Expanding the marine virosphere using metagenomics

Genome sequences of the first Autographiviridae phages infecting marine Roseobacter

The ubiquitous and abundant marine phages play critical roles in shaping the composition and function of bacterial communities, impacting biogeochemical cycling in marine ecosystems. Autographiviridae is among the most abundant and ubiquitous phage families in the ocean. However, studies on the diversity and ecology of Autographiviridae phages in marine environments are restricted to isolates that infect SAR11 bacteria and cyanobacteria. In this study, ten new roseophages that infect marine Roseobacter strains were isolated from coastal waters. These new roseophages have a genome size ranging from 38 917 to 42 634 bp and G+C content of 44.6-50 %. Comparative genomics showed that they are similar to known Autographiviridae phages regarding gene content and architecture, thus representing the first Autographiviridae roseophages. Phylogenomic analysis based on concatenated conserved genes showed that the ten roseophages form three distinct subgroups within the Autographiviridae, and sequence analysis revealed that they belong to eight new genera. Finally, viromic read-mapping showed that these new Autographiviridae phages are widely distributed in global oceans, mostly inhabiting polar and estuarine locations. This study has expanded the current understanding of the genomic diversity, evolution and ecology of Autographiviridae phages and roseophages. We suggest that Autographiviridae phages play important roles in the mortality and community structure of roseobacters, and have broad ecological applications.

Environmental viromes reveal the global distribution signatures of deep-sea DNA viruses

INTRODUCTION

Viruses are abundant and ecologically significant in marine ecosystems. However, the virome of deep-sea sediments is not extensively investigated.

OBJECTIVES

To explore the distribution pattern of deep-sea viruses on a global scale, the viromes of DNA viruses isolated from 138 sediments of 5 deep-sea ecosystems were characterized.

METHODS

The viral particles were purified from each sediment sample. Then the viral DNAs were extracted and subjected to viral metagenomic analysis.

RESULTS

Here, we constructed a global deep-sea environmental virome dataset by analyzing the viral DNA of 138 sediment samples. A total of 347,737 viral operational taxonomic units (vOTUs) were identified, of which 84.94% were hitherto unknown, indicating that deep sea was a reservoir of novel DNA viruses. Furthermore, circular viral genome analysis revealed 98,581 complete genomes. The classified vOTUs included eukaryotic (44.55%) and prokaryotic (25.75%) viruses, and were taxonomically assigned to 63 viral families. The composition and abundance of the deep-sea sediment viromes were dependent on the deep-sea ecosystem as opposed to geographical region. Further analysis revealed that the viral community differentiation in different deep-sea ecosystems was driven by the virus-mediated energy metabolism.

CONCLUSION

Our findings showed that deep-sea ecosystems are a reservoir of novel DNA viruses and the viral community is shaped by the environmental characteristics of deep-sea ecosystems, thus presenting critical information for determining the ecological significance of viruses in global deep-sea ecosystems.

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.

Viruses under the Antarctic Ice Shelf are active and potentially involved in global nutrient cycles

Viruses play an important role in the marine ecosystem. However, our comprehension of viruses inhabiting the dark ocean, and in particular, under the Antarctic Ice Shelves, remains limited. Here, we mine single-cell genomic, transcriptomic, and metagenomic data to uncover the viral diversity, biogeography, activity, and their role as metabolic facilitators of microbes beneath the Ross Ice Shelf. This is the largest Antarctic ice shelf with a major impact on global carbon cycle. The viral community found in the cavity under the ice shelf mainly comprises endemic viruses adapted to polar and mesopelagic environments. The low abundance of genes related to lysogenic lifestyle (<3%) does not support a predominance of the Piggyback-the-Winner hypothesis, consistent with a low-productivity habitat. Our results indicate a viral community actively infecting key ammonium and sulfur-oxidizing chemolithoautotrophs (e.g. Nitrosopumilus spp, Thioglobus spp.), supporting a "kill-the-winner" dynamic. Based on genome analysis, these viruses carry specific auxiliary metabolic genes potentially involved in nitrogen, sulfur, and phosphorus acquisition. Altogether, the viruses under Antarctic ice shelves are putatively involved in programming the metabolism of ecologically relevant microbes that maintain primary production in these chemosynthetically-driven ecosystems, which have a major role in global nutrient cycles.

