Targeted isolation based on metagenome‐assembled genomes reveals a phylogenetically distinct group of thermophilic spirochetes from deep biosphere

Novel thermophilic genera Geochorda gen. nov. and Carboxydochorda gen. nov. from the deep terrestrial subsurface reveal the ecophysiological diversity in the class Limnochordia

The class Limnochordia harbors a single cultivated member, the mesophilic Limnochorda pilosa, which was isolated from a meromictic lake. Despite numerous molecular signatures reported in various ecosystems, the ecophysiological versatility of this deeply branched lineage of Firmicutes (Bacillota) remains poorly understood. The objective of this study was to use targeted cultivation, based on metagenome-assembled genomes from a deep terrestrial aquifer in Western Siberia, to isolate two new thermophilic members of the class. These isolates, described as Geochorda subterranea gen. nov. sp. nov. and Carboxydochorda subterranea gen. nov. sp. nov. within the Geochordaceae fam. nov., were capable of both anaerobic and aerobic respiration using fumarate and O2, respectively, with simple sugars as electron donors. The cultivated Geochordaceae have demonstrated fermentative growth and degradation of various polymers, including starch, maltose, maltodextrin, xylan, and chitin. The carboxydotrophic C. subterranea sp. nov. exhibited autotrophic growth via the Calvin-Benson-Bassham cycle, using CO, H2, and formate as electron donors and O2 as an electron acceptor, adding metabolic flexibility to the bacterium in the nutrient-depleted "deep biosphere" and supporting the possibility of aerobic metabolism in the deep subsurface. The broad physiological potential deciphered from physiological experiments and comparative genomic data explains the widespread distribution of uncultivated members of the class Limnochordia in various ecosystems, where they can oxidize complex organic substrates through both aerobic and anaerobic respiration, as well as pursue a chemolithotrophic lifestyle through the oxidation of H2 or CO.

Modern microbiology: Embracing complexity through integration across scales

Microbes were the only form of life on Earth for most of its history, and they still account for the vast majority of life's diversity. They convert rocks to soil, produce much of the oxygen we breathe, remediate our sewage, and sustain agriculture. Microbes are vital to planetary health as they maintain biogeochemical cycles that produce and consume major greenhouse gases and support large food webs. Modern microbiologists analyze nucleic acids, proteins, and metabolites; leverage sophisticated genetic tools, software, and bioinformatic algorithms; and process and integrate complex and heterogeneous datasets so that microbial systems may be harnessed to address contemporary challenges in health, the environment, and basic science. Here, we consider an inevitably incomplete list of emergent themes in our discipline and highlight those that we recognize as the archetypes of its modern era that aim to address the most pressing problems of the 21st century.

Meta-omics assisted microbial gene and strain resources mining in contaminant environment

Human activities have led to the release of various environmental pollutants, triggering ecological challenges. In situ, microbial communities in these contaminated environments are usually assumed to possess the potential capacity of pollutant degradation. However, the majority of genes and microorganisms in these environments remain uncharacterized and uncultured. The advent of meta-omics provided culture-independent solutions for exploring the functional genes and microorganisms within complex microbial communities. In this review, we highlight the applications and methodologies of meta-omics in uncovering of genes and microbes from contaminated environments. These findings may assist in future bioremediation research.

Mining strategies for isolating plastic-degrading microorganisms

Plastic waste is a growing global pollutant. Plastic degradation by microorganisms has captured attention as an earth-friendly tactic. Although the mechanisms of plastic degradation by bacteria, fungi, and algae have been explored over the past decade, a large knowledge gap still exists regarding the identification, sorting, and cultivation of efficient plastic degraders, primarily because of their uncultivability. Advances in sequencing techniques and bioinformatics have enabled the identification of microbial degraders and related enzymes and genes involved in plastic biodegradation. In this review, we provide an outline of the situation of plastic degradation and summarize the methods for effective microbial identification using multidisciplinary techniques such as multiomics, meta-analysis, and spectroscopy. This review introduces new strategies for controlling plastic pollution in an environmentally friendly manner. Using this information, highly efficient and colonizing plastic degraders can be mined via targeted sorting and cultivation. In addition, based on the recognized rules and plastic degraders, we can perform an in-depth analysis of the associated degradation mechanism, metabolic features, and interactions.

