent at 9°50'N on the East Pacific Rise.

Metal sulfide minerals, including mercury sulfides (HgS), are widespread in hydrothermal vent systems where sulfur-oxidizing microbes are prevalent. Questions remain as to the impact of mineral composition and structure on sulfur-oxidizing microbial populations at deep-sea hydrothermal vents, including the possible role of microbial activity in remobilizing elemental Hg from HgS. In the present study, metal sulfides varying in metal composition, structure, and surface area were incubated for 13days on and near a diffuse-flow hydrothermal vent at 9°50'N on the East Pacific Rise. Upon retrieval, incubated minerals were examined by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), and epifluorescence microscopy (EFM). DNA was extracted from mineral samples, and the 16S ribosomal RNA gene sequenced to characterize colonizing microbes. Sulfur-oxidizing genera common to newly exposed surfaces (Sulfurimonas, Sulfurovum, and Arcobacter) were present on all samples. Differences in their relative abundance between and within incubation sites point to constraining effects of the immediate environment and the minerals themselves. Greater variability in colonizing community composition on off-vent samples suggests that the bioavailability of mineral-derived sulfide (as influenced by surface area, crystal structure, and reactivity) exerted greater control on microbial colonization in the ambient environment than in the vent environment, where dissolved sulfide is more abundant. The availability of mineral-derived sulfide as an electron donor may thus be a key control on the activity and proliferation of deep-sea chemosynthetic communities, and this interpretation supports the potential for microbial dissolution of HgS at hydrothermal vents.

Impact of microbial activity on the mobility of metallic elements (Fe, Al and Hg) in tropical soils

Dissolved organic carbon (DOC), especially low molecular mass organic acids (LMMOAs) derives principally from biota degradation process in which soil microorganisms are the main actors and from roots exudates. The presence of LMMOAs led to an increase of availability and mobility of metallic elements through the formation of organo-metallic complex. In tropical soils, very few information about LMMOAs quantification and their role in the biogeochemical process related to trace metals cycling was available. Quantification of LMMOAs is limited due to their low concentration and rapid degradation. Until now, the role of microbial activity as well as LMMOAs in the biogeochemical cycle of metallic elements in tropical soils has not been investigated. The present study was conducted to evaluate the effect of microbial activity and biomass on the availability and mobility of metallic elements (Fe, Al and Hg) in two tropical soils, Ferralsol and Acrisol. We also quantified LMMOAs contents in soil solutions and addressed to their role in the mobilization of metals.Utilization of Diffuse Gradient in Thin film (DGT) method permits to analyze bioavailable metal in both fractions: organically complexed and free metals. The results show that the quantity of Fe, Al and Hg labile were higher in Ferralsol than Acrisol soils. This was more accentuated for the 50 cm-depth of soils where the microbial activities and the organic carbon content were important. Concentration of LMMOAs of Ferralsol and Acrisol were lower in compare to coniferous and deciduous forest soils. Proportions of LMMOAs in DOC were very small at 10.5% and 6.85% in the Ferralsol and Acrisol soils, respectively. The mobilization of Fe, Al and Hg in Ferralsol and Acrisol soils appeared to vary depending on the soil physico-chemical characteristics (sorption capacities and metals content) and also on the microbial biomass and activity. Soil pH influences the acidity of the functional groups in organic molecules and consequently their speciation. In addition, low pH increase proton competition within acidic functional groups involved in coordinate bond. The content of CEC in Ferralsol is higher than Acrisol that is related to the high contents of clay and organic carbon. Low CEC content can result in a decrease of retain of the cationic trace metals. Low CEC content led to a decrease of the capacity of retaining of metallic elements in tropical soils in compare to temperate soils.

Roles of Microbial Activity and Anthraquinone-2,7-disulfonate as a Model of Humic Substances in Leaching of Iron from Hematite into Seawater.

Fertilization with a mixture of steelmaking slag and compost can affect the supply of dissolved iron used to restore seaweed beds, however, the mechanisms of iron elution from the fertilizer are not well understood. In the present study, the microorganism was isolated from Fe-fertilizer incubated in coastal seawater for 6 months, and was identified as Exiguobacterium oxidotolerans by 16S rDNA sequencing. The iron elutability of the bacteria was proved based on the increasing of dissolved iron by incubation with Fe2O3 (hematite) under a seawater-like condition. The value of ORP was changed by inoculated of the bacteria from ca. 0 to ca. -400 mV, which is anticipated concerning to reduction of Fe. The concentration of eluted iron was largely depended on those of organic acids produced by bacteria. From the results, it was proved that E. oxidotolerans is capable of Fe reductive eluting of iron from Fe2O3 into seawater. Anthraquinone-2,7-disulfonate (AQDS), which can play as an electron acceptor/donor between microbe and insoluble Fe2O3 particles, enhanced the effect of iron bio-leaching.