Viral diversity and dynamics and CRISPR-Cas-mediated immunity in a robust alkaliphilic cyanobacterial consortium

IMPORTANCE

Biotechnology applications utilizing the function of microbial communities have become increasingly important solutions as we strive for sustainable applications. Although viral infections are known to have a significant impact on microbial turnover and nutrient cycling, viral dynamics have remained largely overlooked in these engineered communities. Predatory perturbations to the functional stability of these microbial biotechnology applications must be investigated in order to design more robust applications. In this study, we closely examine virus-microbe dynamics in a model microbial community used in a biotechnology application. Our findings suggest that viral dynamics change significantly with environmental conditions and that microbial immunity may play an important role in maintaining functional stability. We present this study as a comprehensive template for other researchers interested in exploring predatory dynamics in engineered microbial communities.

Contrasting drivers of abundant phage and prokaryotic communities revealed in diverse coastal ecosystems

Phages (viruses of bacteria and archaea) are a ubiquitous top-down control on microbial communities by selectively infecting and killing cells. As obligate parasites, phages are inherently linked to processes that impact their hosts' distribution and physiology, but phages can also be impacted by external, environmental factors, such as UV radiation degrading their virions. To better understand these complex links of phages to their hosts and the environment, we leverage the unique ecological context of the Isthmus of Panama, which narrowly disconnects the productive Tropical Eastern Pacific (EP) and nutrient-poor Tropical Western Atlantic (WA) provinces. We could thus compare patterns of phage and prokaryotic communities at both global scales (between oceans) and local-scales (between habitats within an ocean). Although both phage and prokaryotic communities differed sharply between the oceans, phage community composition did not significantly differ between mangroves and reefs of the WA, while prokaryotic communities were distinct. These results suggest phages are more shaped by dispersal processes than local conditions regardless of spatial scale, while prokaryotes tend to be shaped by local conditions at smaller spatial scales. Collectively, we provide a framework for addressing the co-variability between phages and prokaryotes in marine systems and identifying factors that drive consistent versus disparate trends in community shifts, essential to informing models of biogeochemical cycles that include these interactions.

Depth-driven patterns in lytic viral diversity, auxiliary metabolic gene content, and productivity in offshore oligotrophic waters

Introduction

Marine viruses regulate microbial population dynamics and biogeochemical cycling in the oceans. The ability of viruses to manipulate hosts' metabolism through the expression of viral auxiliary metabolic genes (AMGs) was recently highlighted, having important implications in energy production and flow in various aquatic environments. Up to now, the presence and diversity of viral AMGs is studied using -omics data, and rarely using quantitative measures of viral activity alongside.

Methods

In the present study, four depth layers (5, 50, 75, and 1,000 m) with discrete hydrographic features were sampled in the Eastern Mediterranean Sea; we studied lytic viral community composition and AMG content through metagenomics, and lytic production rates through the viral reduction approach in the ultra-oligotrophic Levantine basin where knowledge regarding viral actions is rather limited.

Results and Discussion

Our results demonstrate depth-dependent patterns in viral diversity and AMG content, related to differences in temperature, nutrients availability, and host bacterial productivity and abundance. Although lytic viral production rates were similar along the water column, the virus-to-bacteria ratio was higher and the particular set of AMGs was more diverse in the bathypelagic (1,000 m) than the shallow epipelagic (5, 50, and 75 m) layers, revealing that the quantitative effect of viruses on their hosts may be the same along the water column through the intervention of different AMGs. In the resource- and energy-limited bathypelagic waters of the Eastern Mediterranean, the detected AMGs could divert hosts' metabolism toward energy production, through a boost in gluconeogenesis, fatty-acid and glycan biosynthesis and metabolism, and sulfur relay. Near the deep-chlorophyll maximum depth, an exceptionally high percentage of AMGs related to photosynthesis was noticed. Taken together our findings suggest that the roles of viruses in the deep sea might be even more important than previously thought as they seem to orchestrate energy acquisition and microbial community dynamics, and thus, biogeochemical turnover in the oceans.