Herbivorous Fish Microbiome Adaptations to Sulfated Dietary Polysaccharides

Marine herbivorous fish that feed primarily on macroalgae, such as those from the genus Kyphosus, are essential for maintaining coral health and abundance on tropical reefs. Here, deep metagenomic sequencing and assembly of gut compartment-specific samples from three sympatric, macroalgivorous Hawaiian kyphosid species have been used to connect host gut microbial taxa with predicted protein functional capacities likely to contribute to efficient macroalgal digestion. Bacterial community compositions, algal dietary sources, and predicted enzyme functionalities were analyzed in parallel for 16 metagenomes spanning the mid- and hindgut digestive regions of wild-caught fishes. Gene colocalization patterns of expanded carbohydrate (CAZy) and sulfatase (SulfAtlas) digestive enzyme families on assembled contigs were used to identify likely polysaccharide utilization locus associations and to visualize potential cooperative networks of extracellularly exported proteins targeting complex sulfated polysaccharides. These insights into the gut microbiota of herbivorous marine fish and their functional capabilities improve our understanding of the enzymes and microorganisms involved in digesting complex macroalgal sulfated polysaccharides. IMPORTANCE This work connects specific uncultured bacterial taxa with distinct polysaccharide digestion capabilities lacking in their marine vertebrate hosts, providing fresh insights into poorly understood processes for deconstructing complex sulfated polysaccharides and potential evolutionary mechanisms for microbial acquisition of expanded macroalgal utilization gene functions. Several thousand new marine-specific candidate enzyme sequences for polysaccharide utilization have been identified. These data provide foundational resources for future investigations into suppression of coral reef macroalgal overgrowth, fish host physiology, the use of macroalgal feedstocks in terrestrial and aquaculture animal feeds, and the bioconversion of macroalgae biomass into value-added commercial fuel and chemical products.

The media composition as a crucial element in high-throughput metabolic network reconstruction

In recent years, metagenome-assembled genomes (MAGs) have provided glimpses into the intra- and interspecies genetic diversity and interactions that form the bases of complex microbial communities. High-throughput reconstruction of genome-scale metabolic networks (GEMs) from MAGs is a promising avenue to disentangle the myriad trophic interactions stabilizing these communities. However, high-throughput reconstruction of GEMs relies on accurate gap filling of metabolic pathways using automated algorithms. Here, we systematically explore how the composition of the media (specification of the available nutrients and metabolites) during gap filling influences the resulting GEMs concerning predicted auxotrophies for fully sequenced model organisms and environmental isolates. We expand this analysis by using 106 MAGs from the same species with differing quality. We find that although the completeness of MAGs influences the fraction of gap-filled reactions, the composition of the media plays the dominant role in the accurate prediction of auxotrophies that form the basis of myriad community interactions. We propose that constraining the media composition for gap filling through both experimental approaches and computational approaches will increase the reliability of high-throughput reconstruction of genome-scale metabolic models from MAGs and paves the way for culture independent prediction of trophic interactions in complex microbial communities.

Reaching unreachables: Obstacles and successes of microbial cultivation and their reasons

In terms of the number and diversity of living units, the prokaryotic empire is the most represented form of life on Earth, and yet it is still to a significant degree shrouded in darkness. This microbial "dark matter" hides a great deal of potential in terms of phylogenetically or metabolically diverse microorganisms, and thus it is important to acquire them in pure culture. However, do we know what microorganisms really need for their growth, and what the obstacles are to the cultivation of previously unidentified taxa? Here we review common and sometimes unexpected requirements of environmental microorganisms, especially soil-harbored bacteria, needed for their replication and cultivation. These requirements include resuscitation stimuli, physical and chemical factors aiding cultivation, growth factors, and co-cultivation in a laboratory and natural microbial neighborhood.

Active Sulfate-Reducing Bacterial Community in the Camel Gut

The diversity and activity of sulfate-reducing bacteria (SRB) in the camel gut remains largely unexplored. An abundant SRB community has been previously revealed in the feces of Bactrian camels (Camelus bactrianus). This study aims to combine the 16S rRNA gene profiling, sulfate reduction rate (SRR) measurement with a radioactive tracer, and targeted cultivation to shed light on SRB activity in the camel gut. Fresh feces of 55 domestic Bactrian camels grazing freely on semi-arid mountain pastures in the Kosh-Agach district of the Russian Altai area were analyzed. Feces were sampled in early winter at an ambient temperature of -15 °C, which prevented possible contamination. SRR values measured with a radioactive tracer in feces were relatively high and ranged from 0.018 to 0.168 nmol S cm^-3 day^-1. The 16S rRNA gene profiles revealed the presence of Gram-negative Desulfovibrionaceae and spore-forming Desulfotomaculaceae. Targeted isolation allowed us to obtain four pure culture isolates belonging to Desulfovibrio and Desulforamulus. An active SRB community may affect the iron and copper availability in the camel intestine due to metal ions precipitation in the form of sparingly soluble sulfides. The copper-iron sulfide, chalcopyrite (CuFeS₂), was detected by X-ray diffraction in 36 out of 55 analyzed camel feces. In semi-arid areas, gypsum, like other evaporite sulfates, can be used as a solid-phase electron acceptor for sulfate reduction in the camel gastrointestinal tract.

