Life in acid: pH homeostasis in acidophiles

Cyclic Tetra-Adenylate (cA4) Recognition by Csa3; Implications for an Integrated Class 1 CRISPR-Cas Immune Response in Saccharolobus solfataricus

Csa3 family transcription factors are ancillary CRISPR-associated proteins composed of N-terminal CARF domains and C-terminal winged helix-turn-helix domains. The activity of Csa3 transcription factors is thought to be controlled by cyclic oligoadenyate (cOA) second messengers produced by type III CRISPR-Cas surveillance complexes. Here we show that Saccharolobus solfataricus Csa3a recognizes cyclic tetra-adenylate (cA₄) and that Csa3a lacks self-regulating "ring nuclease" activity present in some other CARF domain proteins. The crystal structure of the Csa3a/cA4 complex was also determined and the structural and thermodynamic basis for cA₄ recognition are described, as are conformational changes in Csa3a associated with cA₄ binding. We also characterized the effect of cA₄ on recognition of putative DNA binding sites. Csa3a binds to putative promoter sequences in a nonspecific, cooperative and cA₄-independent manner, suggesting a more complex mode of transcriptional regulation. We conclude the Csa3a/cA₄ interaction represents a nexus between the type I and type III CRISPR-Cas systems present in S. solfataricus, and discuss the role of the Csa3/cA₄ interaction in coordinating different arms of this integrated class 1 immune system to mount a synergistic, highly orchestrated immune response.

Low base-substitution mutation rate and predominance of insertion-deletion events in the acidophilic bacterium Acidobacterium capsulatum

Analyses of spontaneous mutation have shown that total genome-wide mutation rates are quantitatively similar for most prokaryotic organisms. However, this view is mainly based on organisms that grow best around neutral pH values (6.0-8.0). In particular, the whole-genome mutation rate has not been determined for an acidophilic organism. Here, we have determined the genome-wide rate of spontaneous mutation in the acidophilic Acidobacterium capsulatum using a direct and unbiased method: a mutation-accumulation experiment followed by whole-genome sequencing. Evaluation of 69 mutation accumulation lines of A. capsulatum after an average of ~2900 cell divisions yielded a base-substitution mutation rate of 1.22 × 10-10 per site per generation or 4 × 10-4 per genome per generation, which is significantly lower than the consensus value (2.5-4.6 × 10-3) of mesothermophilic (~15-40°C) and neutrophilic (pH 6-8) prokaryotic organisms. However, the insertion-deletion rate (0.43 × 10-10 per site per generation) is high relative to the base-substitution mutation rate. Organisms with a similar effective population size and a similar expected effect of genetic drift should have similar mutation rates. Because selection operates on the total mutation rate, it is suggested that the relatively high insertion-deletion rate may be balanced by a low base-substitution rate in A. capsulatum, with selection operating on the total mutation rate.

Genetic engineering of the acidophilic chemolithoautotroph Acidithiobacillus ferrooxidans

There are several natural and anthropomorphic environments where iron- and/or sulfur-oxidizing bacteria thrive in extremely acidic conditions. These acidophilic chemolithautotrophs play important roles in biogeochemical iron and sulfur cycles, are critical catalysts for industrial metal bioleaching operations, and have underexplored potential in future biotechnological applications. However, their unique growth conditions complicate the development of genetic techniques. Over the past few decades genetic tools have been successfully developed for Acidithiobacillus ferrooxidans, which serves as a model organism that exhibits both iron- and sulfur-oxidizing capabilities. Conjugal transfer of plasmids has enabled gene overexpression, gene knockouts, and some preliminary metabolic engineering. We highlight the development of genetic systems and recent genetic engineering of A. ferrooxidans, and discuss future perspectives.

Biochemical Characterization of Recombinant Isocitrate Dehydrogenase and Its Putative Role in the Physiology of an Acidophilic Micrarchaeon

Despite several discoveries in recent years, the physiology of acidophilic Micrarchaeota, such as "Candidatus Micrarchaeum harzensis A_DKE", remains largely enigmatic, as they highly express numerous genes encoding hypothetical proteins. Due to a lacking genetic system, it is difficult to elucidate the biological function of the corresponding proteins and heterologous expression is required. In order to prove the viability of this approach, A_DKE's isocitrate dehydrogenase (MhIDH) was recombinantly produced in Escherichia coli and purified to electrophoretic homogeneity for biochemical characterization. MhIDH showed optimal activity around pH 8 and appeared to be specific for NADP+ yet promiscuous regarding divalent cations as cofactors. Kinetic studies showed KM-values of 53.03 ± 5.63 µM and 1.94 ± 0.12 mM and kcat-values of 38.48 ± 1.62 and 43.99 ± 1.46 s-1 resulting in kcat/KM-values of 725 ± 107.62 and 22.69 ± 2.15 mM-1 s-1 for DL-isocitrate and NADP+, respectively. MhIDH's exceptionally low affinity for NADP+, potentially limiting its reaction rate, can likely be attributed to the presence of a proline residue in the NADP+ binding pocket, which might cause a decrease in hydrogen bonding of the cofactor and a distortion of local secondary structure.

A review on the recycling of spent lithium-ion batteries (LIBs) by the bioleaching approach

This review discusses the latest trend in recovering valuable metals from spent lithium-ion batteries (LIBs) to meet the technological world's critical metal demands. Spent LIBs are a secondary source of valuable metals such as Li (5%-7%), Ni (5%-10%), Co (5%-25%), Mn (5-11%), and non-metal graphite. Recycling is essential for the battery industry to extract valuable critical metals from secondary sources to develop new and novel high-tech LIBs for various applications such as eco-friendly technologies, renewable energy, emission-free electric vehicles, and energy-saving lightings. LIB waste is currently undergoing high-temperature pyrometallurgical or hydrometallurgical processes to recover valuable metals, and these processes have proven to be successful and feasible. These methods, however, are not preferable due to the difficulties in controlling the process, secondary waste produced, high operational cost, and high risk of scaling up. Biotechnological approaches can be promising alternatives to pyrometallurgical and hydrometallurgical technologies in metal recovery from LIB waste. Microbiological metal dissolution or bioleaching has gained popularity for metal extraction from ores, concentrates, and recycled or residual materials in recent years. This technology is eco-friendly, safe to handle, and reduces operating costs and energy demands. The pre-treatment process (material preparation), microorganisms used in the bioleaching of LIBs, factors influencing the bioleaching process, methods of enhancing the leaching efficiency, regeneration of electrode materials, and future aspects have been discussed in detail.

Phosphine on Venus Cannot Be Explained by Conventional Processes

The recent candidate detection of ∼1 ppb of phosphine in the middle atmosphere of Venus is so unexpected that it requires an exhaustive search for explanations of its origin. Phosphorus-containing species have not been modeled for Venus' atmosphere before, and our work represents the first attempt to model phosphorus species in the venusian atmosphere. We thoroughly explore the potential pathways of formation of phosphine in a venusian environment, including in the planet's atmosphere, cloud and haze layers, surface, and subsurface. We investigate gas reactions, geochemical reactions, photochemistry, and other nonequilibrium processes. None of these potential phosphine production pathways is sufficient to explain the presence of ppb phosphine levels on Venus. If PH₃'s presence in Venus' atmosphere is confirmed, it therefore is highly likely to be the result of a process not previously considered plausible for venusian conditions. The process could be unknown geochemistry, photochemistry, or even aerial microbial life, given that on Earth phosphine is exclusively associated with anthropogenic and biological sources. The detection of phosphine adds to the complexity of chemical processes in the venusian environment and motivates in situ follow-up sampling missions to Venus. Our analysis provides a template for investigation of phosphine as a biosignature on other worlds.

