Microbial iron management mechanisms in extremely acidic environments: comparative genomics evidence for diversity and versatility

Genome features of a novel hydrocarbonoclastic Chryseobacterium oranimense strain and its comparison to bacterial oil-degraders and to other C. oranimense strains

For the first time, we report the whole genome sequence of a hydrocarbonoclastic Chryseobacterium oranimense strain isolated from Trinidad and Tobago (COTT) and its genes involved in the biotransformation of hydrocarbons and xenobiotics through functional annotation. The assembly consisted of 11 contigs with 2,794 predicted protein-coding genes which included a diverse group of gene families involved in aliphatic and polycyclic hydrocarbon degradation. Comparative genomic analyses with 18 crude-oil degrading bacteria in addition to two C. oranimense strains not associated with oil were carried out. The data revealed important differences in terms of annotated genes involved in the hydrocarbon degradation process that may explain the molecular mechanisms of hydrocarbon and xenobiotic biotransformation. Notably, many gene families were expanded to explain COTT's competitive ability to manage habitat-specific stressors. Gene-based evidence of the metabolic potential of COTT supports the application of indigenous microbes for the remediation of polluted terrestrial environments and provides a genomic resource for improving our understanding of how to optimize these characteristics for more effective bioremediation.

Fertilization of Microbial Composts: A Technology for Improving Stress Resilience in Plants

Microbial compost plays a crucial role in improving soil health, soil fertility, and plant biomass. These biofertilizers, based on microorganisms, offer numerous benefits such as enhanced nutrient acquisition (N, P, and K), production of hydrogen cyanide (HCN), and control of pathogens through induced systematic resistance. Additionally, they promote the production of phytohormones, siderophore, vitamins, protective enzymes, and antibiotics, further contributing to soil sustainability and optimal agricultural productivity. The escalating generation of organic waste from farm operations poses significant threats to the environment and soil fertility. Simultaneously, the excessive utilization of chemical fertilizers to achieve high crop yields results in detrimental impacts on soil structure and fertility. To address these challenges, a sustainable agriculture system that ensures enhanced soil fertility and minimal ecological impact is imperative. Microbial composts, developed by incorporating characterized plant-growth-promoting bacteria or fungal strains into compost derived from agricultural waste, offer a promising solution. These biofertilizers, with selected microbial strains capable of thriving in compost, offer an eco-friendly, cost-effective, and sustainable alternative for agricultural practices. In this review article, we explore the potential of microbial composts as a viable strategy for improving plant growth and environmental safety. By harnessing the benefits of microorganisms in compost, we can pave the way for sustainable agriculture and foster a healthier relationship between soil, plants, and the environment.

Genomic insight into iron acquisition by sulfate-reducing bacteria in microaerophilic environments

Historically, sulfate-reducing bacteria (SRB) have been considered to be strict anaerobes, but reports in the past couple of decades indicate that SRB tolerate exposure to O2 and can even grow in aerophilic environments. With the transition from anaerobic to microaerophilic conditions, the uptake of Fe(III) from the environment by SRB would become important. In evaluating the metabolic capability for the uptake of iron, the genomes of 26 SRB, representing eight families, were examined. All SRB reviewed carry genes (feoA and feoB) for the ferrous uptake system to transport Fe(II) across the plasma membrane into the cytoplasm. In addition, all of the SRB genomes examined have putative genes for a canonical ABC transporter that may transport ferric siderophore or ferric chelated species from the environment. Gram-negative SRB have additional machinery to import ferric siderophores and ferric chelated species since they have the TonB system that can work alongside any of the outer membrane porins annotated in the genome. Included in this review is the discussion that SRB may use the putative siderophore uptake system to import metals other than iron.

Insights into Systems for Iron-Sulfur Cluster Biosynthesis in Acidophilic Microorganisms

Fe-S clusters are versatile and essential cofactors that participate in multiple and fundamental biological processes. In Escherichia coli, the biogenesis of these cofactors requires either the housekeeping Isc pathway, or the stress-induced Suf pathway which plays a general role under conditions of oxidative stress or iron limitation. In the present work, the Fe-S cluster assembly Isc and Suf systems of acidophilic Bacteria and Archaea, which thrive in highly oxidative environments, were studied. This analysis revealed that acidophilic microorganisms have a complete set of genes encoding for a single system (either Suf or Isc). In acidophilic Proteobacteria and Nitrospirae, a complete set of isc genes (iscRSUAX-hscBA-fdx), but not genes coding for the Suf system, was detected. The activity of the Isc system was studied in Leptospirillum sp. CF-1 (Nitrospirae). RT-PCR experiments showed that eight candidate genes were co-transcribed and conform the isc operon in this strain. Additionally, RT-qPCR assays showed that the expression of the iscS gene was significantly up-regulated in cells exposed to oxidative stress imposed by 260 mM Fe₂(SO₄)₃ for 1 h or iron starvation for 3 h. The activity of cysteine desulfurase (IscS) in CF-1 cell extracts was also up-regulated under such conditions. Thus, the Isc system from Leptospirillum sp. CF-1 seems to play an active role in stressful environments. These results contribute to a better understanding of the distribution and role of Fe-S cluster protein biogenesis systems in organisms that thrive in extreme environmental conditions.