Thermospira aquatica gen. nov., sp. nov., a novel thermophilic spirochete isolated from a Tunisian hot spring, and description of the novel family Thermospiraceae

A novel thermophilic, anaerobic bacterium, strain F1F22^T, was isolated from hot spring water collected in northern Tunisia. The cells were non-motile, Gram-negative and helical with hooked ends, 0.5×10-32 µm in size. Growth of the strain was observed at 45-70 °C (optimum, 55 °C), in 0.0-1.0 % (w/v) NaCl (optimum without NaCl) and at pH 6.5-8.5 (optimum, pH 7.5). Yeast extract was required for growth, and the strain grew on glucose, sucrose and maltose. The major fatty acids were C_16:0 (40.2 %), iso-C_16: 0 (30.2 %) and C_16 :0 DMA (14.5 %). The genome consisted of a circular chromosome (2.5 Mb) containing 2672 predicted protein-encoding genes with a G+C content of 43.15 mol %. Based on a comparative 16S rRNA gene sequence analysis, strain F1F22^T formed a deeply branching lineage within the phylum Spirochaetota, class Spirochaetia, order Brevinematales, and had only low sequence similarity to other species of the phylum (lower than 83 %). Genome-based analysis of average nucleotide identity and digital DNA-DNA hybridization of strain F1F22^T with Treponema caldarium DSM 7334^T, Brevinema andersonii ATCC 43811^T and Spirochaeta thermophila DSM 6578^T showed values between 63.26 and 63.52 %, and between 20 and 25 %. Hence, we propose strain F1F22^T as a representative of a novel family (Thermospiraceae fam. nov.), genus and species of Brevinematales: Thermospira aquatica gen. nov., sp. nov. (type strain F1F22^T=JCM 31314^T=DSM 101182^T).

Culturomics of Bacteria from Radon-Saturated Water of the World's Oldest Radium Mine

Balneotherapeutic water springs, such as those with thermal, saline, sulfur, or any other characteristics, have recently been the subject of phylogenetic studies with a closer focus on the description and/or isolation of phylogenetically novel or biotechnologically interesting microorganisms. Generally, however, most such microorganisms are rarely obtained in pure culture or are even, for now, unculturable under laboratory conditions. In this culture-dependent study of radioactive water springs of Jáchymov (Joachimstahl), Czech Republic, we investigated a combination of classical cultivation approaches with those imitating sampling source conditions. Using these environmentally relevant cultivation approaches, over 1,000 pure cultures were successfully isolated from 4 radioactive springs. Subsequent dereplication yielded 121 unique taxonomic units spanning 44 genera and 9 taxonomic classes, ~10% of which were identified as hitherto undescribed taxa. Genomes of the latter were sequenced and analyzed, with a special focus on endogenous defense systems to withstand oxidative stress and aid in radiotolerance. Due to their origin from radioactive waters, we determined the resistance of the isolates to oxidative stress. Most of the isolates were more resistant to menadione than the model strain Deinococcus radiodurans DSM 20539T. Moreover, isolates of the Deinococcacecae, Micrococcaceae, Bacillaceae, Moraxellaceae, and Pseudomonadaceae families even exhibited higher resistance in the presence of hydrogen peroxide. In summary, our culturomic analysis shows that subsurface water springs contain diverse bacterial populations, including as-yet-undescribed taxa and strains with promising biotechnological potential. Furthermore, this study suggests that environmentally relevant cultivation techniques increase the efficiency of cultivation, thus enhancing the chance of isolating hitherto uncultured microorganisms. IMPORTANCE The mine Svornost in Jáchymov (Joachimstahl), Czech Republic is a former silver-uranium mine and the world's first and for a long time only radium mine, nowadays the deepest mine devoted to the extraction of water which is saturated with radon and has therapeutic benefits given its chemical properties. This healing water, which is approximately 13 thousand years old, is used under medical supervision for the treatment of patients with neurological and rheumatic disorders. Our culturomic approach using low concentrations of growth substrates or the environmental matrix itself (i.e., water filtrate) in culturing media combined with prolonged cultivation time resulted in the isolation of a broad spectrum of microorganisms from 4 radioactive springs of Jáchymov which are phylogenetically novel and/or bear various adaptive or coping mechanisms to thrive under selective pressure and can thus provide a wide spectrum of capabilities potentially exploitable in diverse scientific, biotechnological, or medical disciplines.