Phosphorus in the Clouds of Venus: Potential for Bioavailability

Aerosol phase elements such as phosphorus (P), sulfur (S), and metals including iron (Fe) are essential nutrients that could help sustain potential biodiversity in the cloud deck of Venus. While the presence of S and Fe in the venusian cloud deck has been broadly discussed (Zasova et al., 1981; Krasnopolsky, 2012, 2013, 2016, 2017; Markiewicz et al., 2014), less attention has been given to the presence of P in the aerosols and its involvement in the multiphase chemistry of venusian clouds and potential sources of P deposition in the venusian atmosphere. A detailed characterization of phosphorus atmospheric chemistry in the cloud deck of Venus is crucial for understanding its solubility and bioavailability for potential venusian cloud microbiota (Schulze-Makuch et al., 2004; Grinspoon and Bullock, 2007; Limaye et al., 2018). We summarize our current understanding of the presence of P in the clouds of Venus and its role in a hypothetical atmospheric (bio)chemical cycle. The results of the VeGa lander measurements are put into perspective with regard to nutrient limitation for a potential biosphere in venusian clouds. Our work combines the results of the VeGa measurements and focuses on P as an inorganic nutrient component and its potential sources and chemical behavior as part of multiple transformations of atmospheric chemistry. The VeGa data indicate that a plentiful phosphorus layer exists within a layer that reaches into the lower venusian clouds and exceeds minimum P abundances for terrestrial microbial life. Extreme acidification of airborne phases in the atmosphere of Venus may facilitate P solubilization and its bioavailability for a potential ecosystem in venusian clouds. Further sampling and P abundance measurements in the atmosphere of Venus would improve our knowledge of P speciation and facilitate determination of a bioavailable fraction of P detected in venusian clouds. The previous results deserve further experimental and modeling analyses to diminish uncertainties and understand the rates of atmospheric deposition of P and its role in a potential venusian cloud ecosystem.

Application of acid and heavy metal resistant bacteria from rat-hole coal mines in bioremediation strategy

The Gram-negative bacteria isolated from acid mine drainage (AMD) of rat-hole coal mines were found to be resistant to acidic conditions as well as Fe, Cd, and Cr. The minimum inhibitory concentration (MIC) and maximum bactericidal concentration (MBC) of iron, cadmium, and chromium against the isolates of Enterobacter huaxiensis KHED8 were 4000, 4096, and 256 mg/L, respectively, while the MIC and MBC of Fe, Cd, and Cr against two isolates (KH5M10 and KHCL12) of Serratia marcescens subsp. sakuensis isolates were 4000, 8192, and 256 mg/L, respectively. It was also found that E. huaxiensis KHED8 was able to remove 89%, 90%, and 82.45% of Fe, Cd, and Cr, respectively, and the two isolates of S. marcescens subsp. sakuensis KH5M10 and KHCL12 were able to remove 90%, 95%, and 85.62% of Fe, Cd, and Cr, respectively. The findings of the study provide lead to the use of these heavy metals resistant bacterial isolates for bioremediation of heavy metals laden AMD prevalent in unique rat-hole coal mines.

Physiology of the Nitrite-Oxidizing Bacterium Candidatus Nitrotoga sp. CP45 Enriched From a Colorado River

Nitrogen cycling microbes, including nitrite-oxidizing bacteria (NOB), perform critical ecosystem functions that help mitigate anthropogenic stresses and maintain ecosystem health. Activity of these beneficial nitrogen cycling microbes is dictated in part by the microorganisms’ response to physicochemical conditions, such as temperature, pH, and nutrient availability. NOB from the newly described Candidatus Nitrotoga genus have been detected in a wide range of habitats across the globe, yet only a few organisms within the genus have been physiologically characterized. For freshwater systems where NOB are critical for supporting aquatic life, Ca. Nitrotoga have been previously detected but little is known about the physiological potential of these organisms or their response to changing environmental conditions. Here, we determined functional response to environmental change for a representative freshwater species of Ca. Nitrotoga (Ca. Nitrotoga sp. CP45, enriched from a Colorado river). The physiological findings demonstrated that CP45 maintained nitrite oxidation at pH levels of 5–8, at temperatures from 4 to 28°C, and when incubated in the dark. Light exposure and elevated temperature (30°C) completely halted nitrite oxidation. Ca. Nitrotoga sp. CP45 maintained nitrite oxidation upon exposure to four different antibiotics, and potential rates of nitrite oxidation by river sediment communities were also resilient to antibiotic stress. We explored the Ca. Nitrotoga sp. CP45 genome to make predictions about adaptations to enable survival under specific conditions. Overall, these results contribute to our understanding of the versatility of a representative freshwater Ca. Nitrotoga sp. Identifying the specific environmental conditions that maximize NOB metabolic rates may ultimately direct future management decisions aimed at restoring impacted systems.

Candidatus Eremiobacterota, a metabolically and phylogenetically diverse terrestrial phylum with acid-tolerant adaptations

Candidatus phylum Eremiobacterota (formerly WPS-2) is an as-yet-uncultured bacterial clade that takes its name from Ca. Eremiobacter, an Antarctic soil aerobe proposed to be capable of a novel form of chemolithoautotrophy termed atmospheric chemosynthesis, that uses the energy derived from atmospheric H₂-oxidation to fix CO₂ through the Calvin-Benson-Bassham (CBB) cycle via type 1E RuBisCO. To elucidate the phylogenetic affiliation and metabolic capacities of Ca. Eremiobacterota, we analysed 63 public metagenome-assembled genomes (MAGs) and nine new MAGs generated from Antarctic soil metagenomes. These MAGs represent both recognized classes within Ca. Eremiobacterota, namely Ca. Eremiobacteria and UBP9. Ca. Eremiobacteria are inferred to be facultatively acidophilic with a preference for peptides and amino acids as nutrient sources. Epifluorescence microscopy revealed Ca. Eremiobacteria cells from Antarctica desert soil to be coccoid in shape. Two orders are recognized within class Ca. Eremiobacteria: Ca. Eremiobacterales and Ca. Baltobacterales. The latter are metabolically versatile, with individual members having genes required for trace gas driven autotrophy, anoxygenic photosynthesis, CO oxidation, and anaerobic respiration. UBP9, here renamed Ca. Xenobia class. nov., are inferred to be obligate heterotrophs with acidophilic adaptations, but individual members having highly divergent metabolic capacities compared to Ca. Eremiobacteria, especially with regard to respiration and central carbon metabolism. We conclude Ca. Eremiobacterota to be an ecologically versatile phylum with the potential to thrive under an array of "extreme" environmental conditions.

Highly tunable TetR-dependent target gene expression in the acetic acid bacterium Gluconobacter oxydans

Abstract

For the acetic acid bacterium (AAB) Gluconobacter oxydans only recently the first tight system for regulatable target gene expression became available based on the heterologous repressor-activator protein AraC from Escherichia coli and the target promoter P_araBAD. In this study, we tested pure repressor-based TetR- and LacI-dependent target gene expression in G. oxydans by applying the same plasmid backbone and construction principles that we have used successfully for the araC-P_araBAD system. When using a pBBR1MCS-5-based plasmid, the non-induced basal expression of the Tn10-based TetR-dependent expression system was extremely low. This allowed calculated induction ratios of up to more than 3500-fold with the fluorescence reporter protein mNeonGreen (mNG). The induction was highly homogeneous and tunable by varying the anhydrotetracycline concentration from 10 to 200 ng/mL. The already strong reporter gene expression could be doubled by inserting the ribosome binding site AGGAGA into the 3’ region of the P_tet sequence upstream from mNG. Alternative plasmid constructs used as controls revealed a strong influence of transcription terminators and antibiotics resistance gene of the plasmid backbone on the resulting expression performance. In contrast to the TetR-P_tet-system, pBBR1MCS-5-based LacI-dependent expression from P_lacUV5 always exhibited some non-induced basal reporter expression and was therefore tunable only up to 40-fold induction by IPTG. The leakiness of P_lacUV5 when not induced was independent of potential read-through from the lacI promoter. Protein-DNA binding simulations for pH 7, 6, 5, and 4 by computational modeling of LacI, TetR, and AraC with DNA suggested a decreased DNA binding of LacI when pH is below 6, the latter possibly causing the leakiness of LacI-dependent systems hitherto tested in AAB. In summary, the expression performance of the pBBR1MCS-5-based TetR-P_tet system makes this system highly suitable for applications in G. oxydans and possibly in other AAB.

Key Points

• A pBBR1MCS-5-based TetR-P_tet system was tunable up to more than 3500-fold induction.

• A pBBR1MCS-5-based LacI-P_lacUV5 system was leaky and tunable only up to 40-fold.

• Modeling of protein-DNA binding suggested decreased DNA binding of LacI at pH < 6.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11473-x.