Integrative Genomics Sheds Light on Evolutionary Forces Shaping the Acidithiobacillia Class Acidophilic Lifestyle

Extreme acidophiles thrive in environments rich in protons (pH values <3) and often high levels of dissolved heavy metals. They are distributed across the three domains of the Tree of Life including members of the Proteobacteria. The Acidithiobacillia class is formed by the neutrophilic genus Thermithiobacillus along with the extremely acidophilic genera Fervidacidithiobacillus, Igneacidithiobacillus, Ambacidithiobacillus, and Acidithiobacillus. Phylogenomic reconstruction revealed a division in the Acidithiobacillia class correlating with the different pH optima that suggested that the acidophilic genera evolved from an ancestral neutrophile within the Acidithiobacillia. Genes and mechanisms denominated as "first line of defense" were key to explaining the Acidithiobacillia acidophilic lifestyle including preventing proton influx that allows the cell to maintain a near-neutral cytoplasmic pH and differ from the neutrophilic Acidithiobacillia ancestors that lacked these systems. Additional differences between the neutrophilic and acidophilic Acidithiobacillia included the higher number of gene copies in the acidophilic genera coding for "second line of defense" systems that neutralize and/or expel protons from cell. Gain of genes such as hopanoid biosynthesis involved in membrane stabilization at low pH and the functional redundancy for generating an internal positive membrane potential revealed the transition from neutrophilic properties to a new acidophilic lifestyle by shaping the Acidithiobacillaceae genomic structure. The presence of a pool of accessory genes with functional redundancy provides the opportunity to "hedge bet" in rapidly changing acidic environments. Although a core of mechanisms for acid resistance was inherited vertically from an inferred neutrophilic ancestor, the majority of mechanisms, especially those potentially involved in resistance to extremely low pH, were obtained from other extreme acidophiles by horizontal gene transfer (HGT) events.

Key Factors Governing Microbial Community in Extremely Acidic Mine Drainage (pH <3)

The microbial community of acid mine drainage (AMD) fascinates researchers by their adaption and roles in shaping the environment. Molecular surveys have recently helped to enhance the understanding of the distribution, adaption strategy, and ecological function of microbial communities in extreme AMD environments. However, the interactions between the environment and microbial community of extremely acidic AMD (pH <3) from different mining areas kept unanswered questions. Here, we measured physicochemical parameters and profiled the microbial community of AMD collected from four mining areas with different mineral types to provide a better understanding of biogeochemical processes within the extremely acidic water environment. The prominent physicochemical differences across the four mining areas were in SO₄²⁻, metal ions, and temperature, and distinct microbial diversity and community assemblages were also discovered in these areas. Mg²⁺ and SO₄²⁻ were the predominant factors determining the microbial structure and prevalence of dominant taxa in AMD. Leptospirillum, Ferroplasma, and Acidithiobacillus were abundant but showed different occurrence patterns in AMD from different mining areas. More diverse communities and functional redundancy were identified in AMD of polymetallic mining areas compared with AMD of copper mining areas. Functional prediction revealed iron, sulfur, nitrogen, and carbon metabolisms driven by microorganisms were significantly correlated with Mg²⁺ and SO₄²⁻, Ca²⁺, temperature, and Fe²⁺, which distinguish microbial communities of copper mine AMD from that of polymetallic mine AMD. In summary, microbial diversity, composition, and metabolic potential were mainly shaped by Mg²⁺ and SO₄²⁻ concentrations of AMD, suggesting that the substrate concentrations may contribute to the distinct microbiological profiles of AMD from different mining areas. These findings highlight the microbial community structure in extremely acidic AMD forming by types of minerals and the interactions of physicochemical parameters and microbiology, providing more clues of the microbial ecological function and adaptation mechanisms in the extremely acidic environment.

Genome-Based Characterization of Plant-Associated Rhodococcus qingshengii RL1 Reveals Stress Tolerance and Plant-Microbe Interaction Traits

Stress tolerant, plant-associated bacteria can play an important role in maintaining a functional plant microbiome and protecting plants against various (a)biotic stresses. Members of the stress tolerant genus Rhodococcus are frequently found in the plant microbiome. Rhodococcus qingshengii RL1 was isolated from Eruca sativa and the complete genome was sequenced, annotated and analyzed using different bioinformatic tools. A special focus was laid on functional analyses of stress tolerance and interactions with plants. The genome annotation of RL1 indicated that it contains a repertoire of genes which could enable it to survive under different abiotic stress conditions for e.g., elevated mercury concentrations, to interact with plants via root colonization, to produce phytohormones and siderophores, to fix nitrogen and to interact with bacterial signaling via a LuxR-solo and quorum quenching. Based on the identified genes, functional analyses were performed in vitro with RL1 under different growth conditions. The R. qingshengii type strain djl6 and a closely related Rhodococcus erythropolis BG43 were included in the experiments to find common and distinct traits between the strains. Genome based phylogenetic analysis of 15 available and complete R. erythropolis and R. qingshengii genome sequences revealed a separation of the R. erythropolis clade in two subgroups. First one harbors only R. erythropolis strains including the R. erythropolis type strain. The second group consisted of the R. qingshengii type strain and a mix of R. qingshengii and R. erythropolis strains indicating that some strains of the second group should be considered for taxonomic re-assignment. However, BG43 was clearly identified as R. erythropolis and RL1 clearly as R. qingshengii and the strains had most tested traits in common, indicating a close functional overlap of traits between the two species.

Functional redundancy imparts process stability to acidic Fe(II)-oxidizing microbial reactors

In previous work, lab-scale reactors designed to study microbial Fe(II) oxidation rates at low pH were found to have stable rates under a wide range of pH and Fe(II) concentrations. Since the stirred reactor environment eliminates many of the temporal and spatial variations that promote high diversity among microbial populations in nature, we were surprised that the reactors supported multiple taxa presumed to be autotrophic Fe(II) oxidizers based on their phylogeny. Metagenomic analyses of the reactor communities revealed differences in the metabolic potential of these taxa with respect to Fe(II) oxidation and carbon fixation pathways, acquisition of potentially growth-limiting substrates and the ability to form biofilms. Our findings support the hypothesis that the long-term co-existence of multiple autotrophic Fe(II)-oxidizing populations in the reactors are due to distinct metabolic potential that supports differential growth in response to limiting resources such as nitrogen, phosphorus and oxygen. Our data also highlight the role of biofilms in creating spatially distinct geochemical niches that enable the co-existence of multiple taxa that occupy the same apparent metabolic niche when the system is viewed in bulk. The distribution of key metabolic functions across different co-existing taxa supported functional redundancy and imparted process stability to these reactors.