Comparative Analysis of Brucepastera parasyntrophica gen. nov., sp. nov. and Teretinema zuelzerae gen. nov., comb. nov. (Treponemataceae) Reveals the Importance of Interspecies Hydrogen Transfer in the Energy Metabolism of Spirochetes

Most members of the family Treponemataceae (Spirochaetales) are associated with vertebrate hosts. However, a diverse clade of uncultured, putatively free-living treponemes comprising several genus-level lineages is present in other anoxic environments. The only cultivated representative to date is Treponema zuelzerae, isolated from freshwater mud. Here, we describe the isolation of strain RmG11 from the intestinal tract of cockroaches. The strain represents a novel genus-level lineage of Treponemataceae and is metabolically distinct from T. zuelzerae. While T. zuelzerae grows well on various sugars, forming acetate and H₂ as major fermentation products, strain RmG11 grew poorly on glucose, maltose, and starch, forming mainly ethanol and only small amounts of acetate and H₂. In contrast to the growth of T. zuelzerae, that of strain RmG11 was strongly inhibited at high H₂ partial pressures but improved considerably when H₂ was removed from the headspace. Cocultures of strain RmG11 with the H₂-consuming Methanospirillum hungatei produced acetate and methane but no ethanol. Comparative genomic analysis revealed that strain RmG11 possesses only a single, electron-confurcating hydrogenase that forms H₂ from NADH and reduced ferredoxin, whereas T. zuelzerae also possesses a second, ferredoxin-dependent hydrogenase that allows the thermodynamically more favorable formation of H₂ from ferredoxin via the Rnf complex. In addition, we found that T. zuelzerae utilizes xylan and possesses the genomic potential to degrade other plant polysaccharides. Based on phenotypic and phylogenomic evidence, we describe strain RmG11 as Brucepastera parasyntrophica gen. nov., sp. nov. and Treponema zuelzerae as Teretinema zuelzerae gen. nov., comb. nov. IMPORTANCE Spirochetes are widely distributed in various anoxic environments and commonly form molecular hydrogen as a major fermentation product. Here, we show that two closely related members of the family Treponemataceae differ strongly in their sensitivity to high hydrogen partial pressure, and we explain the metabolic mechanisms that cause these differences by comparative genome analysis. We demonstrate a strong boost in the growth of the hydrogen-sensitive strain and a shift in its fermentation products to acetate during cocultivation with a H₂-utilizing methanogen. Our results add a hitherto unrecognized facet to the fermentative metabolism of spirochetes and also underscore the importance of interspecies hydrogen transfer in not-obligately-syntrophic interactions among fermentative and hydrogenotrophic guilds in anoxic environments.

Opportunities and challenges of using metagenomic data to bring uncultured microbes into cultivation

Although there is now an extensive understanding of the diversity of microbial life on earth through culture-independent metagenomic DNA sequence analyses, the isolation and cultivation of microbes remains critical to directly study them and confirm their metabolic and physiological functions, and their ecological roles. The majority of environmental microbes are as yet uncultured however; therefore, bringing these rare or poorly characterized groups into culture is a priority to further understand microbiome functions. Moreover, cultivated isolates may find utility in a range of applications, such as new probiotics, biocontrol agents, and agents for industrial processes. The growing abundance of metagenomic and meta-transcriptomic sequence information from a wide range of environments provides more opportunities to guide the isolation and cultivation of microbes of interest. In this paper, we discuss a range of successful methodologies and applications that have underpinned recent metagenome-guided isolation and cultivation of microbe efforts. These approaches include determining specific culture conditions to enrich for taxa of interest, to more complex strategies that specifically target the capture of microbial species through antibody engineering and genome editing strategies. With the greater degree of genomic information now available from uncultivated members, such as via metagenome-assembled genomes, the theoretical understanding of their cultivation requirements will enable greater possibilities to capture these and ultimately gain a more comprehensive understanding of the microbiomes. Video Abstract.

Exploiting unconventional prokaryotic hosts for industrial biotechnology

Developing cost-efficient biotechnological processes is a major challenge in replacing fossil-based industrial production processes. The remarkable progress in genetic engineering ensures efficient and fast tailoring of microbial metabolism for a wide range of bioconversions. However, improving intrinsic properties such as tolerance, handling, growth, and substrate consumption rates is still challenging. At the same time, synthetic biology tools are becoming easier applicable and transferable to nonmodel organisms. These trends have resulted in the exploitation of new and unconventional microbial systems with sophisticated properties, which render them promising hosts for the bio-based industry. Here, we highlight the metabolic and cellular capabilities of representative prokaryotic newcomers and discuss the potential and drawbacks of these hosts for industrial application.

