Biology of archaea from a novel family Cuniculiplasmataceae (Thermoplasmata) ubiquitous in hyperacidic environments

Corrosion Characteristics of Typical Gangue Minerals in Biometallurgical Systems

Electrochemical and shake flask tests were used to examine the corrosion characteristics of typical gangue minerals in biometallurgical systems and their impact on microbial communities. The results show that the solubility order of the three gangue minerals is feldspar, mica, and quartz in descending order. Their corrosion processes are mainly controlled by cathodic electron-donating processes. They are subjected to triple resistance, which is defined as solution-resistant, colloidal silica passivation, and iron precipitation (ferric hydroxide or jarosite passivation). Fe3+ and microorganisms both greatly improve the corrosion capacity of the system for the three gangue minerals. The community diversity may rise to 9.3, 8.6, and 4.4 times that of the initial HQ0211 strain, respectively, in the presence of feldspar, mica, and quartz.. The proportions of autotrophic microorganisms Leptospirillum, Sulfobacillus, and Acidiplasma decreased significantly, and the mixed trophic archaeon Ferroplasma and heterotrophic archaeon Cuniculiplasma became the dominant microorganisms in the system. Finally, the dissolution mechanism of gangue minerals in biometallurgical systems is discussed. The results enrich the theory of the gangue mineral corrosion process, which can lay a foundation for the effective regulation of biometallurgical processes.

Multi-stress adaptive lifestyle of acidophiles enhances their robustness for biotechnological and environmental applications

Acidophiles are a group of organisms typically found in highly acidic environments such as acid mine drainage. These organisms have several physiological features that enable them to thrive in highly acidic environments (pH ≤3). Considering that both acid mine drainage and solfatara fields exhibit extreme and dynamic ecological conditions for acidophiles, it is crucial to gain deeper insights into the adaptive mechanisms employed by these unique organisms. The existing literature reveals a notable gap in understanding the multi-stress conditions confronting acidophiles and their corresponding coping mechanisms. Therefore, the current review aims to illuminate the intricacies of the metabolic lifestyles of acidophiles within these demanding habitats, exploring how their energy demands contribute to habitat acidification. In addition, the unique adaptive mechanisms employed by acidophiles were emphasized, especially the pivotal role of monolayer membrane-spanning lipids, and how these organisms effectively respond to a myriad of stresses. Beyond mere survival, understanding the adaptive mechanisms of these unique organisms could further enhance their use in some biotechnological and environmental applications. Lastly, this review explores the strategies used to engineer these organisms to promote their use in industrial applications.

Moderately thermostable GH1 β-glucosidases from hyperacidophilic archaeon Cuniculiplasma divulgatum S5

Family GH1 glycosyl hydrolases are ubiquitous in prokaryotes and eukaryotes and are utilized in numerous industrial applications, including bioconversion of lignocelluloses. In this study, hyperacidophilic archaeon Cuniculiplasma divulgatum (S5T=JCM 30642T) was explored as a source of novel carbohydrate-active enzymes. The genome of C. divulgatum encodes three GH1 enzyme candidates, from which CIB12 and CIB13 were heterologously expressed and characterized. Phylogenetic analysis of CIB12 and CIB13 clustered them with β-glucosidases from genuinely thermophilic archaea including Thermoplasma acidophilum, Picrophilus torridus, Sulfolobus solfataricus, Pyrococcus furiosus, and Thermococcus kodakarensis. Purified enzymes showed maximal activities at pH 4.5-6.0 (CIB12) and 4.5-5.5 (CIB13) with optimal temperatures at 50°C, suggesting a high-temperature origin of Cuniculiplasma spp. ancestors. Crystal structures of both enzymes revealed a classical (α/β)8 TIM-barrel fold with the active site located inside the barrel close to the C-termini of β-strands including the catalytic residues Glu204 and Glu388 (CIB12), and Glu204 and Glu385 (CIB13). Both enzymes preferred cellobiose over lactose as substrates and were classified as cellobiohydrolases. Cellobiose addition increased the biomass yield of Cuniculiplasma cultures growing on peptides by 50%, suggesting that the cellobiohydrolases expand the carbon substrate range and hence environmental fitness of Cuniculiplasma.