Novel pelagiphage isolate Polarivirus skadi is a polar specialist that dominates SAR11-associated bacteriophage communities at high latitudes

The SAR11 clade are the most abundant members of surface marine bacterioplankton and a critical component of global biogeochemical cycles. Similarly, pelagiphages that infect SAR11 are ubiquitous and highly abundant in the oceans. Pelagiphages are predicted to shape SAR11 community structures and increase carbon turnover throughout the oceans. Yet, ecological drivers of host and niche specificity of pelagiphage populations are poorly understood. Here we report the global distribution of a novel pelagiphage called "Polarivirus skadi", which is the sole representative of a novel genus. P. skadi was isolated from the Western English Channel using a cold-water ecotype of SAR11 as bait. P. skadi is closely related to the globally dominant pelagiphage HTVC010P. Along with other HTVC010P-type viruses, P. skadi belongs to a distinct viral family within the order Caudovirales, for which we propose the name Ubiqueviridae. Metagenomic read recruitment identified P. skadi as one of the most abundant pelagiphages on Earth. P. skadi is a polar specialist, replacing HTVC010P at high latitudes. Experimental evaluation of P. skadi host range against cold- and warm-water SAR11 ecotypes supported cold-water specialism. Relative abundance of P. skadi in marine metagenomes correlated negatively with temperature, and positively with nutrients, available oxygen, and chlorophyll concentrations. In contrast, relative abundance of HTVC010P correlated negatively with oxygen and positively with salinity, with no significant correlation to temperature. The majority of other pelagiphages were scarce in most marine provinces, with a few representatives constrained to discrete ecological niches. Our results suggest that pelagiphage populations persist within a global viral seed bank, with environmental parameters and host availability selecting for a few ecotypes that dominate ocean viromes.

Genomic diversity and biogeographic distributions of a novel lineage of bacteriophages that infect marine OM43 bacteria

The marine methylotrophic OM43 clade is considered an important bacterial group in coastal microbial communities. OM43 bacteria, which are closely related to phytoplankton blooms, have small cell sizes and streamlined genomes. Bacteriophages profoundly shape the evolutionary trajectories, population dynamics, and physiology of microbes. The prevalence and diversity of several phages that infect OM43 bacteria have been reported. In this study, we isolated and sequenced two novel OM43 phages, MEP401 and MEP402. These phages share 90% of their open reading frames (ORFs) and are distinct from other known phage isolates. Furthermore, a total of 99 metagenomic viral genomes (MVGs) closely related to MEP401 and MEP402 were identified. Phylogenomic analyses suggest that MEP401, MEP402, and these identified MVGs belong to a novel subfamily in the family Zobellviridae and that they can be separated into two groups. Group I MVGs show conserved whole-genome synteny with MEP401, while group II MVGs possess the MEP401-type DNA replication module and a distinct type of morphogenesis and packaging module, suggesting that genomic recombination occurred between phages. Most members in these two groups were predicted to infect OM43 bacteria. Metagenomic read-mapping analysis revealed that the phages in these two groups are globally ubiquitous and display distinct biogeographic distributions, with some phages being predominant in cold regions, some exclusively detected in estuarine stations, and others displaying wider distributions. This study expands our knowledge of the diversity and ecology of a novel phage lineage that infects OM43 bacteria by describing their genomic diversity and global distribution patterns. IMPORTANCE OM43 phages that infect marine OM43 bacteria are important for host mortality, community structure, and physiological functions. In this study, two OM43 phages were isolated and characterized. Metagenomic viral genome (MVG) retrieval using these two OM43 phages as baits led to the identification of two phage groups of a new subfamily in the family Zobellviridae. We found that group I MVGs share similar genomic content and arrangement with MEP401 and MEP402, whereas group II MVGs only possess the MEP401-type DNA replication module. Metagenomic mapping analysis suggests that members in these two groups are globally ubiquitous with distinct distribution patterns. This study provides important insights into the genomic diversity and biogeography of the OM43 phages in the global ocean.