Application of Firefly Luciferase (Luc) as a Reporter Gene for the Chemoautotrophic and Acidophilic Acidithiobacillus spp

Acidithiobacillus spp. are the most active bacteria in bioleaching and bioremediation, because of their remarkable extreme environmental adaptabilities and unique metabolic characteristics. The researches on regulatory mechanisms of energy metabolism and stress resistance are critical for the understanding and application of Acidithiobacillus spp. However, the lack of an ideal reporter gene has become an obstacle for studying genes expression and regulatory mechanism in these chemoautotrophic bacteria. In this study, we reported the firefly luciferase as a reporter gene for Acidithiobacillus caldus (A. caldus) and created a firefly luciferase (Luc) reporter system. The Luc system was applied for the quantitative analysis of the transcription strength of the promoters of tetH gene and the feoA gene in A. caldus. Moreover, the regulating effect of ferric uptake regulator (Fur) on the feoP gene in A. caldus was determined using the Luc system. The Luc reporter system is not only used in the study of regulatory mechanism of A. caldus, but also applied in the researches of other Acidithiobacillus species. Therefore, this study provides a new useful tool for the studies on the molecular biological mechanism and synthetic biological modification of these chemoautotrophic bacteria, which would promote the industrial application of Acidithiobacillus spp.

Metagenomic and Metatranscriptomic Study of Microbial Metal Resistance in an Acidic Pit Lake

Cueva de la Mora (CM) is an acidic, meromictic pit lake in the Iberian Pyrite Belt characterized by extremely high metal(loid) concentrations and strong gradients in oxygen, metal, and nutrient concentrations. We hypothesized that geochemical variations with depth would result in differences in community composition and in metal resistance strategies among active microbial populations. We also hypothesized that metal resistance gene (MRG) expression would correlate with toxicity levels for dissolved metal species in the lake. Water samples were collected in the upper oxic layer, chemocline, and deep anoxic layer of the lake for shotgun metagenomic and metatranscriptomic sequencing. Metagenomic analyses revealed dramatic differences in the composition of the microbial communities with depth, consistent with changing geochemistry. Based on relative abundance of taxa identified in each metagenome, Eukaryotes (predominantly Coccomyxa) dominated the upper layer, while Archaea (predominantly Thermoplasmatales) dominated the deep layer, and a combination of Bacteria and Eukaryotes were abundant at the chemocline. We compared metal resistance across communities using a curated list of protein-coding MRGs with KEGG Orthology identifiers (KOs) and found that there were broad differences in the metal resistance strategies (e.g., intracellular metal accumulation) expressed by Eukaryotes, Bacteria, and Archaea. Although normalized abundances of MRG and MRG expression were generally higher in the deep layer, expression of metal-specific genes was not strongly related to variations in specific metal concentrations, especially for Cu and As. We also compared MRG potential and expression in metagenome assembled genomes (MAGs) from the deep layer, where metal concentrations are highest. Consistent with previous work showing differences in metal resistance mechanisms even at the strain level, MRG expression patterns varied strongly among MAG populations from the same depth. Some MAG populations expressed very few MRG known to date, suggesting that novel metal resistance strategies remain to be discovered in uncultivated acidophiles.

Insights into ecological role of a new deltaproteobacterial order Candidatus Acidulodesulfobacterales by metagenomics and metatranscriptomics

Several abundant but yet uncultivated bacterial groups exist in extreme iron- and sulfur-rich environments, and the physiology, biodiversity, and ecological roles of these bacteria remain a mystery. Here we retrieved four metagenome-assembled genomes (MAGs) from an artificial acid mine drainage (AMD) system, and propose they belong to a new deltaproteobacterial order, Candidatus Acidulodesulfobacterales. The distribution pattern of Ca. Acidulodesulfobacterales in AMDs across Southeast China correlated strongly with ferrous iron. Reconstructed metabolic pathways and gene expression profiles showed that they were likely facultatively anaerobic autotrophs capable of nitrogen fixation. In addition to dissimilatory sulfate reduction, encoded by dsrAB, dsrD, dsrL, and dsrEFH genes, these microorganisms might also oxidize sulfide, depending on oxygen concentration and/or oxidation reduction potential. Several genes with homology to those involved in iron metabolism were also identified, suggesting their potential role in iron cycling. In addition, the expression of abundant resistance genes revealed the mechanisms of adaptation and response to the extreme environmental stresses endured by these organisms in the AMD environment. These findings shed light on the distribution, diversity, and potential ecological role of the new order Ca. Acidulodesulfobacterales in nature.

