Iron-oxidizing Microorganisms Research Articles

Osmotic response in Leptospirillum ferriphilum isolated from an industrial copper bioleaching environment to sulfate

Iron and sulfur-oxidizing microorganisms play important roles in several natural and industrial processes. Leptospirillum (L.) ferriphilum, is an iron-oxidizing microorganism with a remarkable adaptability to thrive in extreme acidic environments, including heap bioleaching processes, acid mine drainage (AMD) and natural acidic water. A strain of L. ferriphilum (IESL25) was isolated from an industrial bioleaching process in northern Chile. This strain was challenged to grow at increasing concentrations of sulfate in order to assess changes in protein expression profiles, cells shape and to determine potential compatible solute molecules. The results unveiled changes in three proteins: succinyl CoA (SCoA) synthetase, isocitrate dehydrogenase (IDH) and aspartate semialdehyde dehydrogenase (ASD); which were notably overexpressed when the strain grew at elevated concentrations of sulfate. ASD plays a pivotal role in the synthesis of the compatible solute ectoine, which was identified along with hydroxyectoine by using matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF). The relationship between IDH, SCoA, and ectoine production could be due to the TCA cycle, in which both enzymes produce metabolites that can be utilized as precursors or intermediates in the biosynthesis of ectoine. In addition, distinct filamentous cellular morphology in L. ferriphilum IESL25 was observed when growing under sulfate stress conditions. This study highlights a new insight into the possible cellular responses of L. ferriphilum under the presence of high sulfate levels, commonly found in bioleaching of sulfide minerals or AMD environments.

Информационно-энергетическая оценка окислительно- восстановительного состояния почв, генезис и плодородие почв

Soil gluing increases the content of mobile compounds of iron and manganese, reduces the values of Eh(mV). CaCO3, MnO2, KMnO4 increase Eh and reduce the content of mobile compounds of iron in soils. Iron-oxidizing micro-organisms lead to the same effect. The necessity of estimating the rate of fall of Eh soils with excessive moisture is proved. This is necessary to clarify crop rotation systems, the use of fertilizers and tillage. Keywords: SOIL ANAEROBIOSIS, GLUING, SOIL FERTILITY

Indirect in situ bioleaching is an emerging tool for accessing deeply buried metal reserves, but can the process be managed? – A case study of copper leaching at 1 km depth

Copper is a strategic raw material widely needed for electrification. One possibility to diversify the supply to answer the market demand is to produce copper with in situ technology. In this study, feasibility of in situ bioleaching of copper was tested in a deep subsurface deposit. During in situ bioleaching of copper, copper is leached using a biologically produced ferric iron solution, which is recycled back to the in situ reactor after valuable metals are recovered, after which the solution is re-oxidized by iron-oxidizing microorganisms (IOB). A rock reactor was constructed in the Rudna Mine at ca 1 km depth and the microbiology and hydrogeochemistry of the water circulated through the reactor after blasting for fracturing the rock was monitored over time. The test site was rich in carbonates requiring large quantities of acid to remove the buffering capacity. The bacterial, archaeal and fungal communities in the rock reactor were monitored and characterized by quantitative polymerase chain reaction (qPCR) and amplicon sequencing, and acidophilic, iron oxidizing activity of the microbial communities during operation and pre- and post-operation phases was tested by cultivation. No acidophilic iron oxidizers were detected in the water samples during construction of the pilot reactor. Acidic leaching solution originating from the underground ferric iron generating bioreactor (FIGB) contained acidophilic IOB, which were also viable after the leach liquor was returned from the rock reactor. In the post-operation phase, when the rock reactor was neutralized with CaCO3/Ca(HCO3)2 solution, to inhibit the acidophilic IOB, iron oxidizing microorganisms were still present in the effluent solution one week after termination of the leaching and start of neutralization. Therefore, the post-operation phase needs further attention to completely stop the activity of added microorganisms. Copper was abundantly leached during the acid wash and leaching phases, proving the concept of deep in situ bioleaching.