Identification of Structural and Morphogenesis Genes of Sulfitobacter Phage ΦGT1 and Placement within the Evolutionary History of the Podoviruses

ΦGT1 is a lytic podovirus of an alphaproteobacterial Sulfitobacter species, with few closely matching sequences among characterized phages, thus defying a useful description by simple sequence clustering methods. The history of the ΦGT1 core structure module was reconstructed using timetrees, including numerous related prospective prophages, to flesh out the evolutionary lineages spanning from the origin of the ejectosomal podovirus >3.2 Gya to the present genes of ΦGT1 and its closest relatives. A peculiarity of the ΦGT1 structural proteome is that it contains two paralogous tubular tail A (tubeA) proteins. The origin of the dual tubeA arrangement was traced to a recombination between two more ancient podoviral lineages occurring ~0.7 Gya in the alphaproteobacterial order Rhizobiales. Descendants of the ancestral dual A recombinant were tracked forward forming both temperate and lytic phage clusters and exhibiting both vertical transmission with patchy persistence and horizontal transfer with respect to host taxonomy. The two ancestral lineages were traced backward, making junctions with a major metagenomic podoviral family, the LUZ24-like gammaproteobacterial phages, and Myxococcal phage Mx8, and finally joining near the origin of podoviruses with P22. With these most conservative among phage genes, deviations from uncomplicated vertical and nonrecombinant descent are numerous but countable. The use of timetrees allowed conceptualization of the phage's evolution in the context of a sequence of ancestors spanning the time of life on Earth.

The effects of long-term hexabromocyclododecanes contamination on microbial communities in the microcosms

The adaptation of microbial community to the long-term contamination of hexabromocyclododecanes (HBCDs) has not been well studied. Our previous study found that the HBCDs contamination in the microcosms constructed of sediments from two different mangrove forests in 8 months resulted in serious acidification (pH2-3). This study reanalyzed previous sequencing data and compared them with data after 20 months to investigate the adaptive properties of microbial communities in the stress of HBCDs and acidification. It hypothesized that the reassembly was based on the fitness of taxa. The results indicated that eukaryotes and fungi might have better adaptive capacity to these deteriorated habitats. Eukaryotic taxa Eufallia and Syncystis, and fungal taxa Wickerhamomyces were only detected after 20 months of contamination. Moreover, eukaryotic taxa Caloneis and Nitzschia, and fungal taxa Talaromyces were dominant in most of microbial communities (14.467-95.941%). The functional compositions were sediment-dependent and more divergent than community reassemblies. Network and co-occurrence analysis suggested that acidophiles such as Acidisoma and Acidiphilium were gaining more positive relations in the long-term stress. The acidophilic taxa and genes involved in resistance to the acidification and toxicity of HBCDs were enriched, for example, bacteria Acidisoma and Acidiphilium, archaea Thermogymnomonas, and eukaryotes Nitzschia, and genes kdpC, odc1, polA, gst, and sod-2. These genes involved in oxidative stress response, energy metabolism, DNA damage repair, potassium transportation, and decarboxylation. It suggested that the microbial communities might cope with the stress from HBCDs and acidification via multiple pathways. The present research shed light on the evolution of microbial communities under the long-term stress of HBCDs contamination and acidification.