Identification and Unusual Properties of the Master Regulator FNR in the Extreme Acidophile Acidithiobacillus ferrooxidans

The ability to conserve energy in the presence or absence of oxygen provides a metabolic versatility that confers an advantage in natural ecosystems. The switch between alternative electron transport systems is controlled by the fumarate nitrate reduction transcription factor (FNR) that senses oxygen via an oxygen-sensitive [4Fe-4S]2+ iron-sulfur cluster. Under O2 limiting conditions, FNR plays a key role in allowing bacteria to transition from aerobic to anaerobic lifestyles. This is thought to occur via transcriptional activation of genes involved in anaerobic respiratory pathways and by repression of genes involved in aerobic energy production. The Proteobacterium Acidithiobacillus ferrooxidans is a model species for extremely acidophilic microorganisms that are capable of aerobic and anaerobic growth on elemental sulfur coupled to oxygen and ferric iron reduction, respectively. In this study, an FNR-like protein (FNRAF) was discovered in At. ferrooxidans that exhibits a primary amino acid sequence and major motifs and domains characteristic of the FNR family of proteins, including an effector binding domain with at least three of the four cysteines known to coordinate an [4Fe-4S]2+ center, a dimerization domain, and a DNA binding domain. Western blotting with antibodies against Escherichia coli FNR (FNREC) recognized FNRAF. FNRAF was able to drive expression from the FNR-responsive E. coli promoter PnarG, suggesting that it is functionally active as an FNR-like protein. Upon air exposure, FNRAF demonstrated an unusual lack of sensitivity to oxygen compared to the archetypal FNREC. Comparison of the primary amino acid sequence of FNRAF with that of other natural and mutated FNRs, including FNREC, coupled with an analysis of the predicted tertiary structure of FNRAF using the crystal structure of the related FNR from Aliivibrio fisheri as a template revealed a number of amino acid changes that could potentially stabilize FNRAF in the presence of oxygen. These include a truncated N terminus and amino acid changes both around the putative Fe-S cluster coordinating cysteines and also in the dimer interface. Increased O2 stability could allow At. ferrooxidans to survive in environments with fluctuating O2 concentrations, providing an evolutionary advantage in natural, and engineered environments where oxygen gradients shape the bacterial community.

Assessment of Bioleaching Microbial Community Structure and Function Based on Next-Generation Sequencing Technologies

It is widely known that bioleaching microorganisms have to cope with the complex extreme environment in which microbial ecology relating to community structure and function varies across environmental types. However, analyses of microbial ecology of bioleaching bacteria is still a challenge. To address this challenge, numerous technologies have been developed. In recent years, high-throughput sequencing technologies enabling comprehensive sequencing analysis of cellular RNA and DNA within the reach of most laboratories have been added to the toolbox of microbial ecology. The next-generation sequencing technology allowing processing DNA sequences can produce available draft genomic sequences of more bioleaching bacteria, which provides the opportunity to predict models of genetic and metabolic potential of bioleaching bacteria and ultimately deepens our understanding of bioleaching microorganism. High-throughput sequencing that focuses on targeted phylogenetic marker 16S rRNA has been effectively applied to characterize the community diversity in an ore leaching environment. RNA-seq, another application of high-throughput sequencing to profile RNA, can be for both mapping and quantifying transcriptome and has demonstrated a high efficiency in quantifying the changing expression level of each transcript under different conditions. It has been demonstrated as a powerful tool for dissecting the relationship between genotype and phenotype, leading to interpreting functional elements of the genome and revealing molecular mechanisms of adaption. This review aims to describe the high-throughput sequencing approach for bioleaching environmental microorganisms, particularly focusing on its application associated with challenges.

Extremophilic Microfactories: Applications in Metal and Radionuclide Bioremediation

Metals and radionuclides (M&Rs) are a worldwide concern claiming for resilient, efficient, and sustainable clean-up measures aligned with environmental protection goals and global change constraints. The unique defense mechanisms of extremophilic bacteria and archaea have been proving usefulness towards M&Rs bioremediation. Hence, extremophiles can be viewed as microfactories capable of providing specific and controlled services (i.e., genetic/metabolic mechanisms) and/or products (e.g., biomolecules) for that purpose. However, the natural physiological plasticity of such extremophilic microfactories can be further explored to nourish different hallmarks of M&R bioremediation, which are scantly approached in the literature and were never integrated. Therefore, this review not only briefly describes major valuable extremophilic pathways for M&R bioremediation, as it highlights the advances, challenges and gaps from the interplay of ‘omics’ and biological engineering to improve extremophilic microfactories performance for M&R clean-up. Microfactories’ potentialities are also envisaged to close the M&R bioremediation processes and shift the classical idea of never ‘getting rid’ of M&Rs into making them ‘the belle of the ball’ through bio-recycling and bio-recovering techniques.

Ferrous iron efflux systems in bacteria

Bacteria require iron for growth, with only a few reported exceptions. In many environments, iron is a limiting nutrient for growth and high affinity uptake systems play a central role in iron homeostasis. However, iron can also be detrimental to cells when it is present in excess, particularly under aerobic conditions where its participation in Fenton chemistry generates highly reactive hydroxyl radicals. Recent results have revealed a critical role for iron efflux transporters in protecting bacteria from iron intoxication. Systems that efflux iron are widely distributed amongst bacteria and fall into several categories: P1B-type ATPases, cation diffusion facilitator (CDF) proteins, major facilitator superfamily (MFS) proteins, and membrane bound ferritin-like proteins. Here, we review the emerging role of iron export in both iron homeostasis and as part of the adaptive response to oxidative stress.

Extensive horizontal gene transfer in cheese-associated bacteria

Acquisition of genes through horizontal gene transfer (HGT) allows microbes to rapidly gain new capabilities and adapt to new or changing environments. Identifying widespread HGT regions within multispecies microbiomes can pinpoint the molecular mechanisms that play key roles in microbiome assembly. We sought to identify horizontally transferred genes within a model microbiome, the cheese rind. Comparing 31 newly sequenced and 134 previously sequenced bacterial isolates from cheese rinds, we identified over 200 putative horizontally transferred genomic regions containing 4733 protein coding genes. The largest of these regions are enriched for genes involved in siderophore acquisition, and are widely distributed in cheese rinds in both Europe and the US. These results suggest that HGT is prevalent in cheese rind microbiomes, and that identification of genes that are frequently transferred in a particular environment may provide insight into the selective forces shaping microbial communities.