Leucine-Responsive Regulatory Protein in Acetic Acid Bacteria Is Stable and Functions at a Wide Range of Intracellular pH Levels

Acetic acid bacteria grow while producing acetic acid, resulting in acidification of the culture. Limited reports elucidate the effect of changes in intracellular pH on transcriptional factors. In the present study, the intracellular pH of Komagataeibacter europaeus was monitored with a pH-sensitive green fluorescent protein, showing that the intracellular pH decreased from 6.3 to 4.7 accompanied by acetic acid production during cell growth. The leucine-responsive regulatory protein of K. europaeus (KeLrp) was used as a model to examine pH-dependent effects, and its properties were compared with those of the Escherichia coli ortholog (EcLrp) at different pH levels. The DNA-binding activities of EcLrp and KeLrp with the target DNA (Ec-ilvI and Ke-ilvI) were examined by gel mobility shift assays under various pH conditions. EcLrp showed the highest affinity with the target at pH 8.0 (K_d [dissociation constant], 0.7 μM), decreasing to a minimum of 3.4 μM at pH 4.0. Conversely, KeLrp did not show significant differences in binding affinity between pH 4 and 7 (K_d, 1.0 to 1.5 μM), and the highest affinity was at pH 5.0 (K_d, 1.0 μM). Circular dichroism spectroscopy revealed that the α-helical content of KeLrp was the highest at pH 5.0 (49%) and was almost unchanged while being maintained at >45% over a range of pH levels examined, while that of EcLrp decreased from its maximum (49% at pH 7.0) to its minimum (36% at pH 4.0). These data indicate that KeLrp is stable and functions over a wide range of intracellular pH levels. IMPORTANCE Lrp is a highly conserved transcriptional regulator found in bacteria and archaea and regulates transcriptions of various genes. The intracellular pH of acetic acid bacteria (AAB) changes accompanied by acetic acid production during cell growth. The Lrp of AAB K. europaeus (KeLrp) was structurally stable over a wide range of pH and maintained DNA-binding activity even at low pH compared with Lrp from E. coli living in a neutral environment. An in vitro experiment showed DNA-binding activity of KeLrp to the target varied with changes in pH. In AAB, change of the intracellular pH during a cell growth would be an important trigger in controlling the activity of Lrp in vivo.

The biology of thermoacidophilic archaea from the order Sulfolobales

Thermoacidophilic archaea belonging to the order Sulfolobales thrive in extreme biotopes, such as sulfuric hot springs and ore deposits. These microorganisms have been model systems for understanding life in extreme environments, as well as for probing the evolution of both molecular genetic processes and central metabolic pathways. Thermoacidophiles, such as the Sulfolobales, use typical microbial responses to persist in hot acid (e.g. motility, stress response, biofilm formation), albeit with some unusual twists. They also exhibit unique physiological features, including iron and sulfur chemolithoautotrophy, that differentiate them from much of the microbial world. Although first discovered >50 years ago, it was not until recently that genome sequence data and facile genetic tools have been developed for species in the Sulfolobales. These advances have not only opened up ways to further probe novel features of these microbes but also paved the way for their potential biotechnological applications. Discussed here are the nuances of the thermoacidophilic lifestyle of the Sulfolobales, including their evolutionary placement, cell biology, survival strategies, genetic tools, metabolic processes and physiological attributes together with how these characteristics make thermoacidophiles ideal platforms for specialized industrial processes.

Critical Factors for Optimum Biodegradation of Bast Fiber’s Gums in Bacterial Retting

Bast fiber plants require a post-harvest process to yield useable natural cellulosic fibers, denoted as retting or degumming. It encompasses the degradation of the cell wall’s non-cellulosic gummy substances (NCGs), facilitating fibers separations, setting the fiber’s quality, and determining downstream usages. Due to the inconvenience of traditional retting practices, bacterial inoculum and enzyme applications for retting gained attention. Therefore, concurrent changes of agroclimatic and socioeconomic conditions, the conventional water retting confront multiple difficulties, bast industries become vulnerable, and bacterial agents mediated augmented bio-retting processes trying to adapt to sustainability. However, this process’s success demands a delicate balance among substrates and retting-related biotic and abiotic factors. These critical factors were coupled to degrade bast fibers NCGs in bacterial retting while holistically disregarded in basic research. In this study, a set of factors were defined that critically regulates the process and requires to be comprehended to achieve optimum retting without failure. This review presents the bacterial strain characteristics, enzyme potentials, specific bast plant cell wall’s structure, compositions, solvents, and interactions relating to the maximum NCGs removal. Among plants, associated factors pectin is the primary biding material that determines the process’s dynamics, while its degree of esterification has a proficient effect through bacterial enzymatic degradation. The accomplished bast plant cell wall’s structure, macerating solvents pH, and temperature greatly influence the bacterial retting process. This article also highlights the remediation process of water retting pollution in a biocompatible manner concerning the bast fiber industry’s endurance.

Lactobacillus, Bifidobacterium and Lactococcus response to environmental stress: Mechanisms and application of cross-protection to improve resistance against freeze-drying

The review deals with lactic acid bacteria in characterizing the stress adaptation with cross-protection effects, mainly associated with Lactobacillus, Bifidobacterium and Lactococcus. It focuses on adaptation and cross-protection in Lactobacillus, Bifidobacterium and Lactococcus, including heat shocking, cold stress, acid stress, osmotic stress, starvation effect, etc. Web of Science, Google Scholar, Science Direct, and PubMed databases were used for the systematic search of literature up to the year 2020. The literature suggests that a lower survival rate during freeze-drying is linked to environmental stress. Protective pretreatment under various mild stresses can be applied to lactic acid bacteria which may enhance resistance in a strain-dependent manner. We investigate the mechanism of damage and adaptation under various stresses including heat, cold, acidic, osmotic, starvation, oxidative and bile stress. Adaptive mechanisms include synthesis of stress-induced proteins, adjusting the composition of cell membrane fatty acids, accumulating compatible substances, etc. Next, we reveal the cross-protective effect of specific stress on the other environmental stresses. Freeze-drying is discussed from three perspectives including the regulation of membrane, accumulation of compatible solutes and the production of chaperones and stress-responsive proteases. The resistance of lactic acid bacteria against technological stress can be enhanced via cross-protection, which improves industrial efficiency concerning the survival of probiotics. However, the adaptive responses and cross-protection are strain-dependent and should be optimized case by case.

The adaptation mechanisms of Acidithiobacillus caldus CCTCC M 2018054 to extreme acid stress: Bioleaching performance, physiology, and transcriptomics

To understand the acid-resistant mechanism of bioleaching microorganism Acidithiobacillus caldus CCTCC M 2018054, its physiology and metabolic changes at the transcriptional level under extreme acid stress were systemically studied. Scanning electron microscopy (SEM), Fourier transform infrared reflection (FTIR) and X-ray diffraction (XRD) showed that with an increase in acidity, the absorption peak of sulfur oxidation-related functional groups such as S-O decreased significantly, and a dense sulfur passivation film appeared on the surface of the ore. Confocal laser scanning microscopy (CLSM) revealed that coverage scale of extracellular polymeric substance (EPS) and biofilm fluctuated accordingly along with the increasing acid stress (pH-stat 1.5, 1.2 0.9 and 0.6) during the bioleaching process. In response to acid stress, the increased levels of intracellular glutamic acid, alanine, cysteine, and proline contributed to the maintenance of intracellular pH homeostasis via decarboxylation and alkaline neutralization. Higher unsaturated fatty acid content was closely related to membrane fluidity. Up to 490 and 447 differentially expressed genes (DEGs) were identified at pH 1.5 vs pH 1.2 and pH 1.2 vs pH 0.9, respectively, and 177 common DEGs were associated with two-component system (TCS) regulation, transporter regulation, energy metabolism, and stress response. The upregulation of kdpB helped cells defend against proton invasion, whereas the downregulation of cysB and cbl implied stronger oxidation of sulfur compounds. The transcriptional level of sqr, sor, and soxA was significantly increased and consolidated the energy supply needed for resisting acid stress. Furthermore, eight of the identified DEGs (sor, cbl, ompA, atpF, nuoH, nuoC, sqr, grxB) were verified as being related to the acid stress response process. This study contributes toward expanding the application of these acidophiles in industrial bioleaching.

pH dependent electrical properties of the inner- and outer- leaflets of biomimetic cell membranes

Composition and asymmetry of lipid membranes provide a means for regulation of trans-membrane permeability of ions and small molecules. The pH dependence of these processes plays an important role in the functioning and survival of cells. In this work, we study the pH dependence of membrane electrical resistance and capacitance using electrochemical impedance spectroscopy (EIS), surface plasmon resonance (SPR) and neutron reflectometry (NR) measurements of biomimetic tethered bilayer lipid membranes (tBLMs). tBLMs were prepared with single-component phospholipid compositions, as well as mixtures of phospholipids (phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, sphingomyelin and cholesterol) that mimic the inner- and outer- leaflets of plasma cell membranes. We found that all studied tBLMs have a resistance maximum at pHs near the pK_as of the phospholipids. SPR and NR indicated that surface concentration of phospholipids and the thickness of the hydrophobic part of the membrane did not change versus pH. We postulate that these maxima are the result of protonation of the phosphate oxygen of the phospholipids and that hydronium ions play a major role in the conductance at pHs < pK_as while sodium ions play the major role at pHs > pK_as. An additional sharp resistance maximum of the PE tBLMs found at pH 5.9 and most likely represents the phosphatidylethanolamine's isoelectric point. The data show the key roles of the characteristic parts of phospholipid molecules: terminal group (choline, carboxyl, amine), phosphate, glycerol and ester oxygens on the permeability and selectivity of ions through the membrane. The interactions between these groups lead to significant differences in the electrical properties of biomimetic models of inner- and outer- leaflets of the plasma cell membranes.