Oxidation rates and redox stabilization of ferrous iron in trioctahedral smectites

Iron(II)-bearing trioctahedral smectites (saponites) form during anoxic alteration of basaltic rock. They are predicted to have been widespread on the early Earth and are observed in the oceanic subsurface today. Smectite structures, including the occupancy of sites in the octahedral sheet, affect iron redox behavior but the rates and products of trioctahedral smectite oxidation have been largely unexplored to date. In this study we synthesized two Fe(II)-bearing trioctahedral smectites, one moderate (22 wt% Fe) and one high (27 wt% Fe) in iron content. We then examined the rate, extent, and products of their oxidation by dissolved oxygen, nitrite, and hydrogen peroxide. Dissolved oxygen caused partial oxidation of Fe(II) in the smectites with 14 to 43% of Fe(II) unoxidized after 20 to 30 days of exposure. The rate and extent of oxidation correlated with the dissolved oxygen concentration and the Fe(II) content of the clay. The incomplete oxidation in these experiments is consistent with the mixed-valent trioctahedral smectites observed in oxidized natural samples but contrasts with the complete reoxidation by oxygen shown by chemically- or microbially-reduced dioctahedral smectites. Oxidation of structural Fe(II) by 5 mmol L−1 nitrite was negligible for the moderate-iron smectite and yielded only ∼17% oxidation after 54 days of reaction for the high-iron smectite. Hydrogen peroxide caused rapid and near-complete oxidation of both clays. Powder X-ray diffraction, variable-temperature Mössbauer spectroscopy, and extended X-ray absorption fine structure spectroscopy together detected no crystalline or short-range-ordered secondary phases and show that oxidized iron remained in the trioctahedral smectite structure. The recalcitrant Fe(II) pool in oxidized trioctahedral smectites exists in less distorted sites than Fe(II) in the initial clays. Its unreactive nature at prolonged reaction times indicates an elevated redox potential generated by the local coordination environment. Slower oxidation rates create a larger recalcitrant Fe(II) pool, suggesting kinetic competition between oxidation and a process involved in redox stabilization, such as electron exchange between octahedral iron sites or deprotonation of hydroxyl groups in the structure. The resistance to complete oxidation of trioctahedral ferrous smectites and their full retention of iron demonstrates that transitions from anoxic to oxic conditions generate mixed-valence smectites rather than a mixture of new phases. Identifying the diagenetic products of mixed-valent trioctahedral smectites may provide an indicator in the rock record of past redox cycling. Substantial portions of structural Fe(II) in trioctahedral smectites display slow abiotic oxidation kinetics and represent potential electron donors for both microaerophilic iron oxidizing and nitrate-reducing, iron-oxidizing microorganisms in altered mafic rocks and related settings.

BIOKINETIC CHARACTERIZATION OF FERROPLASMA ACIDIPHILUM

Abstract. Recently a mixed culture dominated by the iron-oxidizing microorganisms Leptospirillum and Ferroplasma has been used in a large-scale microbial fuel cell for electrical power generation. There are many factors that affect the kinetics of iron oxidation by the mixotroph Ferroplasma acidiphilum. This study investigated the effects of pH, temperature, and substrate and yeast extract concentrations in order to arrive at kinetically favorable operating conditions with minimal jarosite precipitation. Furthermore, F. acidiphilum was cultured with Leptospirillum sp. in order to determine its viability as a species capable of limiting the organic by-products of chemolithotrophic microbial growth. Bacterial characterization in the culture was accomplished using fluorescent in situ hybridization (FISH). It was found that the ferrous iron oxidation was most favorable at yeast extract concentration of 0.02% (w/v), pH of 1.6, temperature of 35 oC and an initial ferrous iron concentration of 1 g/L yielding a maximum specific growth rate of 0.0351-0.042 h-1. Moreover, F. acidiphilum displayed a symbiotic relationship with its chemolithotrophic counterpart, Leptospirillum sp. in that they were able to utilize the metabolic organic products of the chemolithotroph and limit the organic concentration to ~20 ppm total organic carbon (TOC), well below the threshold concentration of 250 ppm for chemolithotroph activity.

