Genomes of six viruses that infect Asgard archaea from deep-sea sediments

Ancient Host-Virus Gene Transfer Hints at a Diverse Pre-LECA Virosphere

The details surrounding the early evolution of eukaryotes and their viruses are largely unknown. Several key enzymes involved in DNA synthesis and transcription are shared between eukaryotes and large DNA viruses in the phylum Nucleocytoviricota, but the evolutionary relationships between these genes remain unclear. In particular, previous studies of eukaryotic DNA and RNA polymerases often show deep-branching clades of eukaryotes and viruses indicative of ancient gene exchange. Here, we performed updated phylogenetic analysis of eukaryotic and viral family B DNA polymerases, multimeric RNA polymerases, and mRNA-capping enzymes to explore their evolutionary relationships. Our results show that viral enzymes form clades that are typically adjacent to eukaryotes, suggesting that they originate prior to the emergence of the Last Eukaryotic Common Ancestor (LECA). The machinery for viral DNA replication, transcription, and mRNA capping are all key processes needed for the maintenance of virus factories, which are complex structures formed by many nucleocytoviruses during infection, indicating that viruses capable of making these structures are ancient. These findings hint at a diverse and complex pre-LECA virosphere and indicate that large DNA viruses may encode proteins that are relics of extinct proto-eukaryotic lineages.

Discovery and characterization of complete genomes of 38 head-tailed proviruses in four predominant phyla of archaea

Archaea play a significant role in natural ecosystems and the human body. Archaeal viruses exert a considerable influence on the structure and composition of archaeal communities and their associated ecological environments. The present study revealed the complete genomes of 38 archaeal head-tailed proviruses through comprehensive data mining. The hosts of these proviruses were identified as belonging to the following four dominant phyla: Halobacteriota, Thermoplasmatota, Thermoproteota, and Nanoarchaeota. In addition to the 14 proviruses of halophilic archaea related to the Graaviviridae family, the remaining proviruses exhibited limited genetic similarities to known (pro)viruses, suggesting the existence of 14 potential novel families. Of the 38 archaeal proviruses, 30 have the potential to lyse host cells. Eleven proviruses contain genes linked to antiviral defense mechanisms, including those involved in restriction modification (RM), clustered regularly interspaced short palindromic repeat (CRISPR)-associated (CRISPR-Cas) nucleases, defense island system associated with restriction-modification (DISARM), and DNA degradation (Dnd). Moreover, auxiliary metabolic genes were identified in the proviruses of Bathyarchaeia and Halobacteriota archaea, including those involved in carbohydrate and amino acid metabolism. Our findings indicate the diversity of archaeal viruses, their interactions with archaeal hosts, and their roles in the adaptation of the host.IMPORTANCEThe field of archaeal virology has seen a rapid expansion through the use of metagenomics, yet the diversity of these viruses remains largely uncharted. In this study, the complete genomes of 38 novel archaeal proviruses were identified for the following four dominant phyla: Halobacteriota, Thermoplasmatota, Thermoproteota, and Nanoarchaeota. Two families and six genera of Archaea were the first to be identified as hosts for viruses. The proviruses were found to contain diverse genes that were involved in distinct adaptation strategies of viruses to hosts. Our findings contribute to the expansion of the lineages of archaeal viruses and highlight their intricate interactions and essential roles in enabling host survival and adaptation to diverse environmental conditions.

Metagenomic characterization of viruses and mobile genetic elements associated with the DPANN archaeal superphylum

The archaeal superphylum DPANN (an acronym formed from the initials of the first five phyla discovered: Diapherotrites, Parvarchaeota, Aenigmarchaeota, Nanohaloarchaeota and Nanoarchaeota) is a group of ultrasmall symbionts able to survive in extreme ecosystems. The diversity and dynamics between DPANN archaea and their virome remain largely unknown. Here we use a metagenomic clustered regularly interspaced short palindromic repeats (CRISPR) screening approach to identify 97 globally distributed, non-redundant viruses and unclassified mobile genetic elements predicted to infect hosts across 8 DPANN phyla, including 7 viral groups not previously characterized. Genomic analysis suggests a diversity of viral morphologies including head-tailed, tailless icosahedral and spindle-shaped viruses with the potential to establish lytic, chronic or lysogenic infections. We also find evidence of a virally encoded Cas12f1 protein (probably originating from uncultured DPANN archaea) and a mini-CRISPR array, which could play a role in modulating host metabolism. Many metagenomes have virus-to-host ratios >10, indicating that DPANN viruses play an important role in controlling host populations. Overall, our study illuminates the underexplored diversity, functional repertoires and host interactions of the DPANN virome.