Role of microbial activity in Fe(III) hydroxysulfate mineral transformations in an acid mine drainage-impacted site from the Dabaoshan Mine

Fe(III) hydroxysulfate minerals are secondary minerals commonly found in acid mine drainage (AMD) sites and have a major impact on water and soil quality in these environments. While previous studies showed that the Fe(III) hydroxysulfate mineral transformation could be mediated by some bacterial strains under laboratory conditions, the role of indigenous microbial activity in Fe(III) hydroxysulfate mineral transformation in natural environment has received little attention. In this study, microcosms were constructed with AMD-affected river water and sediment from the Dabaoshan Mine that was either left unamended or enriched with nutrients (lactate, nitrogen, and phosphorus (LNP)) and biosynthetic minerals (schwertmannite or jarosite). The results show that microbial activity played a decisive role in the mineralogical transformation of schwertmannite/jarosite in the AMD-contaminated site when organic carbon was available. The accumulation of Fe(II) and sulfide in microcosms amended with LNP indicates that schwertmannite/jarosite transformation is mediated by microbial reduction. XRD, SEM and FTIR analyses suggest that schwertmannite was completely transformed to goethite in the Sch-LNP microcosms at the end of their incubation. Jarosite in the Jar-LNP microcosms was also transformed to goethite, but at a much slower rate than schwertmannite. Bacterial community analysis reveals that the stimulated indigenous bacteria promote the mineralogical transformation of schwertmannite/jarosite. Most of these bacteria, including Geobacter, Desulfosporosinus, Geothrix, Desulfurispora, Desulfovibrio, and Anaeromyxobacter, are known to reduce iron and/or sulfate. The mineralogical transformation of schwertmannite and jarosite exerts significant control on the geochemistry of AMD-contaminated systems.

Siderophore and Organic Acid Promoted Dissolution and Transformation of Cr(III)-Fe(III)-(oxy)hydroxides.

The role of microbial activities on the transformation of chromium (Cr) remediation products has generally been overlooked. This study investigated the stability of Cr(III)-Fe(III)-(oxy)hydroxides, common Cr(VI) remediation products, with a range of compositions in the presence of common microbial exudates, siderophores and small organic acids. In the presence of a representative siderophore, desferrioxamine B (DFOB), iron (Fe) was released at higher rates and to greater extents relative to Cr from all solid phases. The presence of oxalate alone caused the release of Cr, but not of Fe, from all solid phases. In the presence of both DFOB and oxalate, oxalate acted synergistically with DFOB to increase the Fe, but not the Cr, release rate. Upon reaction with DFOB or DFOB + oxalate, the remaining solids became enriched in Cr relative to Fe. Such incongruent dissolution led to solid phases with different compositions and increased solubility relative to the initial solid phases. Thus, the presence of microbial exudates can promote the release of Cr(III) from remediation products via both ligand complexation and increased solid solubility. Understanding the potential reaction kinetics and pathways of Cr(VI) remediation products in the presence of microbial activities is necessary to assess their long-term stability.

Role of Microbial Activity in Bioleaching of a Pyritic and a Pure Chalcopyrite Concentrate

Leaching of a pyritic and a pure chalcopyrite concentrate was carried out in stirred tank reactors in the absence and presence of a mixed culture of moderately thermophilic microorganisms at 45°C and pH 1.5. To study the effect of microbial activity on copper dissolution, the abiotic experiments were performed under accurately controlled redox potential conditions to reproduce the same oxidising conditions recorded during the bioleaching experiments. X-ray photoelectron spectrometry (XPS) was used to study the surface of chalcopyrite chips leached for different durations. The results showed that the microorganisms in cases of both concentrates did not have any effect in the copper leaching efficiency other than oxidation of ferrous to ferric ions. Biooxidation of elemental sulphur did not improve the leaching efficiency and bulk and surface jarosite had no negative effect on dissolution. A composite layer composed of mainly elemental sulphur and iron-oxy-hydroxide was found to be responsible for the hindered dissolution.