Изучение марганецокисляющих микроорганизмов, выделенных из микробоценозов высокомагнитных почв

From the stand point of the possible use of microorganisms in water purification biotechnologies, studying microorganisms capable of removing dangerous pollutants such as heavy metals from the aquatic environment is of great importance. Microbial screening was conducted on isolates from microbiocenoses of highly magnetic soils of the city Mednogorsk (Orenburg region, Russia), to assess their resistance to manganese (II) ions. The objects of this research were nine strains of manganese-oxidizing microorganisms and two strains of iron-oxidizing microorganisms: Bacillus megaterium 69.3 and B. megaterium 69.5. It was observed that, most of the studied microorganisms were characterized by increased resistance to manganese (II) ions when grown on an agar medium. Maximum resistance to Mn (II) was observed in microbial strains: 55.2 and B. megaterium 69.5. The maximum tolerant concentration (MTC) of Mn (II) for these microorganisms was 300 and 350 mmol/L; the minimum inhibitory concentration (MIC) was 350 and 450 mmol/L, respectively. When studying the growth of the two microbial strains: 55.2 and B. megaterium 69.5 in liquid media for 5 days. It was discovered that, when cultured in a selective medium, microbial resistance of the strains to Mn (II) was higher compared to when cultured in a nutrient LB medium. The results showed high resistance of microorganisms to a range of Mn (II) concentrations: from 0,5 to 250 mmol/L in a liquid medium. Maximum growth of the studied microbial strains was observed at Mn (II) concentration of 10 mmol/L. These microbial strains with high resistance to Mn (II) open up the prospect of their practical use for biotechnological purposes involving the purification of drinking and wastewater from high manganese content.

Effect of Environmental pH on Mineralization of Anaerobic Iron-Oxidizing Bacteria.

Freshwater lakes are often polluted with various heavy metals in the Anthropocene. The iron-oxidizing microorganisms and their mineralized products can coprecipitate with many heavy metals, including Al, Zn, Cu, Cd, and Cr. As such, microbial iron oxidation can exert a profound impact on environmental remediation. The environmental pH is a key determinant regulating microbial growth and mineralization and then influences the structure of the final mineralized products of anaerobic iron-oxidizing bacteria. Freshwater lakes, in general, are neutral-pH environments. Understanding the effects of varying pH on the mineralization of iron-oxidizing bacteria under neutrophilic conditions could aid in finding out the optimal pH values that promote the coprecipitation of heavy metals. Here, two typical neutrophilic Fe(II)-oxidizing bacteria, the nitrate-reducing Acidovorax sp. strain BoFeN1 and the anoxygenic phototrophic Rhodobacter ferrooxidans strain SW2, were selected for studying how their growth and mineralization response to slight changes in circumneutral pH. By employing focused ion beam/scanning electron microscopy (FIB–SEM) and transmission electron microscopy (TEM), we examined the interplay between pH changes and anaerobic iron-oxidizing bacteria and observed that pH can significantly impact the microbial mineralization process and vice versa. Further, pH-dependent changes in the structure of mineralized products of bacterial iron oxidation were observed. Our study could provide mechanical insights into how to manipulate microbial iron oxidation for facilitating remediation of heavy metals in the environment.

Jarosites: Formation, Structure, Reactivity and Environmental

Jarosite, beudantite and alunite are members of the alunite supergroup. Minerals like those have been detected in different environments on Earth. These jarosite-type compounds are common in acid rock drainage environments and acid sulfate soils, resulting from the weathering of sulfide ores; they are also present in bioleaching systems because they are found in cultures of iron-oxidizing microorganisms. Jarosite is also generated in hydrometallurgical circuits, mainly in zinc hydrometallurgy. These minerals can be used to immobilize different elements such as arsenic and lead, among others. Jarosite and alunite have also been detected on the surface of Mars; the presence of jarosite and alunite and other sulfates provides evidence for the existence of water on Mars. In this work, an exhaustive review of the natural formation, synthesis, structure, thermodynamics, and reactivity of jarosite, beudantite and alunite are included. The capacity of jarosites for the immobilization of the elements, such as lead and arsenic, and information about studies related to jarosite formation on Mars are also included.