Revitalized abyssal ancient viruses trigger viral pandemic in terrestrial soil

Viruses are the most abundant biological entities on the earth. Some ancient viruses can revive from permafrost along with melting to infect the current hosts. The "zombie viruses" trapped in the ancient deep-sea sediments become the public health concern due to the environmental changes and human activities in deep oceans. However, the biosecurity risk of benthic viruses has not been explored. Here, two viruses purified from the ancient deep-sea sediments were infectious to the bacteria of terrestrial soil. Furthermore, the benthic viruses were purified from each of 106 deep-sea sediments with 1,900-17,300 years old and then the biothreats of deep-sea viruses to terrestrial soil were evaluated on a global scale. The results revealed that the viruses purified from each of 9 sediments could disturb the native bacterial communities in soil and destroy the soil functions. These viruses with the capacity to invade soil were widely distributed in the abyssal sea. Therefore, our findings highlighted the revitalized risks of deep-sea ancient viruses to terrestrial soil ecosystems for the first time. The biosecurity of deep-sea viruses to terrestrial soil should be assessed before performing deep-sea mining and scientific activities.

A metagenomic catalogue of the ruminant gut archaeome

While the ruminant gut archaeome regulates the gut microbiota and hydrogen balance, it is also a major producer of the greenhouse gas methane. However, ruminant gut archaeome diversity within the gastrointestinal tract (GIT) of ruminant animals worldwide remains largely underexplored. Here, we construct a catalogue of 998 unique archaeal genomes recovered from the GITs of ruminants, utilizing 2270 metagenomic samples across 10 different ruminant species. Most of the archaeal genomes (669/998 = 67.03%) belong to Methanobacteriaceae and Methanomethylophilaceae (198/998 = 19.84%). We recover 47/279 previously undescribed archaeal genomes at the strain level with completeness of >80% and contamination of <5%. We also investigate the archaeal gut biogeography across various ruminants and demonstrate that archaeal compositional similarities vary significantly by breed and gut location. The catalogue contains 42,691 protein clusters, and the clustering and methanogenic pathway analysis reveal strain- and host-specific dependencies among ruminant animals. We also find that archaea potentially carry antibiotic and metal resistance genes, mobile genetic elements, virulence factors, quorum sensors, and complex archaeal viromes. Overall, this catalogue is a substantial repository for ruminant archaeal recourses, providing potential for advancing our understanding of archaeal ecology and discovering strategies to regulate methane production in ruminants.

Complete genomes of Asgard archaea reveal diverse integrated and mobile genetic elements

Asgard archaea are of great interest as the progenitors of Eukaryotes, but little is known about the mobile genetic elements (MGEs) that may shape their ongoing evolution. Here, we describe MGEs that replicate in Atabeyarchaeia, a wetland Asgard archaea lineage represented by two complete genomes. We used soil depth-resolved population metagenomic data sets to track 18 MGEs for which genome structures were defined and precise chromosome integration sites could be identified for confident host linkage. Additionally, we identified a complete 20.67 kbp circular plasmid and two family-level groups of viruses linked to Atabeyarchaeia, via CRISPR spacer targeting. Closely related 40 kbp viruses possess a hypervariable genomic region encoding combinations of specific genes for small cysteine-rich proteins structurally similar to restriction-homing endonucleases. One 10.9 kbp integrative conjugative element (ICE) integrates genomically into the Atabeyarchaeum deiterrae-1 chromosome and has a 2.5 kbp circularizable element integrated within it. The 10.9 kbp ICE encodes an expressed Type IIG restriction-modification system with a sequence specificity matching an active methylation motif identified by Pacific Biosciences (PacBio) high-accuracy long-read (HiFi) metagenomic sequencing. Restriction-modification of Atabeyarchaeia differs from that of another coexisting Asgard archaea, Freyarchaeia, which has few identified MGEs but possesses diverse defense mechanisms, including DISARM and Hachiman, not found in Atabeyarchaeia. Overall, defense systems and methylation mechanisms of Asgard archaea likely modulate their interactions with MGEs, and integration/excision and copy number variation of MGEs in turn enable host genetic versatility.