Involvement of CspC in response to diverse environmental stressors in Escherichia coli

The ability of Escherichia coli surviving a cold shock lies mainly with the induction of a few Csps termed as 'Major cold shock proteins'. Regardless of high sequence similarity among the nine homologous members, CspC appears to be functionally diverse in conferring the cell adaptability to various stresses based on fundamental properties of the protein including nucleic acid binding, nucleic acid melting and regulatory activity. Spanning three different stress regulons of acid, oxidative and heat, CspC regulates gene expression and transcript stability of stress proteins and bestows upon the cell tolerance to lethal-inducing agents ultimately helping it adapt to severe environmental assaults. While its exact role in cellular physiology is still to be detailed, understanding the transcriptional and translational control will likely provide insights into the mechanistic role of CspC under stress conditions. To this end, we review the knowledge on stress protein regulation by CspC and highlight its activity in response to stressors thereby elucidating its role as a major Csp player in response to one too many environmental triggers. The knowledge presented here could see various downstream applications in engineering microbes for industrial, agricultural and research applications in order to achieve high product efficiency and to aid bacteria cope with environmentally harsh conditions.

Physiological, Genomic and Transcriptomic Analyses Reveal the Adaptation Mechanisms of Acidiella bohemica to Extreme Acid Mine Drainage Environments

Fungi in acid mine drainage (AMD) environments are of great concern due to their potentials of decomposing organic carbon, absorbing heavy metals and reducing AMD acidity. Based on morphological analysis and ITS/18S high-throughput sequencing technology, previous studies have provided deep insights into the diversity and community composition of fungi in AMD environments. However, knowledge about physiology, metabolic potential and transcriptome profiles of fungi inhabiting AMD environments is still scarce. Here, we reported the physiological, genomic, and transcriptomic characterization of Acidiella bohemica SYSU C17045 to improve our understanding of the physiological, genomic, and transcriptomic mechanisms underlying fungal adaptation to AMD environments. A. bohemica was isolated from an AMD environment, which has been proved to be an acidophilic fungus in this study. The surface of A. bohemica cultured in AMD solutions was covered with a large number of minerals such as jarosite. We thus inferred that the A. bohemica might have the potential of biologically induced mineralization. Taking advantage of PacBio single-molecule real-time sequencing, we obtained the high-quality genome sequences of A. bohemica (50 Mbp). To our knowledge, this was the first attempt to employ a third-generation sequencing technology to explore the genomic traits of fungi isolated from AMD environments. Moreover, our transcriptomic analysis revealed that a series of genes in the A. bohemica genome were related to its metabolic pathways of C, N, S, and Fe as well as its adaptation mechanisms, including the response to acid stress and the resistance to heavy metals. Overall, our physiological, genomic, and transcriptomic data provide a foundation for understanding the metabolic potential and adaptation mechanisms of fungi in AMD environments.

Life in hot acid: a genome-based reassessment of the archaeal order Sulfolobales

The order Sulfolobales was one of the first named Archaeal lineages, with globally distributed members from terrestrial thermal acid springs (pH < 4; T > 65°C). The Sulfolobales represent broad metabolic capabilities, ranging from lithotrophy, based on inorganic iron and sulfur biotransformations, to autotrophy, to chemoheterotrophy in less acidophilic species. Components of the 3-hydroxypropionate/4-hydroxybutyrate carbon fixation cycle, as well as sulfur oxidation, are nearly universally conserved, although dissimilatory sulfur reduction and disproportionation (Acidianus, Stygiolobus and Sulfurisphaera) and iron oxidation (Acidianus, Metallosphaera, Sulfurisphaera, Sulfuracidifex and Sulfodiicoccus) are limited to fewer lineages. Lithotrophic marker genes appear more often in highly acidophilic lineages. Despite the presence of facultative anaerobes and one confirmed obligate anaerobe, oxidase complexes (fox, sox, dox and a new putative cytochrome bd) are prevalent in many species (even facultative/obligate anaerobes), suggesting a key role for oxygen among the Sulfolobales. The presence of fox genes tracks with a putative antioxidant OsmC family peroxiredoxin, an indicator of oxidative stress derived from mixing reactive metals and oxygen. Extreme acidophily appears to track inversely with heterotrophy but directly with lithotrophy. Recent phylogenetic re-organization efforts are supported by the comparative genomics here, although several changes are proposed, including the expansion of the genus Saccharolobus.

Prokaryotic and eukaryotic diversity in hydrothermal continental systems

The term extremophile was suggested more than 30 years ago and represents microorganisms that are capable of developing and living under extreme conditions, these conditions being particularly hostile to other types of microorganisms and to humankind. In terrestrial hydrothermal sites, like hot springs, "mud pools", solfataras, and geysers, the dominant extreme conditions are high temperature, low or high pH, and high levels of salinity. The diversity of microorganisms inhabiting these sites is determined by the conditions of the environment. Organisms belonging to the domains Archaea and Bacteria are more represented than the one belonging to Eukarya. Eukarya members tend to be less present because of their lower tolerance to higher temperatures, however, they perform important ecosystem processes when present. Both prokaryotes and eukaryotes have morphological and physical adaptations that allow them to colonize extreme environments. Microbial mats are complex associations of microorganisms that help the colonization of more extreme systems. In this review, a characterization of prokaryotic and eukaryotic organisms that populate terrestrial hydrothermal systems are made.

Genomic adaptations enabling Acidithiobacillus distribution across wide-ranging hot spring temperatures and pHs

BACKGROUND

Terrestrial hot spring settings span a broad spectrum of physicochemistries. Physicochemical parameters, such as pH and temperature, are key factors influencing differences in microbial composition across diverse geothermal areas. Nonetheless, analysis of hot spring pools from the Taupo Volcanic Zone (TVZ), New Zealand, revealed that some members of the bacterial genus, Acidithiobacillus, are prevalent across wide ranges of hot spring pHs and temperatures. To determine the genomic attributes of Acidithiobacillus that inhabit such diverse conditions, we assembled the genomes of 19 uncultivated hot spring Acidithiobacillus strains from six geothermal areas and compared these to 37 publicly available Acidithiobacillus genomes from various habitats.

RESULTS

Analysis of 16S rRNA gene amplicons from 138 samples revealed that Acidithiobacillus comprised on average 11.4 ± 16.8% of hot spring prokaryotic communities, with three Acidithiobacillus amplicon sequence variants (ASVs) (TVZ_G1, TVZ_G2, TVZ_G3) accounting for > 90% of Acidithiobacillus in terms of relative abundance, and occurring in 126 out of 138 samples across wide ranges of temperature (17.5-92.9 °C) and pH (1.0-7.5). We recovered 19 environmental genomes belonging to each of these three ASVs, as well as a fourth related group (TVZ_G4). Based on genome average nucleotide identities, the four groups (TVZ_G1-TVZ_G4) constitute distinct species (ANI < 96.5%) of which three are novel Acidithiobacillus species (TVZ_G2-TVZ_G4) and one belongs to Acidithiobacillus caldus (TVZ_G1). All four TVZ Acidithiobacillus groups were found in hot springs with temperatures above the previously known limit for the genus (up to 40 °C higher), likely due to significantly higher proline and GC contents than other Acidithiobacillus species, which are known to increase thermostability. Results also indicate hot spring-associated Acidithiobacillus have undergone genome streamlining, likely due to thermal adaptation. Moreover, our data suggest that Acidithiobacillus prevalence across varied hot spring pHs is supported by distinct strategies, whereby TVZ_G2-TVZ_G4 regulate pH homeostasis mostly through Na⁺/H⁺ antiporters and proton-efflux ATPases, whereas TVZ_G1 mainly relies on amino acid decarboxylases.