Evolutionary patterns of archaea predominant in acidic environment

BACKGROUND

Archaea of the order Thermoplasmatales are widely distributed in natural acidic areas and are amongst the most acidophilic prokaryotic organisms known so far. These organisms are difficult to culture, with currently only six genera validly published since the discovery of Thermoplasma acidophilum in 1970. Moreover, known great diversity of uncultured Thermoplasmatales represents microbial dark matter and underlines the necessity of efforts in cultivation and study of these archaea. Organisms from the order Thermoplasmatales affiliated with the so-called "alphabet-plasmas", and collectively dubbed "E-plasma", were the focus of this study. These archaea were found predominantly in the hyperacidic site PM4 of Parys Mountain, Wales, UK, making up to 58% of total metagenomic reads. However, these archaea escaped all cultivation attempts.

RESULTS

Their genome-based metabolism revealed its peptidolytic potential, in line with the physiology of the previously studied Thermoplasmatales isolates. Analyses of the genome and evolutionary history reconstruction have shown both the gain and loss of genes, that may have contributed to the success of the "E-plasma" in hyperacidic environment compared to their community neighbours. Notable genes among them are involved in the following molecular processes: signal transduction, stress response and glyoxylate shunt, as well as multiple copies of genes associated with various cellular functions; from energy production and conversion, replication, recombination, and repair, to cell wall/membrane/envelope biogenesis and archaella production. History events reconstruction shows that these genes, acquired by putative common ancestors, may determine the evolutionary and functional divergences of "E-plasma", which is much more developed than other representatives of the order Thermoplasmatales. In addition, the ancestral hereditary reconstruction strongly indicates the placement of Thermogymnomonas acidicola close to the root of the Thermoplasmatales.

CONCLUSIONS

This study has analysed the metagenome-assembled genome of "E-plasma", which denotes the basis of their predominance in Parys Mountain environmental microbiome, their global ubiquity, and points into the right direction of further cultivation attempts. The results suggest distinct evolutionary trajectories of organisms comprising the order Thermoplasmatales, which is important for the understanding of their evolution and lifestyle.

A Metagenome from a Steam Vent in Los Azufres Geothermal Field Shows an Abundance of Thermoplasmatales archaea and Bacteria from the Phyla Actinomycetota and Pseudomonadota

Los Azufres National Park is a geothermal field that has a wide number of thermal manifestations; nevertheless, the microbial communities in many of these environments remain unknown. In this study, a metagenome from a sediment sample from Los Azufres National Park was sequenced. In this metagenome, we found that the microbial diversity corresponds to bacteria (Actinomycetota, Pseudomonadota), archaea (Thermoplasmatales and Candidatus Micrarchaeota and Candidatus Parvarchaeota), eukarya (Cyanidiaceae), and viruses (Fussellovirus and Caudoviricetes). The functional annotation showed genes related to the carbon fixation pathway, sulfur metabolism, genes involved in heat and cold shock, and heavy-metal resistance. From the sediment, it was possible to recover two metagenome-assembled genomes from Ferrimicrobium and Cuniculiplasma. Our results showed that there are a large number of microorganisms in Los Azufres that deserve to be studied.

Microbial carbon, sulfur, iron, and nitrogen cycling linked to the potential remediation of a meromictic acidic pit lake

Cueva de la Mora is a permanently stratified acidic pit lake and a model system for extreme acid mine drainage (AMD) studies. Using a combination of amplicon sequencing, metagenomics and metatranscriptomics we performed a taxonomically resolved analysis of microbial contributions to carbon, sulfur, iron, and nitrogen cycling. We found that active green alga Coccomyxa onubensis dominated the upper layer and chemocline. The chemocline had activity for iron(II) oxidation carried out by populations of Ca. Acidulodesulfobacterium, Ferrovum, Leptospirillium, and Armatimonadetes. Predicted activity for iron(III) reduction was only detected in the deep layer affiliated with Proteobacteria. Activity for dissimilatory nitrogen cycling including nitrogen fixation and nitrate reduction was primarily predicted in the chemocline. Heterotrophic archaeal populations with predicted activity for sulfide oxidation related to uncultured Thermoplasmatales dominated in the deep layer. Abundant sulfate-reducing Desulfomonile and Ca. Acidulodesulfobacterium populations were active in the chemocline. In the deep layer, uncultured populations from the bacterial phyla Actinobacteria, Chloroflexi, and Nitrospirae contributed to both sulfate reduction and sulfide oxidation. Based on this information we evaluated the potential for sulfide mineral precipitation in the deep layer as a tool for remediation. We argue that sulfide precipitation is not limited by microbial genetic potential but rather by the quantity and quality of organic carbon reaching the deep layer as well as by oxygen additions to the groundwater enabling sulfur oxidation. Addition of organic carbon and elemental sulfur should stimulate sulfate reduction and limit reoxidation of sulfide minerals.