Host range and cell recognition of archaeal viruses

Archaea are members of a separate domain of life that have unique properties, such as the composition of their cell walls and the structure of their lipid bilayers. Consequently, archaeal viruses face different challenges to infect host cells in comparison with viruses of bacteria and eukaryotes. Despite their significant impact on shaping microbial communities, our understanding of infection processes of archaeal viruses remains limited. Several receptors used by archaeal viruses to infect cells have recently been identified. The interactions between viruses and receptors are one of the determinants of the host range of viruses. Here, we review the current literature on host ranges of archaeal viruses and factors that might impact the width of these host ranges.

Large-scale single-virus genomics uncovers hidden diversity of river water viruses and diversified gene profiles

Environmental viruses (primarily bacteriophages) are widely recognized as playing an important role in ecosystem homeostasis through the infection of host cells. However, the majority of environmental viruses are still unknown as their mosaic structure and frequent mutations in their sequences hinder genome construction in current metagenomics. To enable the large-scale acquisition of environmental viral genomes, we developed a new single-viral genome sequencing platform with microfluidic-generated gel beads. Amplification of individual DNA viral genomes in mass-produced gel beads allows high-throughput genome sequencing compared to conventional single-virus genomics. The sequencing analysis of river water samples yielded 1431 diverse viral single-amplified genomes, whereas viral metagenomics recovered 100 viral metagenome-assembled genomes at the comparable sequence depth. The 99.5% of viral single-amplified genomes were determined novel at the species level, most of which could not be recovered by a metagenomic assembly. The large-scale acquisition of diverse viral genomes identified protein clusters commonly detected in different viral strains, allowing the gene transfer to be tracked. Moreover, comparative genomics within the same viral species revealed that the profiles of various methyltransferase subtypes were diverse, suggesting an enhanced escape from host bacterial internal defense mechanisms. Our use of gel bead-based single-virus genomics will contribute to exploring the nature of viruses by accelerating the accumulation of draft genomes of environmental DNA viruses.

Archaeal virus entry and egress

Archaeal viruses display a high degree of structural and genomic diversity. Few details are known about the mechanisms by which these viruses enter and exit their host cells. Research on archaeal viruses has lately made significant progress due to advances in genetic tools and imaging techniques, such as cryo-electron tomography (cryo-ET). In recent years, a steady output of newly identified archaeal viral receptors and egress mechanisms has offered the first insight into how archaeal viruses interact with the archaeal cell envelope. As more details about archaeal viral entry and egress are unravelled, patterns are starting to emerge. This helps to better understand the interactions between viruses and the archaeal cell envelope and how these compare to infection strategies of viruses in other domains of life. Here, we provide an overview of recent developments in the field of archaeal viral entry and egress, shedding light onto the most elusive part of the virosphere.

Diversity of Bathyarchaeia viruses in metagenomes and virus-encoded CRISPR system components

Bathyarchaeia represent a class of archaea common and abundant in sedimentary ecosystems. Here we report 56 metagenome-assembled genomes of Bathyarchaeia viruses identified in metagenomes from different environments. Gene sharing network and phylogenomic analyses led to the proposal of four virus families, including viruses of the realms Duplodnaviria and Adnaviria, and archaea-specific spindle-shaped viruses. Genomic analyses uncovered diverse CRISPR elements in these viruses. Viruses of the proposed family "Fuxiviridae" harbor an atypical Type IV-B CRISPR-Cas system and a Cas4 protein that might interfere with host immunity. Viruses of the family "Chiyouviridae" encode a Cas2-like endonuclease and two mini-CRISPR arrays, one with a repeat identical to that in the host CRISPR array, potentially allowing the virus to recruit the host CRISPR adaptation machinery to acquire spacers that could contribute to competition with other mobile genetic elements or to inhibit host defenses. These findings present an outline of the Bathyarchaeia virome and offer a glimpse into their counter-defense mechanisms.