Autotrophic Fe-Driven Biological Nitrogen Removal Technologies for Sustainable Wastewater Treatment.

Fe-driven biological nitrogen removal (FeBNR) has become one of the main technologies in water pollution remediation due to its economy, safety and mild reaction conditions. This paper systematically summarizes abiotic and biotic reactions in the Fe and N cycles, including nitrate/nitrite-dependent anaerobic Fe(II) oxidation (NDAFO) and anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox). The biodiversity of iron-oxidizing microorganisms for nitrate/nitrite reduction and iron-reducing microorganisms for ammonium oxidation are reviewed. The effects of environmental factors, e.g., pH, redox potential, Fe species, extracellular electron shuttles and natural organic matter, on the FeBNR reaction rate are analyzed. Current application advances in natural and artificial wastewater treatment are introduced with some typical experimental and application cases. Autotrophic FeBNR can treat low-C/N wastewater and greatly benefit the sustainable development of environmentally friendly biotechnologies for advanced nitrogen control.

Significance of Acid Washing after Biooxidation of Sulfides in Sequential Biotreatment of Double Refractory Gold Ore from the Syama Mine, Mali

Environmentally friendly pretreatment of double refractory gold ores (DRGO) to improve gold recovery without emitting pollutant gas is challenging. Sequential biotreatment, including iron-oxidizing microorganisms to decompose sulfides, followed by the enzymatic decomposition of carbonaceous matter, was recently developed. The effect of acid washing by 1 M HCl for 24 h between two bioprocesses was evaluated using a real double refractory gold ore from the Syama mines, Mali, which includes 24 g/t of Au and 5.27 wt% of carbon with a relatively higher graphitic degree. The addition of the acid washing process significantly improved gold recovery by cyanidation to yield to 84.9 ± 0.7% from 64.4 ± 9.2% (n = 2). The positive effects of acid washing can be explained by chemical alteration of carbonaceous matter to facilitate the accessibility for lignin peroxidase (LiP) and manganese peroxidase (MnP) in cell-free spent medium (CFSM), although the agglomeration was enhanced by an acid attack to structural Fe(III) in clay minerals. Sequential treatment of DRGO basically consists of the oxidative dissolution of sulfides and the degradation of carbonaceous matter prior to the extraction of gold; however, the details should be modified depending on the elemental and mineralogical compositions and the graphitic degree of carbonaceous matter.

Carbonaceous matter degradation by fungal enzyme treatment to improve Ag recovery from an Au-Ag-bearing concentrate

Sequential treatment was applied to carbonaceous Au-Ag-bearing ore concentrate to maximize the Au and Ag recovery. In the preliminary test, the present carbonaceous ore had well liberated and exposed type of gold grains, which are not refractory, but included mainly three types of Ag presented as electrum, hessite (Ag2Te) and Ag-bearing other minerals. Au recovery was ∼100% without any treatment, meanwhile Ag recovery was only 33.3%. The sequential treatment comprises two oxidation steps: (a) mixed culture of iron- and sulfur-oxidizing microorganisms at pH 1.2, followed by (b) cell-free spent medium (CFSM) at pH 4.0 from a white rot-fungus, Phanerochaete chrysosporium, which includes lignin-degrading enzymes. As a result, Ag recovery was 55.5% after the first step and greatly improved to ∼100%, including the dissolved Ag+ concentration in the first step of acid treatment. Although the acidophilic iron-oxidizing microorganisms were inhibited by dissolved Ag+ and Cd2+ ions, the strong acidic conditions dissolved hessite and Ag-bearing oxide minerals. However, the remaining carbonaceous matter acted to sorb Ag(CN)2− in cyanidation, causing the recovery loss. In the next step the lignin-degrading enzymes degraded carbonaceous matter in the ore. This step is necessary to avoid the adsorption of Ag(CN)2− on graphitic carbonaceous matter, leading a mostly perfect recovery of the remaining Ag in the solid residues, without necessity of alkaline washing. The sequential treatment including enzymatic lignin-degrading process was also effective in carbonaceous silver ore avoiding the emission of air pollutants.