Microbial Diversity of a Disused Copper Mine Site (Parys Mountain, UK), Dominated by Intensive Eukaryotic Filamentous Growth

The Parys Mountain copper mine (Wales, UK) contains a wide range of discrete environmental microniches with various physicochemical conditions that shape microbial community composition. Our aim was to assess the microbial community in the sediments and overlying water column in an acidic mine drainage (AMD) site containing abundant filamentous biogenic growth via application of a combination of chemical analysis and taxonomic profiling using 16S rRNA gene amplicon sequencing. Our results were then compared to previously studied sites at Parys Mt. Overall, the sediment microbiome showed a dominance of bacteria over archaea, particularly those belonging to Proteobacteria (genera Acidiphilium and Acidisphaera), Acidobacteriota (subgroup 1), Chloroflexota (AD3 cluster), Nitrospirota (Leptospirillum) and the uncultured Planctomycetota/CPIa-3 termite group. Archaea were only present in the sediment in small quantities, being represented by the Terrestrial Miscellaneous Euryarchaeota Group (TMEG), Thermoplasmatales and Ca. Micrarchaeota (Ca. Micracaldota). Bacteria, mostly of the genera Acidiphilium and Leptospirillum, also dominated within the filamentous streamers while archaea were largely absent. This study found pH and dissolved solutes to be the most important parameters correlating with relative proportions of bacteria to archaea in an AMD environment and revealed the abundance patterns of native acidophilic prokaryotes inhabiting Parys Mt sites and their niche specificities.

Recovery of Lutacidiplasmatales archaeal order genomes suggests convergent evolution in Thermoplasmatota

The Terrestrial Miscellaneous Euryarchaeota Group has been identified in various environments, and the single genome investigated thus far suggests that these archaea are anaerobic sulfite reducers. We assemble 35 new genomes from this group that, based on genome analysis, appear to possess aerobic and facultative anaerobic lifestyles and may oxidise rather than reduce sulfite. We propose naming this order (representing 16 genera) "Lutacidiplasmatales" due to their occurrence in various acidic environments and placement within the phylum Thermoplasmatota. Phylum-level analysis reveals that Thermoplasmatota evolution had been punctuated by several periods of high levels of novel gene family acquisition. Several essential metabolisms, such as aerobic respiration and acid tolerance, were likely acquired independently by divergent lineages through convergent evolution rather than inherited from a common ancestor. Ultimately, this study describes the terrestrially prevalent Lutacidiciplasmatales and highlights convergent evolution as an important driving force in the evolution of archaeal lineages.

The importance of biofilm formation for cultivation of a Micrarchaeon and its interactions with its Thermoplasmatales host

Micrarchaeota is a distinctive lineage assigned to the DPANN archaea, which includes poorly characterised microorganisms with reduced genomes that likely depend on interactions with hosts for growth and survival. Here, we report the enrichment of a stable co-culture of a member of the Micrarchaeota (Ca. Micrarchaeum harzensis) together with its Thermoplasmatales host (Ca. Scheffleriplasma hospitalis), as well as the isolation of the latter. We show that symbiont-host interactions depend on biofilm formation as evidenced by growth experiments, comparative transcriptomic analyses and electron microscopy. In addition, genomic, metabolomic, extracellular polymeric substances and lipid content analyses indicate that the Micrarchaeon symbiont relies on the acquisition of metabolites from its host. Our study of the cell biology and physiology of a Micrarchaeon and its host adds to our limited knowledge of archaeal symbioses.

The Micrarchaeota lineage includes poorly characterized archaea with reduced genomes that likely depend on host interactions for survival. Here, the authors report a stable co-culture of a member of the Micrarchaeota and its host, and use multi-omic and physiological analyses to shed light on this symbiosis.

