Microbes of biotechnological importance in acidic saline lakes in the Yilgarn Craton, Western Australia.

Acidic salt lakes are environments that harbor an array of biologically challenging conditions. Through 16S rRNA, 18S rRNA, and ITS amplicon sequencing of eight such lakes across the Yilgarn Craton of Western Australia, we aim to understand the microbial ecology of these lakes with a focus on iron- and sulfur-oxidizing and reducing microorganisms that have theoretical application in biomining industries. In spite of the biological challenges to life in these lakes, the microbial communities were highly diverse. Redundancy analysis of soil samples revealed sulfur, ammonium, organic carbon, and potassium were significant diversities of the microbial community composition. The most abundant microbes with a hypothetical application in biomining include the genus 9 M32 of the Acidithiobacillus family, Alicyclobacillus and Acidiphilium, all of which are possible iron- and/or sulfur-oxidizing bacteria. It is evident through this study that these lakes harbor multiple organisms with potential in biomining industries that should be exploited and studied further.

Blending bauxite residues with multiple byproducts improves capping materials for tailings storage facilities.

Amelioration and management of large volumes of tailings resulting from alumina refining is a major challenge owing to the high alkalinity and salinity of residues. Blended byproduct caps are a potential new and more cost-effective approach to tailings management, where tailings are blended with other local byproducts in order to reduce pH, salinity and toxic elements. Here, alkaline bauxite residue was blended with four byproducts (waste acid, sewage water, fly ash and eucalypt mulch) to create a range of potential capping materials. We leached and weathered materials in the glasshouse with deionized water over nine weeks to investigate if byproducts on their own or in combination improved cap conditions. Combining all four byproducts (10 wt % waste acid, 5 wt % sewage water, 20 wt % fly ash and 10 wt % eucalypt mulch) achieved lower pH (9.60) compared to any byproduct applied individually, or un-remediated bauxite residue (pH 10.7). Leaching decreased EC by dissolving and exporting salts and minerals from the bauxite residue. Fly ash addition increased organic carbon (likely from non-combusted organic material) and nitrogen, while eucalypt mulch increased inorganic phosphorus. Addition of byproducts also decreased the concentration of potentially toxic elements (e.g., Al, Na, Mo and V) and enhanced pH neutralisation. Initial pH with single byproduct treatments was 10.4-10.5, which decreased to between 9.9-10.0. Further lowering of pH and salinity as well as increased nutrient concentrations may be possible through higher addition rates of byproducts, incorporation of other materials such as gypsum, and increasing leaching/weathering time of tailings in situ.

Coupling microbial and abiotic amendments accelerates in situ remediation of bauxite residue at field scale.

Bauxite residue is a highly saline-sodic tailings material formed as a by-product of the Bayer process for alumina production. In situ remediation of bauxite residue has the potential to provide an effective means for accelerated rehabilitation of residue storage areas. However, previous work has predominantly only used chemical and physical amendments to date, limiting rates of pH neutralisation and extent of remediation. Combining these abiotic amendments with recently developed microbial biotechnology for pH neutralisation may transform bauxite residue into a productive soil material in a shorter timeframe. Here we investigated the effects of microbial and abiotic amendments (compost plus tillage), both in isolation and combined, on remediation of key bauxite residue properties in field scale trials (10 × 15 m × 2 m deep field plots). Triplicate residue samples were collected to 30 cm depth from each plot in quarterly field sampling campaigns. Changes in chemical and physical properties were monitored to assess remediation performance under different amendments. After one year, field plots amended with a microbial treatment had significantly (p < 0.05) lower average pH (8.99-9.46) in the upper 20 cm than the control (10.3). The combined microbial-abiotic treatment also had improved physical structure, higher organic C and lower electrical conductivity than the microbial treatment alone. The strong performance of the microbial-abiotic treatment is attributed to the combined benefits of bioneutralisation from microbial fermentation products, enhanced leaching of alkaline pore water and salts due to tillage and compost, and addition of highly stable C and N in compost. Combining novel microbial biotechnology with common abiotic amendments is therefore suggested for accelerating in situ remediation progress towards a material amenable for plant growth.