Astrobiology of life on Earth

Astrobiology is mistakenly regarded by some as a field confined to studies of life beyond Earth. Here, we consider life on Earth through an astrobiological lens. Whereas classical studies of microbiology historically focused on various anthropocentric sub-fields (such as fermented foods or commensals and pathogens of crop plants, livestock and humans), addressing key biological questions via astrobiological approaches can further our understanding of all life on Earth. We highlight potential implications of this approach through the articles in this Environmental Microbiology special issue 'Ecophysiology of Extremophiles'. They report on the microbiology of places/processes including low-temperature environments and chemically diverse saline- and hypersaline habitats; aspects of sulphur metabolism in hypersaline lakes, dysoxic marine waters, and thermal acidic springs; biology of extremophile viruses; the survival of terrestrial extremophiles on the surface of Mars; biological soils crusts and rock-associated microbes of deserts; subsurface and deep biosphere, including a salticle formed within Triassic halite; and interactions of microbes with igneous and sedimentary rocks. These studies, some of which we highlight here, contribute to our understanding of the spatiotemporal reach of Earth'sfunctional biosphere, and the tenacity of terrestrial life. Their findings will help set the stage for future work focused on the constraints for life, and how organisms adapt and evolve to circumvent these constraints.

A novel β-glucosidase from a hot-spring metagenome shows elevated thermal stability and tolerance to glucose and ethanol

β-glucosidase causes hydrolysis of β-1,4-glycosidic bond in glycosides and oligosaccharides. It is an industrially important enzyme owing to its potential in biomass processing applications. In this study, computational screening of an extreme temperature aquatic habitat metagenomic resource was done, leading to the identification of a novel gene, bgl_M, encoding a β-glucosidase. The comparative protein sequence and homology structure analyses designated it as a GH1 family β-glucosidase. The bgl_M gene was expressed in a heterologous host, Escherichia coli. The purified protein, Bgl_M, was biochemically characterized for β-glucosidase activity. Bgl_M exhibited noteworthy hydrolytic potential towards cellobiose and lactose. Bgl_M, showed substantial catalytic activity in the pH range of 5.0-7.0 and at the temperature 40 °C-70 °C. The enzyme was found quite stable at 50 °C with a loss of hardly 20% after 40 h of heat exposure. Furthermore, any drastically negative effect was not observed on the enzyme's activity in the presence of metal ions, non-ionic surfactants, metal chelating, and denaturing agents. A significantly high glucose tolerance, retaining 80% relative activity at 1 M, and 40% at 5 M glucose, and ethanol tolerance, exhibiting 80% relative activity in 10% ethanol, enrolled Bgl_M as a promising enzyme for cellulose saccharification. Furthermore, its ability to catalyze the hydrolysis of daidzin and polydatin ascertained it as an admirably suited biocatalyst for enhancement of nutritional values in soya and wine industries.

The role of the "Thiodendron" consortium in postulating the karyomastigont chimaera of the endosymbiosis theory by Lynn Margulis

The endosymbiosis theory of the origin of eukaryotic cell was first proposed more than a hundred years ago. In the second half of the 20th century, Lynn Margulis suggested a new interpretation of the origin of the nucleus in modern eukaryotes. The background was the study of the consortium "Thiodendron", a symbiotic bacterial community, which includes anaerobic aerotolerant motile spirochaetes and sulfidogenic bacteria (sulfidogens) of vibrioid form with a fermentation type of metabolism. Spirochaetes supply sulfidogens with metabolites (pyruvate and, probably, organic nitrogenous products of cell lysis) and get hydrogen sulfide from sulfidogens that helps to maintain a low redox potential. At low oxygen concentrations, spirochaetes are able to assimilate glucose more efficiently. Margulis hypothesized about the symbiotic origin of the nucleus by adding the bacterium Spirochaeta to the Thermoplasma-like archaea. She considered the "Thiodendron"-like consortium to be an intermediate stage in evolution. According to Margulis, the conversion of carbohydrates and the oxidation of Н₂S to S⁰ by the bacterium provided the archaea with electron acceptors for anaerobic respiration, as shown for modern thermoplasmas and products saturated with carbon. The use of carbon sources increased by attaching the floating bacterium to the archaea. More efficient microaerobic oxidation of glucose pre-adapted the spirochaetes for association with Thermoplasma. However, modern "Thiodendron"-like consortia are not in stable symbiosis and a sulfidogenic component of the consortium is capable for fermentation, rather than anaerobic respiration, which makes the theory by Margulis disputable.