Community structure and activity potentials of archaeal communities in hadal sediments of the Mariana and Mussau trenches

Hadal trenches are the least explored marine habitat on earth. Archaea has been shown to be the dominant group in trench sediments. However, the activity potentials and detailed diversity of these communities as well as their inter-trench variations are still not known. In this study, we combined datasets from two pairs of primers to investigate at high resolution the structure and activity potentials of the archaeal communities in vertically sectioned sediment cores taken from the deepest points of the Mariana (10,853 m) and Mussau (7011 m) trenches. The compositions of the potentially active communities revealed, via 16S ribosomal RNA gene (rDNA) and RNA (rRNA), significant differences between samples. Marine Group I (MGI), with nine identified subgroups, was the most dominant class in the active archaeal communities of the two trenches. Significantly different species composition and vertical variations were observed between the two trenches. Vertical transitions from aerobic MGI α to anaerobic MGI η and υ subgroups were observed in MST but not in MT sediments, which might be related to the faster microbial oxygen consumption in MST. These results provide a better understanding on archaeal activity and diversity in trench sediments.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-021-00105-y.

Microorganisms of Two Thermal Pools on Kunashir Island, Russia

The Kuril Archipelago is a part of the Circum-Pacific Belt (Ring of Fire). These islands have numerous thermal springs. There are very few studies on these microbial communities, and none of them have been conducted by modern molecular biological methods. Here we performed the first metagenomic study on two thermophilic microbial communities of Kunashir Island. Faust Lake is hot (48 °C) and highly acidic (pH 2.0). We constructed 28 metagenome-assembled genomes as well as 17 16S ribosomal RNA sequences. We found that bottom sediments of Faust Lake are dominated by a single species of red algae belonging to the Cyanidiaceae family. Archaeans in Faust Lake are more diverse than bacteria but less abundant. The Tretyakovsky Thermal Spring is also hot (52 °C) but only weakly acidic (pH 6.0). It has much higher microbial diversity (233 metagenome-assembled genomes; 93 16S ribosomal RNAs) and is dominated by bacteria, with only several archaeans and one fungus. Despite their geographic proximity, these two thermal springs were found to not share any species. A comparison of these two lakes with other thermal springs of the Circum-Pacific Belt revealed that only a few members of the communities are shared among different locations.

Respiratory Heme A-Containing Oxidases Originated in the Ancestors of Iron-Oxidizing Bacteria

Respiration is a major trait shaping the biology of many environments. Cytochrome oxidase containing heme A (COX) is a common terminal oxidase in aerobic bacteria and is the only one in mammalian mitochondria. The synthesis of heme A is catalyzed by heme A synthase (CtaA/Cox15), an enzyme that most likely coevolved with COX. The evolutionary origin of COX in bacteria has remained unknown. Using extensive sequence and phylogenetic analysis, we show that the ancestral type of heme A synthases is present in iron-oxidizing Proteobacteria such as Acidithiobacillus spp. These bacteria also contain a deep branching form of the major COX subunit (COX1) and an ancestral variant of CtaG, a protein that is specifically required for COX biogenesis. Our work thus suggests that the ancestors of extant iron-oxidizers were the first to evolve COX. Consistent with this conclusion, acidophilic iron-oxidizing prokaryotes lived on emerged land around the time for which there is the earliest geochemical evidence of aerobic respiration on earth. Hence, ecological niches of iron oxidation have apparently promoted the evolution of aerobic respiration.

Thermoplasmata and Nitrososphaeria as dominant archaeal members in acid mine drainage sediment of Malanjkhand Copper Project, India