Acidophilic Microorganisms Leptospirillum sp., Acidithiobacillus sp., Ferroplasma sp. As a Cathodic Bioagents in a MFC

Reducing fuel cells electrical losses is achieved by the use of a catalyst. Being inexpensive and renewable, biocathodes have an advantage over synthetic and chemical ones. Here the microbial fuel cell (MFC)-utilizing biocathode based on the acidophilic chemolythotrophic microorganisms (ACM) mixed culture has been studied. The Leptospirillum sp., Acidithiobacillus sp., Ferroplasma sp. consortium has been used as a cathode biocatalyst. Pyrite enrichment tails of a mining and processing enterprise served as the catholyte substrates. It is experimentally demonstrated that ACM mixed culture Leptospirillum sp., Acidithiobacillus sp., Ferroplasma sp. introduced into the catholyte leads to MFC electrical characteristics increase by approximately two times. With the selected external load (30 Ω) Cathodic bioagents increased the MFC electric power four-fold (from 137 to 579 µW). Catholyte redox potential increased from 431 to 581 mV during 11 d in the presence of acidophilic iron-oxidizing microorganisms. At 33 d, this value increased up to 695 mV. This is an indirect evidence for active processes of iron oxidation. The analysis results for the Fe3+ content in the medium also support this assumption. Thus, in a course of a 33-d experiment Fe3+ concentration increased from 0 to 1.5 g/l. The redox processes intensification in the studied systems also confirmed by the cyclic voltammetry (CV). Under pyrite utilization conditions in the presence of ACM mixed culture cyclic voltammograms show an increase in the reduction current at the cathodic potentials region. The results obtained herein suggest that the studied biochemical system is a perspective catholyte for MFC capable to reduce a cathodic overpotential.

Pyrrhotite Biooxidation by Moderately Thermophilic Acidophilic Microorganisms

The goal of the present work was to study the oxidation of sulfide mineral pyrrhotite by members of microbial groups predominant in biohydrometallurgical processes (mixotrophic iron- and sulfur-oxidizing bacterium Sulfobacillus thermosulfidooxidans strain VKMV 1269T, autotrophic sulfur-oxidizing bacterium Acidithiobacillus caldus strain MBC-1, and heterotrophic iron- and sulfur-oxidizing archaeon Acidiplasma sp. strain MBA-1) to evaluate the role of microorganisms with different physiological properties in the leaching of this mineral. It was shown that pyrrhotite was most actively leached by the sulfur-oxidizing strain Acidithiobacillus caldus MBC-1, as well as by mixed cultures in which this strain was present: S. thermosulfidooxidans VKMV 1269T + A. caldus MBC-1 and Acidiplasma sp. MBA-1 + A. caldus MBC-1. These cultures leached 70, 43, and 60% of pyrrhotite, respectively, after 30 days of the experiment. In experiments with pure cultures S. thermosulfidooxidans VKMV 1269T, Acidiplasma sp. MBA-1 and a mixed culture S. thermosulfidooxidans VKMV 1269T + Acidiplasma sp. MBA-1, the rates of pyrrhotite leaching were significantly lower: 29, 37 and 30%, respectively. An experiment on sulfur biooxidation by pure cultures of the studied microorganisms demonstrated that A. caldus strain MBC-1 was the most active sulfur oxidizer. Thus, it was shown that the bioleaching of pyrrhotite was carried out mainly by sulfur-oxidizing microorganisms, with sulfur biooxidation as the most important process. Active biooxidation of the Fe2+ ion by iron-oxidizing microorganisms did not increase activity of pyrrhotite oxidation, but, on the contrary, led to inhibition of pyrrhotite leaching. Thus, active biooxidation of sulfur, which is formed during the leaching of pyrrhotite, was a factor determining the leaching rate for this sulfide mineral.