Exploring the Archaeal Virosphere by Metagenomics

During the past decade, environmental research has demonstrated that archaea are abundant and widespread in nature and play important ecological roles at a global scale. Currently, however, the majority of archaeal lineages cannot be cultivated under laboratory conditions and are known exclusively or nearly exclusively through metagenomics. A similar trend extends to the archaeal virosphere, where isolated representatives are available for a handful of model archaeal virus-host systems. Viral metagenomics provides an alternative way to circumvent the limitations of culture-based virus discovery and offers insight into the diversity, distribution, and environmental impact of uncultured archaeal viruses. Presently, metagenomics approaches have been successfully applied to explore the viromes associated with various lineages of extremophilic and mesophilic archaea, including Asgard archaea (Asgardarchaeota), ANME-1 archaea (Methanophagales), thaumarchaea (Nitrososphaeria), altiarchaea (Altiarchaeota), and marine group II archaea (Poseidoniales). Here, we provide an overview of methods widely used in archaeal virus metagenomics, covering metavirome preparation, genome annotation, phylogenetic and phylogenomic analyses, and archaeal host assignment. We hope that this summary will contribute to further exploration and characterization of the enigmatic archaeal virome lurking in diverse environments.

Phylogenomic analysis expands the known repertoire of single-stranded DNA viruses in benthic zones of the South Indian Ocean

Single-stranded (ss) DNA viruses are ubiquitous and constitute some of the most diverse entities on Earth. Most studies have focused on ssDNA viruses from terrestrial environments resulting in a significant deficit in benthic ecosystems including aphotic zones of the South Indian Ocean (SIO). Here, we assess the diversity and phylogeny of ssDNA in deep waters of the SIO using a combination of established viral taxonomy tools and a Hidden Markov Model based approach. Replication initiator protein-associated (Rep) phylogenetic reconstruction and sequence similarity networks were used to show that the SIO hosts divergent and as yet unknown circular Rep-encoding ssDNA viruses. Several sequences appear to represent entirely novel families, expanding the repertoire of known ssDNA viruses. Results suggest that a small proportion of these viruses may be circular genetic elements, which may strongly influence the diversity of both eukaryotes and prokaryotes in the SIO. Taken together, our data show that the SIO harbours a diverse assortment of previously unknown ssDNA viruses. Due to their potential to infect a variety of hosts, these viruses may be crucial for marine nutrient recycling through their influence of the biological carbon pump.

Diversity of Bathyarchaeia viruses in metagenomes and virus-encoded CRISPR system components

Bathyarchaeia represent a class of archaea common and abundant in sedimentary ecosystems. The virome of Bathyarchaeia so far has not been characterized. Here we report 56 metagenome-assembled genomes of Bathyarchaeia viruses identified in metagenomes from different environments. Gene sharing network and phylogenomic analyses led to the proposal of four virus families, including viruses of the realms Duplodnaviria and Adnaviria, and archaea-specific spindle-shaped viruses. Genomic analyses uncovered diverse CRISPR elements in these viruses. Viruses of the proposed family 'Fuxiviridae' harbor an atypical type IV-B CRISPR-Cas system and a Cas4 protein that might interfere with host immunity. Viruses of the family 'Chiyouviridae' encode a Cas2-like endonuclease and two mini-CRISPR arrays, one with a repeat identical to that in the host CRISPR array, potentially allowing the virus to recruit the host CRISPR adaptation machinery to acquire spacers that could contribute to competition with other mobile genetic elements or to inhibition of host defenses. These findings present an outline of the Bathyarchaeia virome and offer a glimpse into their counter-defense mechanisms.