Acid mine drainage (AMD) harbors all three life forms in spite of its toxic and hazardous nature. In comparison to bacterial diversity, an in-depth understanding of the archaeal diversity in AMD and their ecological significance remain less explored. Archaeal populations are known to play significant roles in various biogeochemical cycles within the AMD ecosystem, and it is imperative to have a deeper understanding of archaeal diversity and their functional potential in AMD system. The present study is aimed to understand the archaeal diversity of an AMD sediment of Malanjkhand Copper Project, India through archaea specific V6 region of 16S rRNA gene amplicon sequencing. Geochemical data confirmed the acidic, toxic, heavy metal-rich nature of the sample. Archaea specific V6-16S rRNA gene amplicon data showed a predominance of Thermoplasmata (BSLdp215, uncultured Thermoplasmata, and Thermoplasmataceae) and Nitrososphaeria (Nitrosotaleaceae) members constituting ~ 95% of the archaeal community. Uncultured members of Bathyarchaeia, Group 1.1c, Hydrothermarchaeota, and Methanomassiliicoccales along with Methanobacteriaceae, Methanocellaceae, Haloferaceae, Methanosaetaceae, and Methanoregulaceae constituted the part of rare taxa. Analysis of sequence reads indicated that apart from their close ecological relevance, members of the Thermoplasmata present in Malanjkhand AMD were mostly involved in chemoheterotrophy, Fe/S redox cycling, and with heavy metal resistance, while the Nitrososphaeria members were responsible for ammonia oxidation and fixation of HCO₃^- through 3-hydroxypropionate/4-hydroxybutyrate cycle at low pH and oligotrophic environment which subsequently played an important role in nitrification process in AMD sediment. Overall, the present study elucidated the biogeochemical significance of archaeal populations inhabiting the toxic AMD environment.

Metagenomic insights into the metabolism and evolution of a new Thermoplasmata order (Candidatus Gimiplasmatales)

Thermoplasmata is a widely distributed and ecologically important archaeal class in the phylum Euryarchaeota. Because few cultures and genomes are available, uncharacterized Thermoplasmata metabolisms remain unexplored. In this study, we obtained four medium- to high-quality archaeal metagenome-assembled genomes (MAGs) from the filamentous fragments of black-odorous aquatic sediments (Foshan, Guangdong, China). Based on their 16S rRNA gene and ribosomal protein phylogenies, the four MAGs belong to the previously unnamed Thermoplasmata UBA10834 clade. We propose that this clade (five reference genomes from the Genome Taxonomy Database (GTDB) and four MAGs from this study) be considered a new order, Candidatus Gimiplasmatales. Metabolic pathway reconstructions indicated that the Ca. Gimiplasmatales MAGs can biosynthesize isoprenoids and nucleotides de novo. Additionally, some taxa have genes for formaldehyde and acetate assimilation, and the Wood-Ljungdahl CO₂ -fixation pathway, indicating a mixotrophic lifestyle. Sulfur reduction, hydrogen metabolism, and arsenic detoxification pathways were predicted, indicating sulfur-, hydrogen-, and arsenic-transformation potentials. Comparative genomics indicated that the H₄ F Wood-Ljungdahl pathway of both Ca. Gimiplasmatales and Methanomassiliicoccales was likely obtained by the interdomain lateral gene transfer from the Firmicutes. Collectively, this study elucidates the taxonomic and potential metabolic diversity of the new order Ca. Gimiplasmatales and the evolution of this subgroup and its sister lineage Methanomassiliicoccales.

High Representation of Archaea Across All Depths in Oxic and Low-pH Sediment Layers Underlying an Acidic Stream

Parys Mountain or Mynydd Parys (Isle of Anglesey, United Kingdom) is a mine-impacted environment, which accommodates a variety of acidophilic organisms. Our previous research of water and sediments from one of the surface acidic streams showed a high proportion of archaea in the total microbial community. To understand the spatial distribution of archaea, we sampled cores (0-20 cm) of sediment and conducted chemical analyses and taxonomic profiling of microbiomes using 16S rRNA gene amplicon sequencing in different core layers. The taxonomic affiliation of sequencing reads indicated that archaea represented between 6.2 and 54% of the microbial community at all sediment depths. Majority of archaea were associated with the order Thermoplasmatales, with the most abundant group of sequences being clustered closely with the phylotype B_DKE, followed by "E-plasma," "A-plasma," other yet uncultured Thermoplasmatales with Ferroplasma and Cuniculiplasma spp. represented in minor proportions. Thermoplasmatales were found at all depths and in the whole range of chemical conditions with their abundance correlating with sediment Fe, As, Cr, and Mn contents. The bacterial microbiome component was largely composed in all layers of sediment by members of the phyla Proteobacteria, Actinobacteria, Nitrospirae, Firmicutes, uncultured Chloroflexi (AD3 group), and Acidobacteria. This study has revealed a high abundance of Thermoplasmatales in acid mine drainage-affected sediment layers and pointed at these organisms being the main contributors to carbon, and probably to iron and sulfur cycles in this ecosystem.