Mechanism for the Bio-Oxidation and Decomposition of Pentlandite: Implication for Nickel Bioleaching at Elevated pH

This work investigated the effects of Fe3+, H+ and adsorbed leaching bacteria on the bioleaching of pentlandite. Collectively, an integrated model for the oxidation and decomposition of pentlandite was built to describe the behaviors of different components in a bioleaching system. Proton ions and ferric ions could promote the break and oxidation of Ni-S and Fe-S bonds. The iron-oxidizing microorganisms could regenerate ferric ions and maintain a high Eh value. The sulfur-oxidizing microorganisms showed significant importance in the oxidation of polysulfide and elemental sulfur. The atoms in pentlandite show different modification pathways during the bioleaching process: iron transformed through a (Ni,Fe)9S8 → Fe2+ → Fe3+ → KFe3(SO4)2(OH)6 pathway; nickel experienced a transformation of (Ni,Fe)9S8 → NiS → Ni2+; sulfur modified through the pathway of S2−/S22− → Sn2− → S0 → SO32− → SO42−. During bioleaching, a sulfur-rich layer and jarosite layer formed on the mineral surface, and the rise of pH value accelerated the process. However, no evidence for the inhibition of the layers was shown in the bioleaching of pentlandite at pH 3.00. This study provides a novel method for the extraction of nickel from pentlandite by bioleaching at elevated pH values.

Impact of Iron, Copper and Nickel Ions Introduced Into the Soil Separately and in Various Combinations on Soil Microbiota

In a long-term model experiment, the abundance dynamics of soil microorganisms was studied as affected by pollution of southern chernozem soils with various concentrations and combinations of iron, nickel and copper ions. In the course of this study, soil microbiocenoses were seeded on solid nutrient media and the following values were estimated: the total numbers of heterotrophic microorganisms on meat–peptone agar, the numbers of iron-oxidizing microorganisms on a selective medium in 0, 30, 90 and 210 days after the introduction of heavy metal ions into the soil. A characteristic diverse impact of heavy metal ions on soil microorganisms was established, and the degree of stability of soil microbocenoses of southern chernozem was revealed. Iron and copper concentrations of 10 and 50 RGCB/MPC in 30 days after soil contamination by individual metal ions or their combinations stimulated the proliferation of heterotrophic microorganisms in the soil microbocenoses and 90 days later the number of this microbial group decreased to the control levels and below. After 210 days, the microbiocenoses returned to a stable state. Nickel ions, introduced into the soil at a concentration of 50 MPC separately and in a number of combinations with other heavy metal ions, did not stimulate the proliferation of heterotrophic microorganisms. Opposite trends were observed in the abundance dynamics of iron-oxidizing microorganisms. With the exception of some model variants such as 10 and 50 MPC of Cu (II), iron, nickel and their combinations in various concentrations inhibited the proliferation of ironoxidizing microorganisms in the first month after soil contamination. The inhibitory effect of a combination of heavy metal ions was stronger than that of individual metals. After 90 days, the numbers of iron-oxidizing microorganisms restored to the control level or even exceeded it. After 210 days, an inhibition of the proliferation of ironoxidizing microorganisms was observed in the microbocenoses, or their abundance corresponded to the value in the control soil sample.

Bacterial and Archaeal Diversity in Sulfide-Bearing Waste Rock at Faro Mine Complex, Yukon Territory, Canada