Studies on Power Generation and Wastewater Treatment Using Modified Single Chamber Microbial Fuel Cell

A membrane-less and mediator-less system was designed and tested with wastewater sample as fuel to generate electricity. Microorganisms were first isolated from the wastewater sample to pure culture and were used as the ‘machinery’ that converts wastewater into energy. The wastewater samples were treated either by sterilization or non-sterilization methods. These tests were run using a modified air-cathode single chamber microbial fuel cell (MFC). By sterilizing the wastewater, the calculated power density was much lower compared to non-sterilized wastewater indicating a significant role of microbial activity in the SCMFC system and substrate availability. Furthermore, mixed culture was observed to give larger power density compared to an individual microbe (18.42 ± 5.84 mW/m2 for mixed culture and 8.82 ± 4.56 mW/m2 to 9.46 ± 4.87 mW/m2 for individual microbe, Bukholderi capecia and Acidovorax sp. respectively) to prove that larger power value could be achieved with a mixed microbial system. In addition, the system proved to remove 68.57% of chemical oxygen demand (COD) of the wastewater sample tested. In conclusion, the designed SCMFC has been proven capable of power generation and wastewater treatment comparable to other SCMFCs to date.

Uranium isotopes fingerprint biotic reduction

Knowledge of paleo-redox conditions in the Earth's history provides a window into events that shaped the evolution of life on our planet. The role of microbial activity in paleo-redox processes remains unexplored due to the inability to discriminate biotic from abiotic redox transformations in the rock record. The ability to deconvolute these two processes would provide a means to identify environmental niches in which microbial activity was prevalent at a specific time in paleo-history and to correlate specific biogeochemical events with the corresponding microbial metabolism. Here, we demonstrate that the isotopic signature associated with microbial reduction of hexavalent uranium (U), i.e., the accumulation of the heavy isotope in the U(IV) phase, is readily distinguishable from that generated by abiotic uranium reduction in laboratory experiments. Thus, isotope signatures preserved in the geologic record through the reductive precipitation of uranium may provide the sought-after tool to probe for biotic processes. Because uranium is a common element in the Earth's crust and a wide variety of metabolic groups of microorganisms catalyze the biological reduction of U(VI), this tool is applicable to a multiplicity of geological epochs and terrestrial environments. The findings of this study indicate that biological activity contributed to the formation of many authigenic U deposits, including sandstone U deposits of various ages, as well as modern, Cretaceous, and Archean black shales. Additionally, engineered bioremediation activities also exhibit a biotic signature, suggesting that, although multiple pathways may be involved in the reduction, direct enzymatic reduction contributes substantially to the immobilization of uranium.

Fate of 17β-estradiol and 17α-ethinylestradiol in batch and column studies simulating managed aquifer recharge

Laboratory-scale batch and soil columns experiments were conducted to investigate the attenuation of estrogens (17β-estradiol and 17α-ethinylestradiol) during managed aquifer recharge. The role of microbial activity in the removal of selected estrogens was evaluated by comparing the results from biotic and abiotic batch experiments. Moreover, batch experiments were carried out using the sand media prepared over different acclimation periods to investigate the impact of acclimation periods on the removal of selected estrogens. Batch studies showed that adsorption was the dominant removal mechanism in the removal of 17β-estradiol and 17α-ethinylestradiol. 17β-estradiol and 17α-ethinylestradiol were attenuated by 99% and 96%, respectively, in batch experiments under oxic conditions. Redox conditions did not show any significant effect on the attenuation of 17β-estradiol. However, the net estrogenicity of 17β-estradiol remaining was lower under oxic conditions (130 ng estradiol-equivalents/L) than anoxic conditions (970 ng estradiol-equivalents/L). Column studies operated at 17 h of empty bed contact time also demonstrated that removal mechanism of 17α-ethinylestradiol was more dependent on adsorption than biodegradation.