Microbial Community Structure Along a Horizontal Oxygen Gradient in a Costa Rican Volcanic Influenced Acid Rock Drainage System

We describe the geochemistry and microbial diversity of a pristine environment that resembles an acid rock drainage (ARD) but it is actually the result of hydrothermal and volcanic influences. We designate this environment, and other comparable sites, as volcanic influenced acid rock drainage (VARD) systems. The metal content and sulfuric acid in this ecosystem stem from the volcanic milieu and not from the product of pyrite oxidation. Based on the analysis of 16S rRNA gene amplicons, we report the microbial community structure in the pristine San Cayetano Costa Rican VARD environment (pH = 2.94-3.06, sulfate ~ 0.87-1.19 g L^-1, iron ~ 35-61 mg L^-1 (waters), and ~ 8-293 g kg^-1 (sediments)). San Cayetano was found to be dominated by microorganisms involved in the geochemical cycling of iron, sulfur, and nitrogen; however, the identity and abundance of the species changed with the oxygen content (0.40-6.06 mg L^-1) along the river course. The hypoxic source of San Cayetano is dominated by a putative anaerobic sulfate-reducing Deltaproteobacterium. Sulfur-oxidizing bacteria such as Acidithiobacillus or Sulfobacillus are found in smaller proportions with respect to typical ARD. In the oxic downstream, we identified aerobic iron-oxidizers (Leptospirillum, Acidithrix, Ferrovum) and heterotrophic bacteria (Burkholderiaceae bacterium, Trichococcus, Acidocella). Thermoplasmatales archaea closely related to environmental phylotypes found in other ARD niches were also observed throughout the entire ecosystem. Overall, our study shows the differences and similarities in the diversity and distribution of the microbial communities between an ARD and a VARD system at the source and along the oxygen gradient that establishes on the course of the river.

Proteome Cold-Shock Response in the Extremely Acidophilic Archaeon, Cuniculiplasma divulgatum

The archaeon Cuniculiplasma divulgatum is ubiquitous in acidic environments with low-to-moderate temperatures. However, molecular mechanisms underlying its ability to thrive at lower temperatures remain unexplored. Using mass spectrometry (MS)-based proteomics, we analysed the effect of short-term (3 h) exposure to cold. The C. divulgatum genome encodes 2016 protein-coding genes, from which 819 proteins were identified in the cells grown under optimal conditions. In line with the peptidolytic lifestyle of C. divulgatum, its intracellular proteome revealed the abundance of proteases, ABC transporters and cytochrome C oxidase. From 747 quantifiable polypeptides, the levels of 582 proteins showed no change after the cold shock, whereas 104 proteins were upregulated suggesting that they might be contributing to cold adaptation. The highest increase in expression appeared in low-abundance (0.001-0.005 fmol%) proteins for polypeptides' hydrolysis (metal-dependent hydrolase), oxidation of amino acids (FAD-dependent oxidoreductase), pyrimidine biosynthesis (aspartate carbamoyltransferase regulatory chain proteins), citrate cycle (2-oxoacid ferredoxin oxidoreductase) and ATP production (V type ATP synthase). Importantly, the cold shock induced a substantial increase (6% and 9%) in expression of the most-abundant proteins, thermosome beta subunit and glutamate dehydrogenase. This study has outlined potential mechanisms of environmental fitness of Cuniculiplasma spp. allowing them to colonise acidic settings at low/moderate temperatures.

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.

Diversity of "Ca. Micrarchaeota" in Two Distinct Types of Acidic Environments and Their Associations with Thermoplasmatales

“Candidatus Micrarchaeota” are widely distributed in acidic environments; however, their cultivability and our understanding of their interactions with potential hosts are very limited. Their habitats were so far attributed with acidic sites, soils, peats, freshwater systems, and hypersaline mats. Using cultivation and culture-independent approaches (16S rRNA gene clonal libraries, high-throughput amplicon sequencing of V3-V4 region of 16S rRNA genes), we surveyed the occurrence of these archaea in geothermal areas on Kamchatka Peninsula and Kunashir Island and assessed their taxonomic diversity in relation with another type of low-pH environment, acid mine drainage stream (Wales, UK). We detected “Ca. Micrarchaeota” in thermophilic heterotrophic enrichment cultures of Kunashir and Kamchatka that appeared as two different phylotypes, namely “Ca. Mancarchaeum acidiphilum”-, and ARMAN-2-related, alongside their potential hosts, Cuniculiplasma spp. and other Thermoplasmatales archaea without defined taxonomic position. These clusters of “Ca. Micrarchaeota” together with three other groups were also present in mesophilic acid mine drainage community. Present work expands our knowledge on the diversity of “Ca. Micrarchaeota” in thermophilic and mesophilic acidic environments, suggests cultivability patterns of acidophilic archaea and establishes potential links between low-abundance species of thermophilic “Ca. Micrarchaeota” and certain Thermoplasmatales, such as Cuniculiplasma spp. in situ.