DOI:http://dx.doi.org/10.7554/eLife.22144.001

From the depths of the ocean to the lining of the human gut, almost every environment on Earth is home to a unique community of microorganisms referred to as a microbiome. Within these communities, unrelated microorganisms can exchange genetic information through a process known as horizontal gene transfer. For example, genes linked to antibiotic resistance are often transferred between different microorganisms, which can create increasingly drug resistant microbes and has important implications for human health.

Horizontal gene transfer has been studied for almost 100 years, but examining it directly is challenging because, almost by definition, it requires studying a community of microbes rather than one microbe in isolation. As such, researchers are looking for simple models of microbial communities that can be easily manipulated in experiments.

Bonham et al. have now turned to the outer surface of cheese, also known as cheese rind, to better understand horizontal gene transfer. As a model system, the cheese rind microbiome is relatively simple to work with because cheese rind is easy to replicate in the laboratory, and the microbes growing on cheese can be grown on their own or in combinations with other microbes. By comparing the genetic material of 165 cheese-associated bacteria to one another, Bonham et al. identified over 4,000 genes that were shared between the bacteria, including several large clusters of genes that were shared by many species.

Many of the identified genes (about 23% to be precise) appear to help the microorganisms acquire nutrients that are known to be in short supply on the surface of cheese surface, including iron. Bacteria typically use specialized molecules called siderophores to scavenge for iron and uptake systems to carry the iron-bound siderophore back into the cell. Notably, only the genes associated with the uptake systems were found in some of the shared gene clusters. This finding suggests that horizontal gene transfer has allowed some microbes to “cheat” and take up iron-bound siderophores without expending energy to produce the siderophores themselves.

Using the cheese rind microbiome as a model system, it becomes possible to explore how horizontal gene transfer works in more detail than before. A better understanding of this process can then be applied to other important microbiomes, including those where genes conferring antibiotic resistance are commonly exchanged.

DOI:http://dx.doi.org/10.7554/eLife.22144.002

Multiple Osmotic Stress Responses in Acidihalobacter prosperus Result in Tolerance to Chloride Ions

Extremely acidophilic microorganisms (pH optima for growth of ≤3) are utilized for the extraction of metals from sulfide minerals in the industrial biotechnology of “biomining.” A long term goal for biomining has been development of microbial consortia able to withstand increased chloride concentrations for use in regions where freshwater is scarce. However, when challenged by elevated salt, acidophiles experience both osmotic stress and an acidification of the cytoplasm due to a collapse of the inside positive membrane potential, leading to an influx of protons. In this study, we tested the ability of the halotolerant acidophile Acidihalobacter prosperus to grow and catalyze sulfide mineral dissolution in elevated concentrations of salt and identified chloride tolerance mechanisms in Ac. prosperus as well as the chloride susceptible species, Acidithiobacillus ferrooxidans. Ac. prosperus had optimum iron oxidation at 20 g L⁻¹ NaCl while At. ferrooxidans iron oxidation was inhibited in the presence of 6 g L⁻¹ NaCl. The tolerance to chloride in Ac. prosperus was consistent with electron microscopy, determination of cell viability, and bioleaching capability. The Ac. prosperus proteomic response to elevated chloride concentrations included the production of osmotic stress regulators that potentially induced production of the compatible solute, ectoine uptake protein, and increased iron oxidation resulting in heightened electron flow to drive proton export by the F₀F₁ ATPase. In contrast, At. ferrooxidans responded to low levels of Cl⁻ with a generalized stress response, decreased iron oxidation, and an increase in central carbon metabolism. One potential adaptation to high chloride in the Ac. prosperus Rus protein involved in ferrous iron oxidation was an increase in the negativity of the surface potential of Rus Form I (and Form II) that could help explain how it can be active under elevated chloride concentrations. These data have been used to create a model of chloride tolerance in the salt tolerant and susceptible species Ac. prosperus and At. ferrooxidans, respectively.

An overview of siderophores for iron acquisition in microorganisms living in the extreme

Siderophores are iron-chelating molecules produced by microbes when intracellular iron concentrations are low. Low iron triggers a cascade of gene activation, allowing the cell to survive due to the synthesis of important proteins involved in siderophore synthesis and transport. Generally, siderophores are classified by their functional groups as catecholates, hydroxamates and hydroxycarboxylates. Although other chemical structural modifications and functional groups can be found. The functional groups participate in the iron-chelating process when the ferri-siderophore complex is formed. Classified as acidophiles, alkaliphiles, halophiles, thermophiles, psychrophiles, piezophiles, extremophiles have particular iron requirements depending on the environmental conditions in where they grow. Most of the work done in siderophore production by extremophiles is based in siderophore concentration and/or genomic studies determining the presence of siderophore synthesis and transport genes. Siderophores produced by extremophiles are not well known and more work needs to be done to elucidate chemical structures and their role in microorganism survival and metal cycling in extreme environments.

Genomic and metagenomic challenges and opportunities for bioleaching: a mini-review

High-throughput genomic technologies are accelerating progress in understanding the diversity of microbial life in many environments. Here we highlight advances in genomics and metagenomics of microorganisms from bioleaching heaps and related acidic mining environments. Bioleaching heaps used for copper recovery provide significant opportunities to study the processes and mechanisms underlying microbial successions and the influence of community composition on ecosystem functioning. Obtaining quantitative and process-level knowledge of these dynamics is pivotal for understanding how microorganisms contribute to the solubilization of copper for industrial recovery. Advances in DNA sequencing technology provide unprecedented opportunities to obtain information about the genomes of bioleaching microorganisms, allowing predictive models of metabolic potential and ecosystem-level interactions to be constructed. These approaches are enabling predictive phenotyping of organisms many of which are recalcitrant to genetic approaches or are unculturable. This mini-review describes current bioleaching genomic and metagenomic projects and addresses the use of genome information to: (i) build metabolic models; (ii) predict microbial interactions; (iii) estimate genetic diversity; and (iv) study microbial evolution. Key challenges and perspectives of bioleaching genomics/metagenomics are addressed.