CONCLUSIONS

This study provides insights into the distribution of Acidithiobacillus species across diverse hot spring physichochemistries and determines genomic features and adaptations that potentially enable Acidithiobacillus species to colonize a broad range of temperatures and pHs in geothermal environments. Video Abstract.

Persistence and resistance: survival mechanisms of Candidatus Dormibacterota from nutrient-poor Antarctic soils

Candidatus Dormibacterota is an uncultured bacterial phylum found predominantly in soil that is present in high abundances within cold desert soils. Here, we interrogate nine metagenome-assembled genomes (MAGs), including six new MAGs derived from soil metagenomes obtained from two eastern Antarctic sites. Phylogenomic and taxonomic analyses revealed these MAGs represent four genera and five species, representing two order-level clades within Ca. Dormibacterota. Metabolic reconstructions of these MAGs revealed the potential for aerobic metabolism, and versatile adaptations enabling persistence in the 'extreme' Antarctic environment. Primary amongst these adaptations were abilities to scavenge atmospheric H₂ and CO as energy sources, as well as using the energy derived from H₂ oxidation to fix atmospheric CO₂ via the Calvin-Bassham-Benson cycle, using a RuBisCO type IE. We propose that these allow Ca. Dormibacterota to persist using H₂ oxidation and grow using atmospheric chemosynthesis in terrestrial Antarctica. Fluorescence in situ hybridization revealed Ca. Dormibacterota to be coccoid cells, 0.3-1.4 μm in diameter, with some cells exhibiting the potential for a symbiotic or syntrophic lifestyle.

Metabolic potentials of members of the class Acidobacteriia in metal-contaminated soils revealed by metagenomic analysis

The relative abundance of Acidobacteriia correlated positively with the concentrations of arsenic (As), mercury (Hg), chromium (Cr), copper (Cu) and other metals, suggesting their adaptation of the metal-rich environments. Metagenomic binning reconstructed 29 high-quality metagenome-assembled genomes (MAGs) associated with Acidobacteriia, providing an opportunity to study their metabolic potentials. These MAGs contained genes to transform As, Hg and Cr through oxidation, reduction, efflux and demethylation, suggesting the potential of Acidobacteriia to transform such metal(loid)s. Additionally, genes associated with alleviation of acidic and metal stress were also detected in these MAGs. Acidobacteriia may have the capabilities to resist or transform metal(loid)s in acidic metal-contaminated sites. Moreover, these genes encoding metal transformation could be also identified in the Acidobacteriia-associated MAGs from five additional metal-contaminated sites across Southwest China, as well as Acidobacteriia-associated reference genomes from the NCBI database, suggesting that the capability of metal transformation may be widespread among Acidobacteriia members. This discovery provides an understanding of metabolic potentials of the Acidobacteriia in acidic metal-rich sites.

Bacterial community in saline farmland soil on the Tibetan plateau: responding to salinization while resisting extreme environments

Background

Salinization damages the health of soil systems and reduces crop yields. Responses of microbial communities to salinized soils and their functional maintenance under high salt stress are valuable scientific problems. Meanwhile, the microbial community of the salinized soil in the plateau environment is less understood. Here, we applied metagenomics technology to reveal the structure and function of microorganisms in salinized soil of the Tibetan Plateau.

Results

The diversity of composition and function of microbial community in saline soil have changed significantly. The abundances of chemoautotrophic and acidophilic bacteria comprising Rhodanobacter, Acidobacterium, Candidatus Nitrosotalea, and Candidatus Koribacter were significantly higher in saline soil. The potential degradation of organic carbon in the saline soil, as well as the production of NO and N₂O via denitrification, and the production of sulfate by sulfur oxidation were significantly higher than the non-saline soil. Both types of soils were rich in genes encoding resistance to environmental stresses (i.e., cold, ultraviolet light, and hypoxia in Tibetan Plateau). The resistance of the soil microbial communities to the saline environment is based on the absorption of K⁺ as the main mechanism, with cross-protection proteins and absorption buffer molecules as auxiliary mechanisms in our study area. Network analysis showed that functional group comprising chemoautotrophic and acidophilic bacteria had significant positive correlations with electrical conductivity and total sulfur, and significant negative correlations with the total organic carbon, pH, and available nitrogen. The soil moisture, pH, and electrical conductivity are likely to affect the bacterial carbon, nitrogen, and sulfur cycles.

Conclusions

These results indicate that the specific environment of the Tibetan Plateau and salinization jointly shape the structure and function of the soil bacterial community, and that the bacterial communities respond to complex and harsh living conditions. In addition, environmental feedback probably exacerbates greenhouse gas emissions and accelerates the reduction in the soil pH. This study will provide insights into the microbial responses to soil salinization and the potential ecological risks in the special plateau environment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-021-02190-6.

Ammonia oxidation at pH 2.5 by a new gammaproteobacterial ammonia-oxidizing bacterium

Ammonia oxidation was considered impossible under highly acidic conditions, as the protonation of ammonia leads to decreased substrate availability and formation of toxic nitrogenous compounds. Recently, some studies described archaeal and bacterial ammonia oxidizers growing at pH as low as 4, while environmental studies observed nitrification at even lower pH values. In this work, we report on the discovery, cultivation, and physiological, genomic, and transcriptomic characterization of a novel gammaproteobacterial ammonia-oxidizing bacterium enriched via continuous bioreactor cultivation from an acidic air biofilter that was able to grow and oxidize ammonia at pH 2.5. This microorganism has a chemolithoautotrophic lifestyle, using ammonia as energy source. The observed growth rate on ammonia was 0.196 day^-1, with a doubling time of 3.5 days. The strain also displayed ureolytic activity and cultivation with urea as ammonia source resulted in a growth rate of 0.104 day^-1 and a doubling time of 6.7 days. A high ammonia affinity (K_m(app) = 147 ± 14 nM) and high tolerance to toxic nitric oxide could represent an adaptation to acidic environments. Electron microscopic analysis showed coccoid cell morphology with a large amount of intracytoplasmic membrane stacks, typical of gammaproteobacterial ammonia oxidizers. Furthermore, genome and transcriptome analysis showed the presence and expression of diagnostic genes for nitrifiers (amoCAB, hao, nor, ure, cbbLS), but no nirK was identified. Phylogenetic analysis revealed that this strain belonged to a novel bacterial genus, for which we propose the name "Candidatus Nitrosacidococcus tergens" sp. RJ19.

Role of extremophiles and their extremozymes in biorefinery process of lignocellulose degradation

Technological advances in the field of life sciences have led to discovery of organisms that live in harsh environmental conditions referred to as extremophiles. These organisms have adapted themselves to thrive in extreme habitat giving these organisms an advantage over conventional mesophilic organisms in various industrial applications. Extremozymes produced by these extremophiles have high tolerance to inhospitable environmental conditions making them an ideal enzyme system for various industrial processes. A notable application of these extremophiles and extremozymes is their use in the degradation of recalcitrant lignocellulosic biomass and application in biorefineries. For maximum utilization of the trapped carbon source from this obstinate biomass, pretreatment is a necessary step that requires various physiochemical and enzymatic treatments. From search for novel extremophiles and extremozymes to development of various genetic and protein engineering techniques, investigation on extremozymes with enhanced stability and efficiency is been done. Since extremozymes are easily calibrated to work under such conditions, they have become an emerging topic in the research field of biofuel production. The review discusses the various extremozymes that play an important role in lignocellulose degradation along with recent studies on their molecular and genetic evolution for industrial application and production of biofuels and various value-added products.