Archaea dominate the microbial community in an ecosystem with low-to-moderate temperature and extreme acidity

BACKGROUND

The current view suggests that in low-temperature acidic environments, archaea are significantly less abundant than bacteria. Thus, this study of the microbiome of Parys Mountain (Anglesey, UK) sheds light on the generality of this current assumption. Parys Mountain is a historically important copper mine and its acid mine drainage (AMD) water streams are characterised by constant moderate temperatures (8-18 °C), extremely low pH (1.7) and high concentrations of soluble iron and other metal cations.

RESULTS

Metagenomic and SSU rRNA amplicon sequencing of DNA from Parys Mountain revealed a significant proportion of archaea affiliated with Euryarchaeota, which accounted for ca. 67% of the community. Within this phylum, potentially new clades of Thermoplasmata were overrepresented (58%), with the most predominant group being "E-plasma", alongside low-abundant Cuniculiplasmataceae, 'Ca. Micrarchaeota' and 'Terrestrial Miscellaneous Euryarchaeal Group' (TMEG) archaea, which were phylogenetically close to Methanomassilicoccales and clustered with counterparts from acidic/moderately acidic settings. In the sediment, archaea and Thermoplasmata contributed the highest numbers in V3-V4 amplicon reads, in contrast with the water body community, where Proteobacteria, Nitrospirae, Acidobacteria and Actinobacteria outnumbered archaea. Cultivation efforts revealed the abundance of archaeal sequences closely related to Cuniculiplasma divulgatum in an enrichment culture established from the filterable fraction of the water sample. Enrichment cultures with unfiltered samples showed the presence of Ferrimicrobium acidiphilum, C. divulgatum, 'Ca. Mancarchaeum acidiphilum Mia14', 'Ca. Micrarchaeota'-related and diverse minor (< 2%) bacterial metagenomic reads.

CONCLUSION

Contrary to expectation, our study showed a high abundance of archaea in this extremely acidic mine-impacted environment. Further, archaeal populations were dominated by one particular group, suggesting that they are functionally important. The prevalence of archaea over bacteria in these microbiomes and their spatial distribution patterns represents a novel and important advance in our understanding of acidophile ecology. We also demonstrated a procedure for the specific enrichment of cell wall-deficient members of the archaeal component of this community, although the large fraction of archaeal taxa remained unculturable. Lastly, we identified a separate clustering of globally occurring acidophilic members of TMEG that collectively belong to a distinct order within Thermoplasmata with yet unclear functional roles in the ecosystem.

Cuniculiplasmataceae, their ecogenomic and metabolic patterns, and interactions with 'ARMAN'

Recently, the order Thermoplasmatales was expanded through the cultivation and description of species Cuniculiplasma divulgatum and corresponding family Cuniculiplasmataceae. Initially isolated from acidic streamers, signatures of these archaea were ubiquitously found in various low-pH settings. Eight genomes with various levels of completeness are currently available, all of which exhibit very high sequence identities and genomic conservation. Co-existence of Cuniculiplasmataceae with archaeal Richmond Mine acidophilic nanoorganisms (‘ARMAN’)-related archaea representing an intriguing group within the “microbial dark matter” suggests their common fundamental environmental strategy and metabolic networking. The specific case of “Candidatus Mancarchaeum acidiphilum” Mia14 phylogenetically affiliated with “Ca. Micrarchaeota” from the superphylum “Ca. Diapherotrites” along with the presence of other representatives of ‘DPANN’ with significantly reduced genomes points at a high probability of close interactions between the latter and various Thermoplasmatales abundant in situ. This review critically assesses our knowledge on specific functional role and potential of the members of Cuniculiplasmataceae abundant in acidophilic microbiomes through the analysis of distribution, physiological and genomic patterns, and their interactions with ‘ARMAN’-related archaea.