Retaining natural vegetation to safeguard biodiversity and humanity.

Global efforts to deliver internationally agreed goals to reduce carbon emissions, halt biodiversity loss, and retain essential ecosystem services have been poorly integrated. These goals rely in part on preserving natural (e.g., native, largely unmodified) and seminatural (e.g., low intensity or sustainable human use) forests, woodlands, and grasslands. To show how to unify these goals, we empirically derived spatially explicit, quantitative, area-based targets for the retention of natural and seminatural (e.g., native) terrestrial vegetation worldwide. We used a 250-m-resolution map of natural and seminatural vegetation cover and, from this, selected areas identified under different international agreements as being important for achieving global biodiversity, carbon, soil, and water targets. At least 67 million km2 of Earth's terrestrial vegetation (∼79% of the area of vegetation remaining) required retention to contribute to biodiversity, climate, soil, and freshwater conservation objectives under 4 United Nations' resolutions. This equates to retaining natural and seminatural vegetation across at least 50% of the total terrestrial (excluding Antarctica) surface of Earth. Retention efforts could contribute to multiple goals simultaneously, especially where natural and seminatural vegetation can be managed to achieve cobenefits for biodiversity, carbon storage, and ecosystem service provision. Such management can and should co-occur and be driven by people who live in and rely on places where natural and sustainably managed vegetation remains in situ and must be complemented by restoration and appropriate management of more human-modified environments if global goals are to be realized.

Retención de la vegetación natural para salvaguardar la biodiversidad y la humanidad Resumen Hoy en día hay muy poca integración de los esfuerzos mundiales para alcanzar los objetivos internacionales de reducción de las emisiones de carbono, impedimento de la pérdida de biodiversidad y conservación de los servicios ambientales esenciales. Estos objetivos dependen parcialmente de la conservación de los bosques, selvas y praderas naturales (por ejemplo, nativos y en su mayoría sin alteraciones) y seminaturales (por ejemplo, de uso humano sostenible o de baja intensidad). Obtuvimos de manera empírica objetivos espacialmente explícitos, cuantitativos y basados en áreas para la conservación de la vegetación terrestre natural y seminatural (por ejemplo, nativa) en todo el mundo para mostrar cómo unificar los objetivos internacionales. Usamos un mapa de 250 m de resolución de la cubierta vegetal natural y seminatural y, a partir de él, seleccionamos las áreas identificadas como importantes en diferentes acuerdos internacionales para alcanzar los objetivos globales de biodiversidad, carbono, suelo y agua. Al menos 67 millones de km2 de la vegetación terrestre de la Tierra (∼79% de la superficie de vegetación restante) requieren ser conservados para contribuir a los objetivos de conservación de la biodiversidad, el clima, el suelo y el agua dulce en virtud de cuatro de las resoluciones de las Naciones Unidas. Esto equivale a conservar la vegetación natural y seminatural en al menos el 50% de la superficie terrestre total de la Tierra (sin contar a la Antártida). Los esfuerzos de retención podrían contribuir a alcanzar múltiples objetivos simultáneamente, especialmente en donde la vegetación natural y seminatural puede gestionarse para lograr beneficios colaterales para la biodiversidad, el almacenamiento de carbono y la provisión de servicios ambientales. Esta gestión puede y debe ser impulsada y llevada a cabo por las personas que viven en y dependen de los lugares donde la vegetación natural y gestionada de forma sostenible permanece in situ y debe complementarse con la restauración y la gestión adecuada de entornos modificados por el hombre si se quieren alcanzar los objetivos globales.

Microbial Community Structure Is Most Strongly Associated With Geographical Distance and pH in Salt Lake Sediments.