Growth on Formic Acid Is Dependent on Intracellular pH Homeostasis for the Thermoacidophilic Methanotroph Methylacidiphilum sp. RTK17.1

Members of the genus Methylacidiphilum, a clade of metabolically flexible thermoacidophilic methanotrophs from the phylum Verrucomicrobia, can utilize a variety of substrates including methane, methanol, and hydrogen for growth. However, despite sequentially oxidizing methane to carbon dioxide via methanol and formate intermediates, growth on formate as the only source of reducing equivalents (i.e., NADH) has not yet been demonstrated. In many acidophiles, the inability to grow on organic acids has presumed that diffusion of the protonated form (e.g., formic acid) into the cell is accompanied by deprotonation prompting cytosolic acidification, which leads to the denaturation of vital proteins and the collapse of the proton motive force. In this work, we used a combination of biochemical, physiological, chemostat, and transcriptomic approaches to demonstrate that Methylacidiphilum sp. RTK17.1 can utilize formate as a substrate when cells are able to maintain pH homeostasis. Our findings show that Methylacidiphilum sp. RTK17.1 grows optimally with a circumneutral intracellular pH (pH 6.52 ± 0.04) across an extracellular range of pH 1.5–3.0. In batch experiments, formic acid addition resulted in no observable cell growth and cell death due to acidification of the cytosol. Nevertheless, stable growth on formic acid as the only source of energy was demonstrated in continuous chemostat cultures (D = 0.0052 h⁻¹, t_d = 133 h). During growth on formic acid, biomass yields remained nearly identical to methanol-grown chemostat cultures when normalized per mole electron equivalent. Transcriptome analysis revealed the key genes associated with stress response: methane, methanol, and formate metabolism were differentially expressed in response to growth on formic acid. Collectively, these results show formic acid represents a utilizable source of energy/carbon to the acidophilic methanotrophs within geothermal environments. Findings expand the known metabolic flexibility of verrucomicrobial methanotrophs to include organic acids and provide insight into potential survival strategies used by these species during methane starvation.

The Molecular Basis for Life in Extreme Environments

Sampling and genomic efforts over the past decade have revealed an enormous quantity and diversity of life in Earth's extreme environments. This new knowledge of life on Earth poses the challenge of understandingits molecular basis in such inhospitable conditions, given that such conditions lead to loss of structure and of function in biomolecules from mesophiles. In this review, we discuss the physicochemical properties of extreme environments. We present the state of recent progress in extreme environmental genomics. We then present an overview of our current understanding of the biomolecular adaptation to extreme conditions. As our current and future understanding of biomolecular structure-function relationships in extremophiles requires methodologies adapted to extremes of pressure, temperature, and chemical composition, advances in instrumentation for probing biophysical properties under extreme conditions are presented. Finally, we briefly discuss possible future directions in extreme biophysics.

RNA transcript response by an Acidithiobacillus spp. mixed culture reveals adaptations to growth on arsenopyrite

Biooxidation of gold-bearing refractory mineral ores such as arsenopyrite (FeAsS) in stirred tanks produces solutions containing highly toxic arsenic concentrations. In this study, ferrous iron and inorganic sulfur-oxidizing Acidithiobacillus strain IBUN Ppt12 most similar to Acidithiobacillus ferrianus and inorganic sulfur compound oxidizing Acidithiobacillus sp. IBUNS3 were grown in co-culture during biooxidation of refractory FeAsS. Total RNA was extracted and sequenced from the planktonic cells to reveal genes with different transcript counts involved in the response to FeAsS containing medium. The co-culture's response to arsenic release during biooxidation included the ars operon genes that were independently regulated according to the arsenopyrite concentration. Additionally, increased mRNA transcript counts were identified for transmembrane ion transport proteins, stress response mechanisms, accumulation of inorganic polyphosphates, urea catabolic processes, and tryptophan biosynthesis. Acidithiobacillus spp. RNA transcripts also included those encoding the Rus and PetI proteins involved in ferrous iron oxidation and gene clusters annotated as encoding inorganic sulfur compound metabolism enzymes. Finally, mRNA counts of genes related to DNA methylation, management of oxidative stress, chemotaxis, and motility during biooxidation were decreased compared to cells growing without mineral. The results provide insights into the adaptation of Acidithiobacillus spp. to growth during biooxidation of arsenic-bearing sulfides.

Bioprospecting of Novel Extremozymes From Prokaryotes-The Advent of Culture-Independent Methods

Extremophiles are remarkable organisms that thrive in the harshest environments on Earth, such as hydrothermal vents, hypersaline lakes and pools, alkaline soda lakes, deserts, cold oceans, and volcanic areas. These organisms have developed several strategies to overcome environmental stress and nutrient limitations. Thus, they are among the best model organisms to study adaptive mechanisms that lead to stress tolerance. Genetic and structural information derived from extremophiles and extremozymes can be used for bioengineering other nontolerant enzymes. Furthermore, extremophiles can be a valuable resource for novel biotechnological and biomedical products due to their biosynthetic properties. However, understanding life under extreme conditions is challenging due to the difficulties of in vitro cultivation and observation since > 99% of organisms cannot be cultivated. Consequently, only a minor percentage of the potential extremophiles on Earth have been discovered and characterized. Herein, we present a review of culture-independent methods, sequence-based metagenomics (SBM), and single amplified genomes (SAGs) for studying enzymes from extremophiles, with a focus on prokaryotic (archaea and bacteria) microorganisms. Additionally, we provide a comprehensive list of extremozymes discovered via metagenomics and SAGs.

Recovering iron and sulfate in the form of mineral from acid mine drainage by a bacteria-driven cyclic biomineralization system

Acid mine drainage (AMD) is recognized as a challenge encountered by mining industries globally. Cyclic mineralization method, namely Fe²⁺ oxidation/mineralization-residual Fe³⁺ reduction-resultant Fe²⁺ oxidation/mineralization, could precipitate Fe and SO₄^2- present in AMD into iron hydroxysulfate minerals and greatly improve the efficiency of subsequent lime neutralization, but the current Fe⁰-mediated reduction approach increased the mineralization cycles. This study constructed a bacteria-driven biomineralization system based on the reactions of Acidithiobacillus ferrooxidans-mediated Fe²⁺ oxidation and Acidiphilium multivorum-controlled Fe³⁺ reduction, and utilized water-dropping aeration and biofilm technology to satisfy the requirement of practical application. The resultant biofilms showed stable activity for Fe conversion: the efficiency of Fe²⁺-oxidation, Fe-precipitation, and Fe³⁺-reduction maintained at 98%, 32%, and 87%, respectively. Dissolved oxygen for Fe-oxidizing bacteria growth was continuously replenished by water-dropping aeration (4.2-7.2 mg/L), and the added organic carbon was mainly metabolized by Fe-reducing bacteria. About 89% Fe and 60% SO₄^2- were precipitated into jarosite mineral after five biomineralization cycles. Fe was removed via forming secondary mineral precipitates, while SO₄^2- was coprecipitated into mineral within the initial three biomineralization cycles, and then mainly precipitated with Ca²⁺ afterwards. Fe concentration in AMD was proven to directly correlate with subsequent lime neutralization efficiency. Biomineralization for five cycles drastically reduced the amount of required lime and neutralized sludge by 75% and 77%, respectively. The results in this study provided theoretical guidance for practical AMD treatment based on biomineralization technology.

AciDB 1.0: a database of acidophilic organisms, their genomic information and associated metadata

MOTIVATION

There are about 600 available genome sequences of acidophilic organisms (grow at a pH < 5) from the three domains of the Tree of Life. Information about acidophiles is scattered over many heterogeneous sites making it extraordinarily difficult to link physiological traits with genomic data. We were motivated to generate a curated, searchable database to address this problem.

RESULTS

AciDB 1.0 is a curated database of sequenced acidophiles that enables researchers to execute complex queries linking genomic features to growth data, environmental descriptions and taxonomic information.

AVAILABILITY AND IMPLEMENTATION

AciDB 1.0 is freely available online at: http://AciDB.cl. The source code is released under an MIT license at: https://gitlab.com/Hawkline451/acidb/.

Unlocking Survival Mechanisms for Metal and Oxidative Stress in the Extremely Acidophilic, Halotolerant Acidihalobacter Genus

Microorganisms used for the biohydrometallurgical extraction of metals from minerals must be able to survive high levels of metal and oxidative stress found in bioleaching environments. The Acidihalobacter genus consists of four species of halotolerant, iron–sulfur-oxidizing acidophiles that are unique in their ability to tolerate chloride and acid stress while simultaneously bioleaching minerals. This paper uses bioinformatic tools to predict the genes and mechanisms used by Acidihalobacter members in their defense against a wide range of metals and oxidative stress. Analysis revealed the presence of multiple conserved mechanisms of metal tolerance. Ac. yilgarnensis F5^T, the only member of this genus that oxidizes the mineral chalcopyrite, contained a 39.9 Kb gene cluster consisting of 40 genes encoding mobile elements and an array of proteins with direct functions in copper resistance. The analysis also revealed multiple strategies that the Acidihalobacter members can use to tolerate high levels of oxidative stress. Three of the Acidihalobacter genomes were found to contain genes encoding catalases, which are not common to acidophilic microorganisms. Of particular interest was a rubrerythrin genomic cluster containing genes that have a polyphyletic origin of stress-related functions.