'ARMAN' archaea depend on association with euryarchaeal host in culture and in situ

Intriguing, yet uncultured ‘ARMAN’-like archaea are metabolically dependent on other members of the microbial community. It remains uncertain though which hosts they rely upon, and, because of the lack of complete genomes, to what extent. Here, we report the co-culturing of ARMAN-2-related organism, Mia14, with Cuniculiplasma divulgatum PM4 during the isolation of this strain from acidic streamer in Parys Mountain (Isle of Anglesey, UK). Mia14 is highly enriched in the binary culture (ca. 10% genomic reads) and its ungapped 0.95 Mbp genome points at severe voids in central metabolic pathways, indicating dependence on the host, C. divulgatum PM4. Analysis of C. divulgatum isolates from different sites and shotgun sequence data of Parys Mountain samples suggests an extensive genetic exchange between Mia14 and hosts in situ. Within the subset of organisms with high-quality genomic assemblies representing the ‘DPANN’ superphylum, the Mia14 lineage has had the largest gene flux, with dozens of genes gained that are implicated in the host interaction.

In the absence of complete genomes, the metabolic capabilities of uncultured ARMAN-like archaea have been uncertain. Here, Golyshina et al. apply an enrichment culture technique and find that the ungapped genome of the ARMAN-like archaeon Mia14 has lost key metabolic pathways, suggesting dependence on the host archaeon Cuniculiplasma divulgatum.

Metabolic and evolutionary patterns in the extremely acidophilic archaeon Ferroplasma acidiphilum YT

Ferroplasmaceae represent ubiquitous iron-oxidising extreme acidophiles with a number of unique physiological traits. In a genome-based study of Ferroplasma acidiphilum YT, the only species of the genus Ferroplasma with a validly published name, we assessed its central metabolism and genome stability during a long-term cultivation experiment. Consistently with physiology, the genome analysis points to F. acidiphilum YT having an obligate peptidolytic oligotrophic lifestyle alongside with anaplerotic carbon assimilation. This narrow trophic specialisation abridges the sugar uptake, although all genes for glycolysis and gluconeogenesis, including bifunctional unidirectional fructose 1,6-bisphosphate aldolase/phosphatase, have been identified. Pyruvate and 2-oxoglutarate dehydrogenases are substituted by 'ancient' CoA-dependent pyruvate and alpha-ketoglutarate ferredoxin oxidoreductases. In the lab culture, after ~550 generations, the strain exhibited the mutation rate of ≥1.3 × 10-8 single nucleotide substitutions per site per generation, which is among the highest values recorded for unicellular organisms. All but one base substitutions were G:C to A:T, their distribution between coding and non-coding regions and synonymous-to-non-synonymous mutation ratios suggest the neutral drift being a prevalent mode in genome evolution in the lab culture. Mutations in nature seem to occur with lower frequencies, as suggested by a remarkable genomic conservation in F. acidiphilum YT variants from geographically distant populations.

Characterisation of a stable laboratory co-culture of acidophilic nanoorganisms

This study describes the laboratory cultivation of ARMAN (Archaeal Richmond Mine Acidophilic Nanoorganisms). After 2.5 years of successive transfers in an anoxic medium containing ferric sulfate as an electron acceptor, a consortium was attained that is comprised of two members of the order Thermoplasmatales, a member of a proposed ARMAN group, as well as a fungus. The 16S rRNA identity of one archaeon is only 91.6% compared to the most closely related isolate Thermogymnomonas acidicola. Hence, this organism is the first member of a new genus. The enrichment culture is dominated by this microorganism and the ARMAN. The third archaeon in the community seems to be present in minor quantities and has a 100% 16S rRNA identity to the recently isolated Cuniculiplasma divulgatum. The enriched ARMAN species is most probably incapable of sugar metabolism because the key genes for sugar catabolism and anabolism could not be identified in the metagenome. Metatranscriptomic analysis suggests that the TCA cycle funneled with amino acids is the main metabolic pathway used by the archaea of the community. Microscopic analysis revealed that growth of the ARMAN is supported by the formation of cell aggregates. These might enable feeding of the ARMAN by or on other community members.