The Role of Microbial Activity In the Generation of Lower Cretaceous Mixed FE-Oxide-phosphate Ooids from the Provencal Domain, French Maritime Alps

Abstract Early Hauterivian mixed Fe-oxide–phosphate ooids from the Lower Cretaceous condensed succession of the Alpine Provencal Domain are ellipsoidal, 200–2000 µm in diameter, and the main component of a sedimentary body, 0–100 cm thick, that can be traced laterally for tens of kilometers. Cathodoluminescence, EDS microprobe, backscattered electron imaging, epifluorescence, XRD, and micro Raman spectroscopy reveal that ooid cortices consist of variable proportions of Fe-oxide-rich and Ca-phosphate-rich layers. SEM observations show that phosphate-rich layers are composed of a dense matrix of micrometer-size, rod-shaped, commonly curved phosphate grains, whose size and shape strongly suggest a microbial origin. Conversely, Fe-oxide-rich ooid cortical layers consist of aggregates of micrometer-size, plate-like crystals of hematite that point to an inorganic precipitation. We suggest that the ooidal condensed interval records a long sedimentary history in an open shelf environment subject to a background accumulation of organic-rich, fine-grained sediments. It was periodically interrupted by high-energy, storm-related events that resulted in winnowing, reworking, and oxygenation that favored precipitation of Fe-oxides around nuclei of biogenic origin. Such precipitation was not uniform on the outer surface of ooids and influenced their ellipsoidal shape. After the return to background conditions, Fe-ooids were buried and driven from an oxic to a post-oxic subbottom geochemical environment where rod-shaped Fe-reducing bacteria colonized Fe-ooid outer surfaces. Decomposition of organic matter and release of P bound on Fe-oxides increased pore-water concentration of phosphate, which precipitated at the outer surfaces of the ooids, thus preserving the microbial community. The bimineralic composition of these ooids discloses a complex and prolonged history of cyclic alternations of oxic and post-oxic geochemical conditions, which are in turn related to alternating sediment accumulation and winnowing phases with exposure at the seafloor.

Production and elimination of water extractable organic matter in a calcareous soil as assessed by UV/Vis absorption and fluorescence spectroscopy of its fractions isolated on XAD-8/4 resins

Water extractable organic matter (WEOM) in soil refers to a broad mixture of water-soluble structures recoverable in laboratory through predetermined procedures. In this work, we investigate the mechanisms controlling the occurrence of WEOM fractions isolated on XAD-8/4 resins in a calcareous soil profile as a function of soil depth and seasonal climatic changes. WEOM was roughly collected on a bi-weekly basis during eight consecutive months from an experimental field in the Mediterranean region, south-east of France. Each fraction was analysed for organic carbon content and further characterised using UV/Vis absorbance and 3D fluorescence spectroscopy.The concentrations of hydrophobic (HPOA), transphilic (TPIA) and hydrophilic (HPIA) acids fractions isolated from WEOM followed similar decreasing trends with increasing soil depth, but HPOA decreased most sharply. Specific UV absorbance at 254nm (SUVA254) indicated that HPOA's distribution along the soil column is strongly influenced by the selective removal of aromatic structures, which could be controlled by adsorption mechanisms involving organo-mineral interactions and/or by a shift towards less aromatic precursors in deeper soil layers. 3D spectra of fluorescence revealed the presence of four types of fluorophores: two humic-like (A and C) common to all fractions, one lignin-like (H) specific to HPOA particularly in the upper soil layer, and two protein-like (B1 and B2) specific to TPIA. The importance of the humic component in all fractions was verified by the highly significant correlations between their carbon content and the intensity of humic-like peaks. The aging index (IA/IC) and the fluorescence index (FI), two indicators derived from 3D fluorescence spectra, showed an increase in the microbial input and the degree of humification in HPOA as a function of increasing soil depth. By comparison, IA/IC and FI values in HPIA and TPIA indicated that both fractions were composed almost exclusively of microbial-derived structures, regardless of soil depth.Principal component analysis (PCA) showed the association of temperature (T) to the carbon content in all fractions extracted from the soil layers 0–30 and 30–60cm, suggesting possible role of microbial activity on their occurrence. The hydraulic regime in soil, controlled by the frequency of rainfall events, affected most significantly HPIA across the entire soil column. PCA, however, failed to identify any consistent trends relating the qualitative properties of WEOM fractions to climatic variables.

Desert varnish-nature's smallest sedimentary formation

Desert varnish is a surface stain, coating bedrock surfaces as well as surface sediments in arid regions around the world. The desert varnish itself comprises a sedimentary formation containing internal microscopic laminations. The chemical and mineral composition of each layer reflects the climate conditions under which it was deposited. A detailed study of these individual layers is used to both reconstruct local and regional climate history as well as quantitatively date geomorphic surfaces. Although the mineral and chemical components comprising desert varnish consist of wind-blown dust, atmospheric aerosols and microbes, the role of microbial activity in the formation of desert varnish remains a source of active research and lively debate.