Sulfur Reduction at Hyperthermoacidophilic Conditions with Mesophilic Anaerobic Sludge as the Inoculum

Sulfur reduction at hyperthermoacidophilic conditions represents a promising opportunity for metal sulfide precipitation from hot acidic metallurgical streams, avoiding costly cooling down. The suitability of mesophilic anaerobic sludges as the inoculum for sulfur-reducing bioreactors operated at high temperature and low pH was explored. We examined sludges from full-scale anaerobic reactors for sulfur-reducing activity at pH 2.0-3.5 and 70 or 80 °C, with H₂ as an electron donor. At pH 3.5 in batch experiments, sulfidogenesis started within 4 days, reaching up to 100-200 mg·L^-1 of dissolved sulfide produced after 19-24 days, depending on the origin of the sludge. Sulfidogenesis resumed after removing H₂S by flushing with nitrogen gas, indicating that sulfide was limiting the conversion. The best performing sludge was used to inoculate a 4 L gas-lift reactor fed with H₂ as the electron donor, CO₂ as the carbon source, and elemental sulfur as the electron acceptor. The reactor was operated in semibatch mode at a pH 3.5 and 80 °C, and stable sulfide production rates of 60-80 mg·L^-1·d^-1 were achieved for a period of 24 days, without formation of methane or acetate. Our results reveal the potential of mesophilic anaerobic sludges as seed material for sulfur-reducing bioprocesses operated at hyperthermoacidophilic conditions. The process needs further optimization of the volumetric sulfide production rate to gain relevance for practice.

Extremophilic Microorganisms for the Treatment of Toxic Pollutants in the Environment

As concerns about the substantial effect of various hazardous toxic pollutants on the environment and public health are increasing, the development of effective and sustainable treatment methods is urgently needed. In particular, the remediation of toxic components such as radioactive waste, toxic heavy metals, and other harmful substances under extreme conditions is quite difficult due to their restricted accessibility. Thus, novel treatment methods for the removal of toxic pollutants using extremophilic microorganisms that can thrive under extreme conditions have been investigated during the past several decades. In this review, recent trends in bioremediation using extremophilic microorganisms and related approaches to develop them are reviewed, with relevant examples and perspectives.

A tunable L-arabinose-inducible expression plasmid for the acetic acid bacterium Gluconobacter oxydans

Abstract

The acetic acid bacterium (AAB) Gluconobacter oxydans incompletely oxidizes a wide variety of carbohydrates and is therefore used industrially for oxidative biotransformations. For G. oxydans, no system was available that allows regulatable plasmid-based expression. We found that the l-arabinose-inducible P_BAD promoter and the transcriptional regulator AraC from Escherichia coli MC4100 performed very well in G. oxydans. The respective pBBR1-based plasmids showed very low basal expression of the reporters β-glucuronidase and mNeonGreen, up to 480-fold induction with 1% l-arabinose, and tunability from 0.1 to 1% l-arabinose. In G. oxydans 621H, l-arabinose was oxidized by the membrane-bound glucose dehydrogenase, which is absent in the multi-deletion strain BP.6. Nevertheless, AraC-P_BAD performed similar in both strains in the exponential phase, indicating that a gene knockout is not required for application of AraC-P_BAD in wild-type G. oxydans strains. However, the oxidation product arabinonic acid strongly contributed to the acidification of the growth medium in 621H cultures during the stationary phase, which resulted in drastically decreased reporter activities in 621H (pH 3.3) but not in BP.6 cultures (pH 4.4). These activities could be strongly increased quickly solely by incubating stationary cells in d-mannitol-free medium adjusted to pH 6, indicating that the reporters were hardly degraded yet rather became inactive. In a pH-controlled bioreactor, these reporter activities remained high in the stationary phase (pH 6). Finally, we created a multiple cloning vector with araC-P_BAD based on pBBR1MCS-5. Together, we demonstrated superior functionality and good tunability of an AraC-P_BAD system in G. oxydans that could possibly also be used in other AAB.

Key points

• We found the AraC-P_BADsystem from E. coli MC4100 was well tunable in G. oxydans.

•  In the absence of AraC orl-arabinose, expression from P_BADwas extremely low.

• This araC-P_BADsystem could also be fully functional in other acetic acid bacteria.

Electronic supplementary material

The online version of this article (10.1007/s00253-020-10905-4) contains supplementary material, which is available to authorized users.

Understanding How Microorganisms Respond to Acid pH Is Central to Their Control and Successful Exploitation

Microbes from the three domains of life, Bacteria, Archaea, and Eukarya, share the need to sense and respond to changes in the external and internal concentrations of protons. When the proton concentration is high, acidic conditions prevail and cells must respond appropriately to ensure that macromolecules and metabolic processes are sufficiently protected to sustain life. While, we have learned much in recent decades about the mechanisms that microbes use to cope with acid, including the unique challenges presented by organic acids, there is still much to be gained from developing a deeper understanding of the effects and responses to acid in microbes. In this perspective article, we survey the key molecular mechanisms known to be important for microbial survival during acid stress and discuss how this knowledge might be relevant to microbe-based applications and processes that are consequential for humans. We discuss the research approaches that have been taken to investigate the problem and highlight promising new avenues. We discuss the influence of acid on pathogens during the course of infections and highlight the potential of using organic acids in treatments for some types of infection. We explore the influence of acid stress on photosynthetic microbes, and on biotechnological and industrial processes, including those needed to produce organic acids. We highlight the importance of understanding acid stress in controlling spoilage and pathogenic microbes in the food chain. Finally, we invite colleagues with an interest in microbial responses to low pH to participate in the EU-funded COST Action network called EuroMicropH and contribute to a comprehensive database of literature on this topic that we are making publicly available.

Role of microbial diversity for sustainable pyrite oxidation control in acid and metalliferous drainage prevention

Acid and metalliferous drainage (AMD) remains a challenging issue for the mining sector. AMD management strategies have attempted to shift from treatment of acid leachates post-generation to more sustainable at-source prevention. Here, the efficacy of microbial-geochemical at-source control approach was investigated over a period of 84 weeks. Diverse microbial communities were stimulated using organic carbon amendment in a simulated silicate-containing sulfidic mine waste rock environment. Mineral waste in the unamended leach system generated AMD quickly and throughout the study, with known lithotrophic iron- and sulfur-oxidising microbes dominating column communities. The organic-amended mineral waste column showed suppressed metal dissolution and AMD generation. Molecular DNA-based next generation sequencing confirmed a less diverse lithotrophic community in the acid-producing control, with a more diverse microbial community under organic amendment comprising organotrophic iron/sulfur-reducers, autotrophs, hydrogenotrophs and heterotrophs. Time-series multivariate statistical analyses displayed distinct ecological patterns in microbial diversity between AMD- and non-AMD-environments. Focused ion beam-TEM micrographs and elemental mapping showed that silicate-stabilised passivation layers were successfully established across pyrite surfaces in organic-amended treatments, with these layers absent in unamended controls. Organic amendment and resulting increases in microbial abundance and diversity played an important role in sustaining these passivating layers in the long-term.

Life in mine tailings: microbial population structure across the bulk soil, rhizosphere, and roots of boreal species colonizing mine tailings in northwestern Québec

Purpose

Mining activities have negative effects on soil characteristics and can result in low pH, high heavy metal content, and limited levels of essential nutrients. A tailings storage area located in northwestern Québec showed natural colonization by plants from the adjacent natural environment. The objective of the study was to determine the main edaphic parameters that structured microbial populations associated with the indigenous woody plants that had naturally colonized the site.

Methods

Microbial populations were studied in the bulk soil, the rhizosphere, and inside plant roots using Illumina sequencing, ordination analysis (i.e., redundancy analysis (RDA) and principal coordinates analysis (PCoA)), ternary plotting, and statistical analysis (MANOVA).