Salt lakes are globally significant microbial habitats, hosting substantial novel microbial diversity and functional capacity. Extremes of salinity and pH both pose major challenges for survival of microbial life in terrestrial and aquatic environments, and are frequently cited as primary influences on microbial diversity across a wide variety of environments. However, few studies have attempted to identify spatial and geochemical contributions to microbial community composition, functional capacity, and environmental tolerances in salt lakes, limiting exploration of novel halophilic and halotolerant microbial species and their potential biotechnological applications. Here, we collected sediment samples from 16 salt lakes at pH values that ranged from pH 4 to 9, distributed across 48,000 km2 of the Archaean Yilgarn Craton in southwestern Australia to identify associations between environmental factors and microbial community composition, and used a high throughput culturing approach to identify the limits of salt and pH tolerance during iron and sulfur oxidation in these microbial communities. Geographical distance between lakes was the primary contributor to variation in microbial community composition, with pH identified as the most important geochemical contributor to variation in microbial community composition. Microbial community composition split into two clear groups by pH: Bacillota dominated microbial communities in acidic saline lakes, whereas Euryarchaeota dominated microbial communities in alkaline saline lakes. Iron oxidation was observed at salinities up to 160 g L-1 NaCl at pH values as low as pH 1.5, and sulfur oxidation was observed at salinities up to 160 g L-1 NaCl between pH values 2-10, more than doubling previously observed tolerances to NaCl salinity amongst cultivable iron and sulfur oxidizers at these extreme pH values. OTU level diversity in the salt lake microbial communities emerged as the major indicator of iron- and sulfur-oxidizing capacity and environmental tolerances to extremes of pH and salinity. Overall, when bioprospecting for novel microbial functional capacity and environmental tolerances, our study supports sampling from remote, previously unexplored, and maximally distant locations, and prioritizing for OTU level diversity rather than present geochemical conditions.

Simple Organic Carbon Sources and High Diversity Inocula Enhance Microbial Bioneutralization of Alkaline Bauxite Residues.

Bioneutralization of pH by microbial fermentation of added carbon substrates is a promising new method for remediation of the 1.7 GT/yr of alkaline mining tailings produced globally. Here, we present the first study to systematically compare and optimize the efficacy of microbial inocula of varying diversities, structures, and provenance and organic carbon substrates of varying complexities on the rate and extent of pH bioneutralization in alkaline bauxite residue tailings. Laboratory-scale bioreactors inoculated with soda lake sediments or with monosaccharide substrates added had a significantly lower minimum pH (<8) and a significantly higher maximum rate of pH neutralization (>0.02 µmol H+ day-1) and achieved these in significantly less time (<26 days) compared to bioreactors with other inocula or substrates. The soda lake sediment introduced a significantly higher-diversity microbial community with a distinct structure (dominated by Euryarchaeota and Bacteroidetes, rather than Acidobacteria and Actinobacteria), supporting higher acetate and formate-yielding fermentation pathways compared to other inocula. The strong performance of monosaccharides is attributed to widespread microbial capacity for efficient fermentation. Using either monosaccharide carbon substrates or soda lake sediments is recommended to maximize bioneutralization efficiency at the industrial scale.

Extreme Geochemical Conditions and Dispersal Limitation Retard Primary Succession of Microbial Communities in Gold Tailings.