Isolation of phages infecting the abundant freshwater Actinobacteriota order 'Ca. Nanopelagicales'

Low-GC Actinobacteriota of the order 'Ca. Nanopelagicales' (also known as acI or hgcI clade) are abundant in freshwaters around the globe. Extensive predation pressure by phages has been assumed to be the reason for their high levels of microdiversity. So far, however, only a few metagenome-assembled phages have been proposed to infect them and no phages have been isolated. Taking advantage of recent advances in the cultivation of 'Ca. Nanopelagicales' we isolated a novel species of its genus 'Ca. Planktophila'. Using this isolate as bait, we cultivated the first two phages infecting this abundant bacterial order. Both genomes contained a whiB-like transcription factor and a RNA polymerase sigma-70 factor, which might aid in manipulating their host's metabolism. Both phages encoded a glycosyltransferase and one an anti-restriction protein, potential means to evade degradation of their DNA by nucleases present in the host genome. The two phage genomes shared only 6% of their genome with their closest relatives, with whom they form a previously uncultured family of actinophages within the Caudoviricetes. Read recruitment analyses against globally distributed metagenomes revealed the endemic distribution of this group of phages infecting 'Ca. Nanopelagicales'. The recruitment pattern against metagenomes from the isolation site and the modular distribution of shared genes between the two phages indicate high levels of horizontal gene transfer, likely mirroring the microdiversity of their host in the evolutionary arms race between host and phage.

Diverse viruses of marine archaea discovered using metagenomics

During the past decade, metagenomics became a method of choice for the discovery of novel viruses. However, host assignment for uncultured viruses remains challenging, especially for archaeal viruses, which are grossly undersampled compared to viruses of bacteria and eukaryotes. Here, we assessed the utility of CRISPR spacer targeting, tRNA gene matching and homology searches for viral signature proteins, such as major capsid proteins, for the assignment of archaeal hosts and validated these approaches on metaviromes from Yangshan Harbor (YSH). We report 35 new genomes of viruses which could be confidently assigned to hosts representing diverse lineages of marine archaea. We show that the archaeal YSH virome is highly diverse, with some viruses enriching the previously described virus groups, such as magroviruses of Marine Group II Archaea (Poseidoniales), and others representing novel groups of marine archaeal viruses. Metagenomic recruitment of Tara Oceans datasets on the YSH viral genomes demonstrated the presence of YSH Poseidoniales and Nitrososphaeria viruses in the global oceans, but also revealed the endemic YSH-specific viral lineages. Furthermore, our results highlight the relationship between the soil and marine thaumarchaeal viruses. We propose three new families within the class Caudoviricetes for the classification of the five complete viral genomes predicted to replicate in marine Poseidoniales and Nitrososphaeria, two ecologically important and widespread archaeal groups. This study illustrates the utility of viral metagenomics in exploring the archaeal virome and provides new insights into the diversity, distribution and evolution of marine archaeal viruses.

High-resolution metagenomic reconstruction of the freshwater spring bloom

BACKGROUND

The phytoplankton spring bloom in freshwater habitats is a complex, recurring, and dynamic ecological spectacle that unfolds at multiple biological scales. Although enormous taxonomic shifts in microbial assemblages during and after the bloom have been reported, genomic information on the microbial community of the spring bloom remains scarce.

RESULTS

We performed a high-resolution spatio-temporal sampling of the spring bloom in a freshwater reservoir and describe a multitude of previously unknown taxa using metagenome-assembled genomes of eukaryotes, prokaryotes, and viruses in combination with a broad array of methodologies. The recovered genomes reveal multiple distributional dynamics for several bacterial groups with progressively increasing stratification. Analyses of abundances of metagenome-assembled genomes in concert with CARD-FISH revealed remarkably similar in situ doubling time estimates for dominant genome-streamlined microbial lineages. Discordance between quantitations of cryptophytes arising from sequence data and microscopic identification suggested the presence of hidden, yet extremely abundant aplastidic cryptophytes that were confirmed by CARD-FISH analyses. Aplastidic cryptophytes are prevalent throughout the water column but have never been considered in prior models of plankton dynamics. We also recovered the first metagenomic-assembled genomes of freshwater protists (a diatom and a haptophyte) along with thousands of giant viral genomic contigs, some of which appeared similar to viruses infecting haptophytes but owing to lack of known representatives, most remained without any indication of their hosts. The contrasting distribution of giant viruses that are present in the entire water column to that of parasitic perkinsids residing largely in deeper waters allows us to propose giant viruses as the biological agents of top-down control and bloom collapse, likely in combination with bottom-up factors like a nutrient limitation.