Acid mine/rock drainage (AMD/ARD) is generated by the microbially-accelerated oxidative dissolution of sulfide minerals in working and abandoned mine sites, and some natural environments. Iron-oxidizing microorganisms (IOM) regenerate the oxidant Fe3+, while sulfur-oxidizing microorganisms (SOM) contribute to AMD/ARD by generating H2SO4 via the oxidation of elemental S and reduced inorganic sulfur compounds. Bacterial and archaeal diversity in 34 samples of sulfide-bearing waste rock recovered from three boreholes at the Faro Mine Complex (Yukon Territory, Canada; Pb/Zn production from 1969 to 1998) was investigated using high-throughput amplicon sequencing of 16S rRNA genes. A majority of the borehole pore water samples had circum-neutral or mildly alkaline pH (6.3 − 8.7), while some had low pH (3.3 − 5.2). Mean relative abundance of prokaryotic SOM/IOM accounted for 3.4% of the total amplicons. The acidophilic genera Alicyclobacillus and Acidithiobacillus were the most abundant sulfur- and iron-metabolizing prokaryotes detected, followed by neutrophilic and moderately acidophilic (non-iron-oxidizing) SOM. Sulfate-reducing bacteria (SRB) were also detected, accounting for 0.6% of total reads. The presence of both acidophilic and neutrophilic prokaryotes catalyzing transformations of sulfur and iron in the same samples suggests development of microenvironments within the waste rock where dynamic biogeochemical transformations of these elements occur.

Microbial community structure within a weathered waste-rock pile overlain by a monolayer soil cover

A historic waste-rock stockpile (WRS) at the Detour Lake Mine (DLM), covered with a thin layer (<1 m) of local overburden, was studied to determine the potential for microbially-mediated generation of acid rock drainage (ARD). The sulfur content of the waste rock ranged from 0 to 2.2 wt %, with pyrite and pyrrhotite identified as the principal sulfide minerals. Acidity released through sulfide oxidation in the WRS has been neutralized through carbonate mineral dissolution, and has resulted in the generation of neutral mine drainage (pH 6–8). However, the WRS is heterogeneous, and localized water samples collected from discrete zones within the stockpile were more highly oxidized and acidic (i.e., pH ≥ 2.5). Enumerations of acidophilic sulfur- (aSOM) and iron-oxidizing microorganisms (aIOM) were performed, yielding mean abundances of 1.2 × 103 and 9.0 × 105 cells g−1, respectively. The mean abundance of neutrophilic sulfur-oxidizing microorganisms (nSOM) was 5.5 × 105 cells g−1. Fungi and bacteria present in the waste rock were identified using high-throughput amplicon sequencing of 18S and 16S rRNA genes, respectively. Sequencing confirmed the presence of Thiobacillus and Acidithiobacillus species. Bacterial diversity was greatest in samples from the cover material. Unoxidized waste rock samples were characterized by neutrophilic iron- or sulfur-oxidizing genera (i.e., Thiobacillus), whereas samples collected from oxidized and acidic zones in the WRS showed greater abundances of acidophilic taxa (i.e., Acidithiobacillus). None of the fungal genera identified in this study have been shown to oxidize sulfide minerals directly, other than indirectly through creation of a suitable environment for the prokaryotes involved in the processes. Although installation of a simple, non-engineered cover is anticipated to have slowed the generation of ARD, evidence of ongoing sulfide oxidation within the covered WRS was observed. High abundances and activities of sulfur- and iron-oxidizing microorganisms indicate that the soil cover has not prevented the growth of microorganisms that catalyze sulfide-mineral oxidation.

Biodiagnostics of Soil under a Low Level of Anthropogenic Impact (by the Example of the Urban Settlement of Stepnoe, Saratov Oblast)