Microbial community succession in tailings materials is poorly understood at present, and likely to be substantially different from similar processes in natural primary successional environments due to the unusual geochemical properties of tailings and the isolated design of tailings storage facilities. This is the first study to evaluate processes of primary succession in microbial communities colonizing unamended tailings, and compare the relative importance of stochastic (predominantly dust-borne dispersal) and deterministic (strong selection pressures from extreme geochemical properties) processes in governing community assembly rates and trajectories to those observed in natural environments. Dispersal-based recruitment required > 6 months to shift microbial community composition in unamended, field-weathered gold tailings; and in the absence of targeted inoculants, recruitment was dominated by salt- and alkali-tolerant species. In addition, cell numbers were less than 106 cells/g tailings until > 6 months after deposition. Laboratory experiments simulating microbial cell addition via dust revealed that high (>6 months' equivalent) dust addition rates were required to effect stabilization of microbial cell counts in tailings. In field-weathered tailings, topsoil addition during rehabilitation works exerted a double effect, acting as a microbial inoculant and correcting geochemical properties of tailings. However, microbial communities in rehabilitated tailings remained compositionally distinct from those of reference soils in surrounding environments. pH, water extractable Mg, and water extractable Fe emerged as major controls on microbial community composition in the field-weathered gold tailings. Overall, this study highlights the need for application of targeted microbial inoculants to accelerate rates of microbial community succession in tailings, which are limited primarily by slow dispersal due to physical and spatial isolation of tailings facilities from inoculant sources; and for geochemical properties of tailings to be amended to moderate values to encourage microbial community diversification and succession.

Microbial Fermentation of Organic Carbon Substrates Drives Rapid pH Neutralization and Element Removal in Bauxite Residue Leachate.

Globally, mineral processing activities produce an estimated 680 GL/yr of alkaline wastewater. Neutralizing pH and removing dissolved elements are the main goals of wastewater treatment prior to discharge. Here, we present the first study to explicitly evaluate the role of microbial communities in driving pH neutralization and element removal in alkaline wastewaters by fermentation of organic carbon, using bauxite residue leachate as a model system, and evaluate the effects of organic carbon complexity and microbial inoculum addition rates on the performance of these treatment systems at laboratory scale. Rates and extents of pH neutralization were higher in bioreactors fed with simpler organic carbon substrates (glucose and banana: 6 days to reach pH ≤ 8) than those fed with more complex organic carbon substrates (eucalyptus mulch: 15 days to reach pH ≤ 8; woodchips: equilibrium pH around 9). Concentrations of dissolved Al, As, B, Mo, Na, S, and V all significantly decreased after bioremediation. Increasing soil inoculant addition rate accelerated rates and extent of pH neutralization and element removal up to 0.1 wt %; further increases had little effect. Overall, glucose added at 1.8 wt % and soil inoculum added at 0.1 wt % provided the most effective minimal combination of carbon substrate and inoculum to drive pH neutralization and element removal.

pH and Organic Carbon Dose Rates Control Microbially Driven Bioremediation Efficacy in Alkaline Bauxite Residue.

Bioremediation of alkaline tailings, based on fermentative microbial metabolisms, is a novel strategy for achieving rapid pH neutralization and thus improving environmental outcomes associated with mining and refining activities. Laboratory-scale bioreactors containing bauxite residue (an alkaline, saline tailings material generated as a byproduct of alumina refining), to which a diverse microbial inoculum was added, were used in this study to identify key factors (pH, salinity, organic carbon supply) controlling the rates and extent of microbially driven pH neutralization (bioremediation) in alkaline tailings. Initial tailings pH and organic carbon dose rates both significantly affected bioremediation extent and efficiency with lower minimum pHs and higher extents of pH neutralization occurring under low initial pH or high organic carbon conditions. Rates of pH neutralization (up to 0.13 mM H+ produced per day with pH decreasing from 9.5 to ≤6.5 in three days) were significantly higher in low initial pH treatments. Representatives of the Bacillaceae and Enterobacteriaceae, which contain many known facultative anaerobes and fermenters, were identified as key contributors to 2,3-butanediol and/or mixed acid fermentation as the major mechanism(s) of pH neutralization. Initial pH and salinity significantly influenced microbial community successional trajectories, and microbial community structure was significantly related to markers of fermentation activity. This study provides the first experimental demonstration of bioremediation in bauxite residue, identifying pH and organic carbon dose rates as key controls on bioremediation efficacy, and will enable future development of bioreactor technologies at full field scale.

Microbial Diversity in Engineered Haloalkaline Environments Shaped by Shared Geochemical Drivers Observed in Natural Analogues.