Influence of N-Glycosylation on Virus-Host Interactions in Halorubrum lacusprofundi

N-glycosylation is a post-translational modification of proteins that occurs across all three domains of life. In Archaea, N-glycosylation is crucial for cell stability and motility, but importantly also has significant implications for virus-host interactions. While some archaeal viruses present glycosylated proteins or interact with glycosylated host proteins, the direct influence of N-glycosylation on archaeal virus-host interactions remains to be elucidated. In this study, we generated an N-glycosylation-deficient mutant of Halorubrum lacusprofundi, a halophilic archaeon commonly used to study cold adaptation, and examined the impact of compromised N-glycosylation on the infection dynamics of two very diverse viruses. While compromised N-glycosylation had no influence on the life cycle of the head-tailed virus HRTV-DL1, we observed a significant effect on membrane-containing virus HFPV-1. Both intracellular genome numbers and extracellular virus particle numbers of HFPV-1 were increased in the mutant strain, which we attribute to instability of the surface-layer which builds the protein envelope of the cell. When testing the impact of compromised N-glycosylation on the life cycle of plasmid vesicles, specialized membrane vesicles that transfer a plasmid between host cells, we determined that plasmid vesicle stability is strongly dependent on the host glycosylation machinery. Our study thus provides important insight into the role of N-glycosylation in virus-host interactions in Archaea, while pointing to how this influence strongly differs amongst various viruses and virus-like elements.

The virome of the last eukaryotic common ancestor and eukaryogenesis

All extant eukaryotes descend from the last eukaryotic common ancestor (LECA), which is thought to have featured complex cellular organization. To gain insight into LECA biology and eukaryogenesis-the origin of the eukaryotic cell, which remains poorly understood-we reconstructed the LECA virus repertoire. We compiled an inventory of eukaryotic hosts of all major virus taxa and reconstructed the LECA virome by inferring the origins of these groups of viruses. The origin of the LECA virome can be traced back to a small set of bacterial-not archaeal-viruses. This provenance of the LECA virome is probably due to the bacterial origin of eukaryotic membranes, which is most compatible with two endosymbiosis events in a syntrophic model of eukaryogenesis. In the first endosymbiosis, a bacterial host engulfed an Asgard archaeon, preventing archaeal viruses from entry owing to a lack of archaeal virus receptors on the external membranes.

Potential Auxiliary Metabolic Capabilities and Activities Reveal Biochemical Impacts of Viruses in Municipal Wastewater Treatment Plants

Viruses influence biogeochemical cycles in oceans, freshwater, soil, and human gut through infection and by modulating virocell metabolism through virus-encoded auxiliary metabolic genes (vAMGs). However, the geographical distribution, potential metabolic function, and engineering significance of vAMGs in wastewater treatment plants (WWTPs) remain to be explored. Here, 752 single-contig viral genomes with high confidence, 510 of which belonged to Caudovirales, were recovered from the activated sludge metagenomes of 32 geographically distributed WWTPs. A total of 101 vAMGs involved in various metabolic pathways were identified, the most common of which were the queuosine biosynthesis genes folE, queD, and queE and the sulfur metabolism gene cysH. Phylogenetic analysis and virus-host relationship prediction revealed the probable evolutionary histories of vAMGs involved in carbon (acpP and prsA), nitrogen (amoC), sulfur (cysH), and phosphate (phoH) metabolism, which potentially mediate microbial carbon and nutrient cycling. Notably, 11 of the 38 (28.3%) vAMGs identified in the metagenomes with corresponding metatranscriptomes were transcriptionally expressed, implying an active functional state. This meta-analysis provides the first broad catalog of vAMGs in municipal WWTPs and how they may assist in the basic physiological reactions of their microbial hosts or nutrient cycling in the WWTPs, and therefore, may have important effects on the engineering of wastewater treatment processes.

Metagenomic analysis reveals unexplored diversity of archaeal virome in the human gut

The human gut microbiome has been extensively explored, while the archaeal viruses remain largely unknown. Here, we present a comprehensive analysis of the archaeal viruses from the human gut metagenomes and the existing virus collections using the CRISPR spacer and viral signature-based approach. This results in 1279 viral species, of which, 95.2% infect Methanobrevibacteria_A, 56.5% shared high identity (>95%) with the archaeal proviruses, 37.2% have a host range across archaeal species, and 55.7% are highly prevalent in the human population (>1%). A methanogenic archaeal virus-specific gene for pseudomurein endoisopeptidase (PeiW) frequently occurs in the viral sequences (n = 150). Analysis of 33 Caudoviricetes viruses with a complete genome often discovers the genes (integrase, n = 29; mazE, n = 10) regulating the viral lysogenic-lytic cycle, implying the dominance of temperate viruses in the archaeal virome. Together, our work uncovers the unexplored diversity of archaeal viruses, revealing the novel facet of the human gut microbiome.