Adaptation to environmental factors shapes the organization of regulatory regions in microbial communities

Background

It has been shown in a number of metagenomic studies that the addition and removal of specific genes have allowed microbiomes to adapt to specific environmental conditions by losing and gaining specific functions. But it is not known whether and how the regulation of gene expression also contributes to adaptation.

Results

We have here characterized and analyzed the metaregulome of three different environments, as well as their impact in the adaptation to particular variable physico-chemical conditions. For this, we have developed a computational protocol to extract regulatory regions and their corresponding transcription factors binding sites directly from metagenomic reads and applied it to three well known environments: Acid Mine, Whale Fall, and Waseca Farm. Taking the density of regulatory sites in promoters as a measure of the potential and complexity of gene regulation, we found it to be quantitatively the same in all three environments, despite their different physico-chemical conditions and species composition. However, we found that each environment distributes their regulatory potential differently across their functional space. Among the functions with highest regulatory potential in each niche, we found significant enrichment of processes related to sensing and buffering external variable factors specific to each environment, like for example, the availability of co-factors in deep sea, of oligosaccharides in soil and the regulation of pH in the acid mine.

Conclusions

These results highlight the potential impact of gene regulation in the adaptation of bacteria to the different habitats through the distribution of their regulatory potential among specific functions, and point to critical environmental factors that challenge the growth of any microbial community.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-877) contains supplementary material, which is available to authorized users.

α-fur, an antisense RNA gene to fur in the extreme acidophile Acidithiobacillus ferrooxidans

A large non-coding RNA, termed α-Fur, of ~1000 nt has been detected in the extreme acidophile Acidithiobacillus ferrooxidans encoded on the antisense strand to the iron-responsive master regulator fur (ferric uptake regulator) gene. A promoter for α-fur was predicted bioinformatically and validated using gene fusion experiments. The promoter is situated within the coding region and in the same sense as proB, potentially encoding a glutamate 5-kinase. The 3′ termination site of the α-fur transcript was determined by 3′ rapid amplification of cDNA ends to lie 7 nt downstream of the start of transcription of fur. Thus, α-fur is antisense to the complete coding region of fur, including its predicted ribosome-binding site. The genetic context of α-fur is conserved in several members of the genus Acidithiobacillus but not in all acidophiles, indicating that it is monophyletic but not niche specific. It is hypothesized that α-Fur regulates the cellular level of Fur. This is the fourth example of an antisense RNA to fur, although it is the first in an extreme acidophile, and underscores the growing importance of cis-encoded non-coding RNAs as potential regulators involved in the microbial iron-responsive stimulon.

New copper resistance determinants in the extremophile acidithiobacillus ferrooxidans: a quantitative proteomic analysis

Acidithiobacillus ferrooxidans is an extremophilic bacterium used in biomining processes to recover metals. The presence in A. ferrooxidans ATCC 23270 of canonical copper resistance determinants does not entirely explain the extremely high copper concentrations this microorganism is able to stand, suggesting the existence of other efficient copper resistance mechanisms. New possible copper resistance determinants were searched by using 2D-PAGE, real time PCR (qRT-PCR) and quantitative proteomics with isotope-coded protein labeling (ICPL). A total of 594 proteins were identified of which 120 had altered levels in cells grown in the presence of copper. Of this group of proteins, 76 were up-regulated and 44 down-regulated. The up-regulation of RND-type Cus systems and different RND-type efflux pumps was observed in response to copper, suggesting that these proteins may be involved in copper resistance. An overexpression of most of the genes involved in histidine synthesis and several of those annotated as encoding for cysteine production was observed in the presence of copper, suggesting a possible direct role for these metal-binding amino acids in detoxification. Furthermore, the up-regulation of putative periplasmic disulfide isomerases was also seen in the presence of copper, suggesting that they restore copper-damaged disulfide bonds to allow cell survival. Finally, the down-regulation of the major outer membrane porin and some ionic transporters was seen in A. ferrooxidans grown in the presence of copper, indicating a general decrease in the influx of the metal and other cations into the cell. Thus, A. ferrooxidans most likely uses additional copper resistance strategies in which cell envelope proteins are key components. This knowledge will not only help to understand the mechanism of copper resistance in this extreme acidophile but may help also to select the best fit members of the biomining community to attain more efficient industrial metal leaching processes.

Architecture and gene repertoire of the flexible genome of the extreme acidophile Acidithiobacillus caldus

BACKGROUND

Acidithiobacillus caldus is a sulfur oxidizing extreme acidophile and the only known mesothermophile within the Acidithiobacillales. As such, it is one of the preferred microbes for mineral bioprocessing at moderately high temperatures. In this study, we explore the genomic diversity of A. caldus strains using a combination of bioinformatic and experimental techniques, thus contributing first insights into the elucidation of the species pangenome.

PRINCIPAL FINDINGS

Comparative sequence analysis of A. caldus ATCC 51756 and SM-1 indicate that, despite sharing a conserved and highly syntenic genomic core, both strains have unique gene complements encompassing nearly 20% of their respective genomes. The differential gene complement of each strain is distributed between the chromosomal compartment, one megaplasmid and a variable number of smaller plasmids, and is directly associated to a diverse pool of mobile genetic elements (MGE). These include integrative conjugative and mobilizable elements, genomic islands and insertion sequences. Some of the accessory functions associated to these MGEs have been linked previously to the flexible gene pool in microorganisms inhabiting completely different econiches. Yet, others had not been unambiguously mapped to the flexible gene pool prior to this report and clearly reflect strain-specific adaption to local environmental conditions.