Microbial communities in engineered terrestrial haloalkaline environments have been poorly characterized relative to their natural counterparts and are geologically recent in formation, offering opportunities to explore microbial diversity and assembly in dynamic, geochemically comparable contexts. In this study, the microbial community structure and geochemical characteristics of three geographically dispersed bauxite residue environments along a remediation gradient were assessed and subsequently compared with other engineered and natural haloalkaline systems. In bauxite residues, bacterial communities were similar at the phylum level (dominated by Proteobacteria and Firmicutes) to those found in soda lakes, oil sands tailings, and nuclear wastes; however, they differed at lower taxonomic levels, with only 23% of operational taxonomic units (OTUs) shared with other haloalkaline environments. Although being less diverse than natural analogues, bauxite residue harbored substantial novel bacterial taxa, with 90% of OTUs nonmatchable to cultured representative sequences. Fungal communities were dominated by Ascomycota and Basidiomycota, consistent with previous studies of hypersaline environments, and also harbored substantial novel (73% of OTUs) taxa. In bauxite residues, community structure was clearly linked to geochemical and physical environmental parameters, with 84% of variation in bacterial and 73% of variation in fungal community structures explained by environmental parameters. The major driver of bacterial community structure (salinity) was consistent across natural and engineered environments; however, drivers differed for fungal community structure between natural (pH) and engineered (total alkalinity) environments. This study demonstrates that both engineered and natural terrestrial haloalkaline environments host substantial repositories of microbial diversity, which are strongly shaped by geochemical drivers.

Microbially-driven strategies for bioremediation of bauxite residue.

Globally, 3 Gt of bauxite residue is currently in storage, with an additional 120 Mt generated every year. Bauxite residue is an alkaline, saline, sodic, massive, and fine grained material with little organic carbon or plant nutrients. To date, remediation of bauxite residue has focused on the use of chemical and physical amendments to address high pH, high salinity, and poor drainage and aeration. No studies to date have evaluated the potential for microbial communities to contribute to remediation as part of a combined approach integrating chemical, physical, and biological amendments. This review considers natural alkaline, saline environments that present similar challenges for microbial survival and evaluates candidate microorganisms that are both adapted for survival in these environments and have the capacity to carry out beneficial metabolisms in bauxite residue. Fermentation, sulfur oxidation, and extracellular polymeric substance production emerge as promising pathways for bioremediation whether employed individually or in combination. A combination of bioaugmentation (addition of inocula from other alkaline, saline environments) and biostimulation (addition of nutrients to promote microbial growth and activity) of the native community in bauxite residue is recommended as the approach most likely to be successful in promoting bioremediation of bauxite residue.

Spontaneous vegetation encroachment upon bauxite residue (red mud) as an indicator and facilitator of in situ remediation processes.

The spontaneous colonization of a bauxite residue (alumina refining tailings) deposit by local vegetation in Linden, Guyana, over 30 years, indicates that natural weathering processes can ameliorate tailings to the extent that it can support vegetation. Samples were collected from vegetated and unvegetated areas to investigate the relationships between bauxite residue properties and vegetation cover. Compared to unvegetated areas, bauxite residue in vegetated areas had lower pH (mean pH 7.9 vs 10.9), lower alkalinity (mean titratable alkalinity 0.4 vs 1.4 mol H(+) kg(-1)), lower electrical conductivity (mean EC 0.3 vs 2.1 mS cm(-1)), lower total Al (mean Al2O3 19.8 vs 25.8% wt) and Na (mean Na2O 0.9 vs 3.7% wt), and less sodalite and calcite. Accumulation of N, NH4(+), and organic C occurred under vegetation, demonstrating the capacity for plants to modify residue to suit their requirements as a soil-like growth medium. Aeolian redistribution of coarse grained tailings appeared to support vegetation establishment by providing a thin zone of enhanced drainage at the surface. Natural pedogenic processes may be supplemented by irrigation, enhanced drainage, and incorporation of sand and organic matter at other tailings deposits to accelerate the remediation process and achieve similar results in a shorter time frame.