Abstract—Biodiagnostics of soil in a settlement with low technogenic impact was performed by the example of the urban settlement of Stepnoe. The number of microorganisms was evaluated in 30 soil samples by inoculation on solid nutrition media: on meat peptone agar for the calculation of the total heterotrophic microorganisms and on the corresponding selective media for hydrocarbon-oxidizing and iron-oxidizing microorganisms. We also analyzed the activity of several soil redox and hydrolytic enzymes (dehydrogenases, catalases, peroxidases, and invertases). The content of the mobile forms of copper, zinc, lead, cadmium, chromium, and nickel was determined to assess the anthropogenic impact on the soil, and the total soil contamination coefficient (Zc) was calculated. The excess of the MPC of mobile forms of Ni, Cu, and Pb was revealed in the area studied. In general, the total soil contamination coefficient did not exceed 16, indicating favorable environmental and geochemical conditions in the settlement. The analysis of the total number of heterotrophic microorganisms and the amounts of hydrocarbon-oxidizing and iron-oxidizing microorganisms showed deviations from the norm in some regions of the area surveyed, which pointed to anthropogenic transformation of the soil biocenoses. The maximum deviations were typical for the index of hydrocarbon-oxidizing microorganisms, which pointed to soil contamination by hydrocarbons. Our data on the evaluation of the activity of indicator soil enzymes did not exclude possible functional disturbances in the soil. It was found that the activity of soil dehydrogenases and invertases corresponded to low and very low levels of soil enrichment with these enzymes. The activity of soil catalases and peroxidases corresponded to the moderate and high levels of the enzymes, which indicated the absence of pathological changes in the soil. It has been shown that the studied soil microbiological and biochemical parameters are promising for soil health biodiagnostics in areas with low anthropogenic impact.

Numerical modelling of column experiments to investigate in-situ bioleaching as an alternative mining technology

This publication investigates indirect in-situ bioleaching as an alternative mining technology for minimizing waste production, environmental impact, and chemical consumption. The process consists in injecting a leaching solution into a targeted ore body for dissolving base metal bearing minerals, while iron-oxidizing microorganisms regenerate the solution. In this contribution, we present laboratory column experiments that investigate the impact of grain size on the action of an acidic oxidizing solution in contact with ore samples crushed at different grain sizes. These results are used for developing a one-dimensional reactive transport model based on PhreeqC software. In this model, porous and fractured media are approached by a dual porosity reactive transport model where dissolution reactions are described by kinetics. Column experiments are used as a reference for calibrating the key parameters of the numerical models, which include the relative volume of mobile and immobile zones within the dual porosity medium and the exchange rate between these two volumes. This model is then adapted to in-situ conditions by considering the preferential flow of fluids through natural or artificial fractures of enhanced hydraulic conductivity. These models are used to discuss key elements affecting the feasibility of coupling bioleaching and in-situ recovery for improving the sustainability of mining, especially for deep and complex ore deposits.

Microbial diversity involved in iron and cryptic sulfur cycling in the ferruginous, low-sulfate waters of Lake Pavin.

Both iron- and sulfur- reducing bacteria strongly impact the mineralogy of iron, but their activity has long been thought to be spatially and temporally segregated based on the higher thermodynamic yields of iron over sulfate reduction. However, recent evidence suggests that sulfur cycling can predominate even under ferruginous conditions. In this study, we investigated the potential for bacterial iron and sulfur metabolisms in the iron-rich (1.2 mM dissolved Fe2+), sulfate-poor (< 20 μM) Lake Pavin which is expected to host large populations of iron-reducing and iron-oxidizing microorganisms influencing the mineralogy of iron precipitates in its permanently anoxic bottom waters and sediments. 16S rRNA gene amplicon libraries from at and below the oxycline revealed that highly diverse populations of sulfur/sulfate-reducing (SRB) and sulfur/sulfide-oxidizing bacteria represented up to 10% and 5% of the total recovered sequences in situ, respectively, which together was roughly equivalent to the fraction of putative iron cycling bacteria. In enrichment cultures amended with key iron phases identified in situ (ferric iron phosphate, ferrihydrite) or with soluble iron (Fe2+), SRB were the most competitive microorganisms, both in the presence and absence of added sulfate. The large fraction of Sulfurospirillum, which are known to reduce thiosulfate and sulfur but not sulfate, present in all cultures was likely supported by Fe(III)-driven sulfide oxidation. These results support the hypothesis that an active cryptic sulfur cycle interacts with iron cycling in the lake. Analyses of mineral phases showed that ferric phosphate in cultures dominated by SRB was transformed to vivianite with concomitant precipitation of iron sulfides. As colloidal FeS and vivianite have been reported in the monimolimnion, we suggest that SRB along with iron-reducing bacteria strongly influence iron mineralogy in the water column and sediments of Lake Pavin.