SIGNIFICANCE

For many years, and because of DNA instability at low pH and recurrent failure to genetically transform acidophilic bacteria, gene transfer in acidic environments was considered negligible. Findings presented herein imply that a more or less conserved pool of actively excising MGEs occurs in the A. caldus population and point to a greater frequency of gene exchange in this econiche than previously recognized. Also, the data suggest that these elements endow the species with capacities to withstand the diverse abiotic and biotic stresses of natural environments, in particular those associated with its extreme econiche.

Analysis of Gene Expression in Response to Copper Stress in Acidithiobacillus ferrooxidans Strain D2, Isolated from a Copper Bioleaching Operation

Acidithiobacillus ferrooxidans strain D2 was isolated from a copper bioleaching operation in Atacama Desert, Chile. Copper is widely used as cofactor in proteins but high concentrations of copper are toxic. Cells require certain mechanisms to maintain the copper homeostasis and avoid toxic effects of high intracellular concentration. The molecular response of A. ferrooxidans strain D2 grown in the presence/absence of copper was examined using a A. ferrooxidans whole-genome DNA microarrays. Roughly 23% of 3,147 genes represented on the microarray were differentially expressed; about 9% of them were upregulated in the presence of copper. Among the upregulated genes, those encoding for the copper efflux protein (CusA) and for the copper-translocating P-type ATPase (CopA) were upregulated. The expression of genes encoding proteins related to iron transport was repressed. Similarly, genes related with assimilative metabolism of sulfur (L-cysteine biosynthesis) cysB, cysJ, cysI, CysD-2 and cysN were upregulated. Our results show that when A. ferrooxidans strain D2 was challenged with high copper concentrations, genes related to copper stress response were upregulated as well as others that have not been reported to be related to that mechanism. In addition, some genes related to other metabolic pathways were repressed, probably because of the energy cost of the stress response.

A basic set of homeostatic controller motifs

Adaptation and homeostasis are essential properties of all living systems. However, our knowledge about the reaction kinetic mechanisms leading to robust homeostatic behavior in the presence of environmental perturbations is still poor. Here, we describe, and provide physiological examples of, a set of two-component controller motifs that show robust homeostasis. This basic set of controller motifs, which can be considered as complete, divides into two operational work modes, termed as inflow and outflow control. We show how controller combinations within a cell can integrate uptake and metabolization of a homeostatic controlled species and how pathways can be activated and lead to the formation of alternative products, as observed, for example, in the change of fermentation products by microorganisms when the supply of the carbon source is altered. The antagonistic character of hormonal control systems can be understood by a combination of inflow and outflow controllers.

Plant growth-promoting rhizobacteria (PGPR): Their potential as antagonists and biocontrol agents

Bacteria that colonize plant roots and promote plant growth are referred to as plant growth-promoting rhizobacteria (PGPR). PGPR are highly diverse and in this review we focus on rhizobacteria as biocontrol agents. Their effects can occur via local antagonism to soil-borne pathogens or by induction of systemic resistance against pathogens throughout the entire plant. Several substances produced by antagonistic rhizobacteria have been related to pathogen control and indirect promotion of growth in many plants, such as siderophores and antibiotics. Induced systemic resistance (ISR) in plants resembles pathogen-induced systemic acquired resistance (SAR) under conditions where the inducing bacteria and the challenging pathogen remain spatially separated. Both types of induced resistance render uninfected plant parts more resistant to pathogens in several plant species. Rhizobacteria induce resistance through the salicylic acid-dependent SAR pathway, or require jasmonic acid and ethylene perception from the plant for ISR. Rhizobacteria belonging to the genera Pseudomonas and Bacillus are well known for their antagonistic effects and their ability to trigger ISR. Resistance-inducing and antagonistic rhizobacteria might be useful in formulating new inoculants with combinations of different mechanisms of action, leading to a more efficient use for biocontrol strategies to improve cropping systems.

Phylogenetic relationships and selective pressure on gene families related to iron homeostasis in land plants

Iron is involved in many metabolic processes, such as respiration and photosynthesis, and therefore an essential element for plant development. Comparative analysis of gene copies between crops and lower plant groups can shed light on the evolution of genes important to iron homeostasis. A phylogenetic analysis of five metal homeostasis gene families (NAS, NRAMP, YSL, FRO, and IRT) selected in monocots, dicots, gymnosperms, and bryophytes was performed. The homologous genes were found using known iron homeostasis gene sequences of Oryza sativa, Arabidopsis thaliana, and Physcomitrella patens as queries. The phylogeny was constructed using bioinfomatics tools. A total of 243 gene sequences for 30 plant species were found. The evolutionary fingerprint analysis suggested a purifying selective pressure of iron homeostasis genes for most of the plant gene homologues. The NAS and YSL genes appear to accumulate more negative selection sites, suggesting a strong selective pressure on these two gene families. The divergence time analysis indicates IRT as the most ancient gene family and FRO as the most recent. NRAMP and YSL genes appear to share a close relationship in the evolution of iron homeostasis gene families.