Non-negligible roles of archaea in coastal carbon biogeochemical cycling

Coastal zones are among the world's most productive ecosystems. They store vast amounts of organic carbon, as 'blue carbon' reservoirs, and impact global climate change. Archaeal communities are integral components of coastal microbiomes but their ecological roles are often overlooked. However, archaeal diversity, metabolism, evolution, and interactions, revealed by recent studies using rapidly developing cutting-edge technologies, place archaea as important players in coastal carbon biogeochemical cycling. We here summarize the latest advances in the understanding of archaeal carbon cycling processes in coastal ecosystems, specifically, archaeal involvement in CO₂ fixation, organic biopolymer transformation, and methane metabolism. We also showcase the potential to use of archaeal communities to increase carbon sequestration and reduce methane production, with implications for mitigating climate change.

The expanding Asgard archaea invoke novel insights into Tree of Life and eukaryogenesis

The division of organisms on the Tree of Life into either a three-domain (3D) tree or a two-domain (2D) tree has been disputed for a long time. Ever since the discovery of Archaea by Carl Woese in 1977 using 16S ribosomal RNA sequence as the evolutionary marker, there has been a great advance in our knowledge of not only the growing diversity of Archaea but also the evolutionary relationships between different lineages of living organisms. Here, we present this perspective to summarize the progress of archaeal diversity and changing notion of the Tree of Life. Meanwhile, we provide the latest progress in genomics/physiology-based discovery of Asgard archaeal lineages as the closest relative of Eukaryotes. Furthermore, we propose three major directions for future research on exploring the "next one" closest Eukaryote relative, deciphering the function of archaeal eukaryotic signature proteins and eukaryogenesis from both genomic and physiological aspects, and understanding the roles of horizontal gene transfer, viruses, and mobile elements in eukaryogenesis.

Three families of Asgard archaeal viruses identified in metagenome-assembled genomes

Asgardarchaeota harbour many eukaryotic signature proteins and are widely considered to represent the closest archaeal relatives of eukaryotes. Whether similarities between Asgard archaea and eukaryotes extend to their viromes remains unknown. Here we present 20 metagenome-assembled genomes of Asgardarchaeota from deep-sea sediments of the basin off the Shimokita Peninsula, Japan. By combining a CRISPR spacer search of metagenomic sequences with phylogenomic analysis, we identify three family-level groups of viruses associated with Asgard archaea. The first group, verdandiviruses, includes tailed viruses of the class Caudoviricetes (realm Duplodnaviria); the second, skuldviruses, consists of viruses with predicted icosahedral capsids of the realm Varidnaviria; and the third group, wyrdviruses, is related to spindle-shaped viruses previously identified in other archaea. More than 90% of the proteins encoded by these viruses of Asgard archaea show no sequence similarity to proteins encoded by other known viruses. Nevertheless, all three proposed families consist of viruses typical of prokaryotes, providing no indication of specific evolutionary relationships between viruses infecting Asgard archaea and eukaryotes. Verdandiviruses and skuldviruses are likely to be lytic, whereas wyrdviruses potentially establish chronic infection and are released without host cell lysis. All three groups of viruses are predicted to play important roles in controlling Asgard archaea populations in deep-sea ecosystems.

A closed Candidatus Odinarchaeum chromosome exposes Asgard archaeal viruses

Asgard archaea have recently been identified as the closest archaeal relatives of eukaryotes. Their ecology, and particularly their virome, remain enigmatic. We reassembled and closed the chromosome of Candidatus Odinarchaeum yellowstonii LCB_4, through long-range PCR, revealing CRISPR spacers targeting viral contigs. We found related viruses in the genomes of diverse prokaryotes from geothermal environments, including other Asgard archaea. These viruses open research avenues into the ecology and evolution of Asgard archaea.