Results

The main variables that drove the microbial community patterns were plant species and the tailings pH. Indeed, the main bacterial classes were Gammaproteobacteria and Deltaproteobacteria in both the rhizosphere and root endosphere. Analysis revealed that some dominant operational taxonomic units (e.g., Pseudomonas sp., Acinetobacter sp., and Delftia sp.) were present in increased proportions in roots for each plant species under study. This study also revealed that many of the most abundant fungal genera (e.g., Claussenomyces, Eupenicillium, and Trichoderma) were more abundant in the rhizosphere than in the root endosphere.

Conclusions

This comprehensive study of the microbial community dynamics in the bulk soil, rhizosphere, and root endosphere of boreal trees and shrubs could be beneficial in facilitating the rehabilitation of disturbed ecosystems.

The Cell Membrane of Sulfolobus spp.-Homeoviscous Adaption and Biotechnological Applications

The microbial cell membrane is affected by physicochemical parameters, such as temperature and pH, but also by the specific growth rate of the host organism. Homeoviscous adaption describes the process of maintaining membrane fluidity and permeability throughout these environmental changes. Archaea, and thereby, Sulfolobus spp. exhibit a unique lipid composition of ether lipids, which are altered in regard to the ratio of diether to tetraether lipids, number of cyclopentane rings and type of head groups, as a coping mechanism against environmental changes. The main biotechnological application of the membrane lipids of Sulfolobus spp. are so called archaeosomes. Archaeosomes are liposomes which are fully or partly generated from archaeal lipids and harbor the potential to be used as drug delivery systems for vaccines, proteins, peptides and nucleic acids. This review summarizes the influence of environmental parameters on the cell membrane of Sulfolobus spp. and the biotechnological applications of their membrane lipids.

Ferric Uptake Regulator Provides a New Strategy for Acidophile Adaptation to Acidic Ecosystems

Acidophiles play a dominant role in driving elemental cycling in natural acid mine drainage (AMD) habitats and exhibit important application value in bioleaching and bioremediation. Acidity is an inevitable environmental stress and a key factor that affects the survival of acidophiles in their acidified natural habitats; however, the regulatory strategies applied by acidophilic bacteria to withstand low pH are unclear. We identified the significance of the ferric uptake regulator (Fur) in acidophiles adapting to acidic environments and discovered that Fur is ubiquitous as well as highly conserved in acidophilic bacteria. Mutagenesis of the fur gene of Acidithiobacillus caldus, a prototypical acidophilic sulfur-oxidizing bacterium found in AMD, revealed that Fur is required for the acid resistance of this acidophilic bacterium. Phenotypic characterization, transcriptome sequencing (RNA-seq), mutagenesis, and biochemical assays indicated that the Acidithiobacillus caldus ferric uptake regulator (AcFur) is involved in extreme acid resistance by regulating the expression of several key genes of certain cellular activities, such as iron transport, biofilm formation, sulfur metabolism, chemotaxis, and flagellar biosynthesis. Finally, a Fur-dependent acid resistance regulatory strategy in A. caldus was proposed to illustrate the ecological behavior of acidophilic bacteria under low pH. This study provides new insights into the adaptation strategies of acidophiles to AMD ecosystems and will promote the design and development of engineered biological systems for the environmental adaptation of acidophiles.IMPORTANCE This study advances our understanding of the acid tolerance mechanism of A. caldus, identifies the key fur gene responsible for acid resistance, and elucidates the correlation between fur and acid resistance, thus contributing to an understanding of the ecological behavior of acidophilic bacteria. These findings provide new insights into the acid resistance process in Acidithiobacillus species, thereby promoting the study of the environmental adaptation of acidophilic bacteria and the design of engineered biological systems.

Evolution of Predicted Acid Resistance Mechanisms in the Extremely Acidophilic Leptospirillum Genus

Organisms that thrive in extremely acidic environments (≤pH 3.5) are of widespread importance in industrial applications, environmental issues, and evolutionary studies. Leptospirillum spp. constitute the only extremely acidophilic microbes in the phylogenetically deep-rooted bacterial phylum Nitrospirae. Leptospirilli are Gram-negative, obligatory chemolithoautotrophic, aerobic, ferrous iron oxidizers. This paper predicts genes that Leptospirilli use to survive at low pH and infers their evolutionary trajectory. Phylogenetic and other bioinformatic approaches suggest that these genes can be classified into (i) "first line of defense", involved in the prevention of the entry of protons into the cell, and (ii) neutralization or expulsion of protons that enter the cell. The first line of defense includes potassium transporters, predicted to form an inside positive membrane potential, spermidines, hopanoids, and Slps (starvation-inducible outer membrane proteins). The "second line of defense" includes proton pumps and enzymes that consume protons. Maximum parsimony, clustering methods, and gene alignments are used to infer the evolutionary trajectory that potentially enabled the ancestral Leptospirillum to transition from a postulated circum-neutral pH environment to an extremely acidic one. The hypothesized trajectory includes gene gains/loss events driven extensively by horizontal gene transfer, gene duplications, gene mutations, and genomic rearrangements.

Computational Analysis of the Primary and Secondary Structure of Amidases in Relation to their pH Adaptation

Background:

Amidases are ubiquitous enzymes and biological functions of these enzymes vary widely. They are considered to be synergistically involved in the synthesis of a wide variety of carboxylic acids, hydroxamic acids and hydrazides, which find applications in commodity chemicals synthesis, pharmaceuticals agrochemicals and wastewater treatments.

Methods:

They hydrolyse a wide variety of amides (short-chain aliphatic amides, mid-chain amides, arylamides, α-aminoamides and α-hydroxyamides) and can be grouped on the basis of their catalytic site and preferred substrate. Despite their economic importance, we lack knowledge as to how these amidases withstand elevated pH and temperature whereas others cannot.

Results:

The present study focuses on the statistical comparison between the acid-tolerant, alkali tolerant and neutrophilic organisms. In silico analysis of amidases of acid-tolerant, alkali tolerant and neutrophilic organisms revealed some striking trends as to how amino acid composition varies significantly. Statistical analysis of primary and secondary structure revealed amino acid trends in amidases of these three groups of bacteria. The abundance of isoleucine (Ile, I) in acid-tolerant and leucine (Leu, L) in alkali tolerant showed the aliphatic amino acid dominance in extreme conditions of pH in acidtolerant and alkali tolerant amidases.

Conclusion:

The present investigation insights physiochemical properties and dominance of some crucial amino acid residues in the primary and secondary structure of some amidases from acid-tolerant, alkali tolerant and neutrophilic microorganisms.

Remarks on Computational Method for Identifying Acid and Alkaline Enzymes

The catalytic efficiency of the enzyme is thousands of times higher than that of ordinary catalysts. Thus, they are widely used in industrial and medical fields. However, enzymes with protein structure can be destroyed and inactivated in high temperature, over acid or over alkali environment. It is well known that most of enzymes work well in an environment with pH of 6-8, while some special enzymes remain active only in an alkaline environment with pH > 8 or an acidic environment with pH < 6. Therefore, the identification of acidic and alkaline enzymes has become a key task for industrial production. Because of the wide varieties of enzymes, it is hard work to determine the acidity and alkalinity of the enzyme by experimental methods, and even this task cannot be achieved. Converting protein sequences into digital features and building computational models can efficiently and accurately identify the acidity and alkalinity of enzymes. This review summarized the progress of the digital features to express proteins and computational methods to identify acidic and alkaline enzymes. We hope that this paper will provide more convenience, ideas, and guides for computationally classifying acid and alkaline enzymes.

Microbial response to acid stress: mechanisms and applications

Microorganisms encounter acid stress during multiple bioprocesses. Microbial species have therefore developed a variety of resistance mechanisms. The damage caused by acidic environments is mitigated through the maintenance of pH homeostasis, cell membrane integrity and fluidity, metabolic regulation, and macromolecule repair. The acid tolerance mechanisms can be used to protect probiotics against gastric acids during the process of food intake, and can enhance the biosynthesis of organic acids. The combination of systems and synthetic biology technologies offers new and wide prospects for the industrial applications of microbial acid tolerance mechanisms. In this review, we summarize acid stress response mechanisms of microbial cells, illustrate the application of microbial acid tolerance in industry, and prospect the introduction of systems and synthetic biology to further explore the acid tolerance mechanisms and construct a microbial cell factory for valuable chemicals.