Thiol/Disulfide system plays a crucial role in redox protection in the acidophilic iron-oxidizing bacterium Leptospirillum ferriphilum

Thiol/disulfide systems are involved in the maintenance of the redox status of proteins and other molecules that contain thiol/disulfide groups. Leptospirillum ferriphilum DSM14647, an acidophilic bacterium that uses Fe²⁺ as electron donor, and withstands very high concentrations of iron and other redox active metals, is a good model to study how acidophiles preserve the thiol/disulfide balance. We studied the composition of thiol/disulfide systems and their role in the oxidative stress response in this extremophile bacterium. Bioinformatic analysis using genomic data and enzymatic assays using protein extracts from cells grown under oxidative stress revealed that the major thiol/disulfide system from L. ferriphilum are a cytoplasmic thioredoxin system (composed by thioredoxins Trx and thioredoxin reductase TR), periplasmic thiol oxidation system (DsbA/DsbB) and a c-type cytochrome maturation system (DsbD/DsbE). Upon exposure of L. ferriphilum to reactive oxygen species (ROS)-generating compounds, transcriptional activation of the genes encoding Trxs and the TR enzyme, which results in an increase of the corresponding activity, was observed. Altogether these data suggest that the thioredoxin-based thiol/disulfide system plays an important role in redox protection of L. ferriphilum favoring the survival of this microorganism under extreme environmental oxidative conditions.

Kinetics of anaerobic elemental sulfur oxidation by ferric iron in Acidithiobacillus ferrooxidans and protein identification by comparative 2-DE-MS/MS

Elemental sulfur oxidation by ferric iron in Acidithiobacillus ferrooxidans was investigated. The apparent Michaelis constant for ferric iron was 18.6 mM. An absence of anaerobic ferric iron reduction ability was observed in bacteria maintained on elemental sulfur for an extended period of time. Upon transition from ferrous iron to elemental sulfur medium, the cells exhibited similar kinetic characteristics of ferric iron reduction under anaerobic conditions to those of cells that were originally maintained on ferrous iron. Nevertheless, a total loss of anaerobic ferric iron reduction ability after the sixth passage in elemental sulfur medium was demonstrated. The first proteomic screening of total cell lysates of anaerobically incubated bacteria resulted in the detection of 1599 protein spots in the master two-dimensional electrophoresis gel. A set of 59 more abundant and 49 less abundant protein spots that changed their protein abundances in an anaerobiosis-dependent manner was identified and compared to iron- and sulfur-grown cells, respectively. Proteomic analysis detected a significant increase in abundance under anoxic conditions of electron transporters, such as rusticyanin and cytochrome c(552), involved in the ferrous iron oxidation pathway. Therefore we suggest the incorporation of rus-operon encoded proteins in the anaerobic respiration pathway. Two sulfur metabolism proteins were identified, pyridine nucleotide-disulfide oxidoreductase and sulfide-quinone reductase. The important transcription regulator, ferric uptake regulation protein, was anaerobically more abundant. The anaerobic expression of several proteins involved in cell envelope formation indicated a gradual adaptation to elemental sulfur oxidation.

Identification of differentially expressed genes in Sulfobacillus sp. TPY grown on either elemental sulphur or Fe(2+)

Sulfobacillus sp. TPY is a moderately thermophilic and acidophilic bacterium found in hydrothermal vents in the Pacific Ocean. This bacterium can oxidize ferrous sulfate (Fe(2+)) and elemental sulfur (S(0)) under separate conditions. We used random arbitrarily primed polymerase chain reaction (RAP-PCR) to screen and identify differentially expressed genes from bacteria grown on Fe(2+) or S(0) as the energy source. Fifty-five differential cDNA fragments were isolated and subjected to single-pass sequencing. Thirty-five fragments were identified as orthologs of known genes in the GenBank databases, of which 19 were confirmed to be differentially expressed at the transcriptional level by Northern blot analysis. Among these 19 genes, 14 genes, including isocitrate dehydrogenase, formyltetrahydrofolate deformylase, 3-hydroxybutyryl-CoA dehydrogenase, and GTP-binding protein, were upregulated in TPY grown on Fe(2+) or downregulated in TPY grown on S(0), while five genes such as the outer membrane adhesion-like protein, phosphomannomutase, and cysteine desulfurase sufS were upregulated in TPY strain grown on S(0) or downregulated in TPY grown on Fe(2+). These altered genes are involved in metabolism, osmotic stress, cell membrane alterations, oxidative stress, and the regulatory adaptive response. These results will aid our understanding of the molecular basis of Fe(2+) or S(0) oxidation by the moderately thermophilic and acidophilic bacteria.

Acidithiobacillus ferrooxidans metabolism: from genome sequence to industrial applications

BACKGROUND

Acidithiobacillus ferrooxidans is a major participant in consortia of microorganisms used for the industrial recovery of copper (bioleaching or biomining). It is a chemolithoautrophic, gamma-proteobacterium using energy from the oxidation of iron- and sulfur-containing minerals for growth. It thrives at extremely low pH (pH 1-2) and fixes both carbon and nitrogen from the atmosphere. It solubilizes copper and other metals from rocks and plays an important role in nutrient and metal biogeochemical cycling in acid environments. The lack of a well-developed system for genetic manipulation has prevented thorough exploration of its physiology. Also, confusion has been caused by prior metabolic models constructed based upon the examination of multiple, and sometimes distantly related, strains of the microorganism.

RESULTS

The genome of the type strain A. ferrooxidans ATCC 23270 was sequenced and annotated to identify general features and provide a framework for in silico metabolic reconstruction. Earlier models of iron and sulfur oxidation, biofilm formation, quorum sensing, inorganic ion uptake, and amino acid metabolism are confirmed and extended. Initial models are presented for central carbon metabolism, anaerobic metabolism (including sulfur reduction, hydrogen metabolism and nitrogen fixation), stress responses, DNA repair, and metal and toxic compound fluxes.

CONCLUSION

Bioinformatics analysis provides a valuable platform for gene discovery and functional prediction that helps explain the activity of A. ferrooxidans in industrial bioleaching and its role as a primary producer in acidic environments. An analysis of the genome of the type strain provides a coherent view of its gene content and metabolic potential.