CONCLUSION

We reconstructed thousands of genomes of microbes and viruses from a freshwater spring bloom and show that such large-scale genome recovery allows tracking of planktonic succession in great detail. However, integration of metagenomic information with other methodologies (e.g., microscopy, CARD-FISH) remains critical to reveal diverse phenomena (e.g., distributional patterns, in situ doubling times) and novel participants (e.g., aplastidic cryptophytes) and to further refine existing ecological models (e.g., factors affecting bloom collapse). This work provides a genomic foundation for future approaches towards a fine-scale characterization of the organisms in relation to the rapidly changing environment during the course of the freshwater spring bloom. Video Abstract.

FastViromeExplorer-Novel: Recovering Draft Genomes of Novel Viruses and Phages in Metagenomic Data

Despite the recent surge of viral metagenomic studies, recovering complete virus/phage genomes from metagenomic data is still extremely difficult and most viral contigs generated from de novo assembly programs are highly fragmented, posing serious challenges to downstream analysis and inference. In this study, we develop FastViromeExplorer (FVE)-novel, a computational pipeline for reconstructing complete or near-complete viral draft genomes from metagenomic data. The FVE-novel deploys FVE to efficiently map metagenomic reads to viral reference genomes, performs de novo assembly of the mapped reads to generate contigs, and extends the contigs through iterative assembly to produce final viral scaffolds. We applied FVE-novel to an ocean metagenomic sample and obtained 268 viral scaffolds that potentially come from novel viruses. Through manual examination and validation of the 10 longest scaffolds, we successfully recovered 4 complete viral genomes, 2 are novel as they cannot be found in the existing databases and the other 2 are related to known phages. This hybrid reference-based and de novo assembly approach used by FVE-novel represents a powerful new approach for uncovering near-complete viral genomes in metagenomic data.

Genomic analysis and characterization of phages infecting the marine Roseobacter CHAB-I-5 lineage reveal a globally distributed and abundant phage genus

Marine phages play an important role in marine biogeochemical cycles by regulating the death, physiological metabolism, and evolutionary trajectory of bacteria. The Roseobacter group is an abundant and important heterotrophic bacterial group in the ocean, and plays an important role in carbon, nitrogen, sulfur and phosphorus cycling. The CHAB-I-5 lineage is one of the most dominant Roseobacter lineages, but remains largely uncultured. Phages infecting CHAB-I-5 bacteria have not yet been investigated due to the lack of culturable CHAB-I-5 strains. In this study, we isolated and sequenced two new phages (CRP-901 and CRP-902) infecting the CHAB-I-5 strain FZCC0083. We applied metagenomic data mining, comparative genomics, phylogenetic analysis, and metagenomic read-mapping to investigate the diversity, evolution, taxonomy, and biogeography of the phage group represented by the two phages. The two phages are highly similar, with an average nucleotide identity of 89.17%, and sharing 77% of their open reading frames. We identified several genes involved in DNA replication and metabolism, virion structure, DNA packing, and host lysis from their genomes. Metagenomic mining identified 24 metagenomic viral genomes closely related to CRP-901 and CRP-902. Genomic comparison and phylogenetic analysis demonstrated that these phages are distinct from other known viruses, representing a novel genus-level phage group (CRP-901-type). The CRP-901-type phages do not contain DNA primase and DNA polymerase genes, but possess a novel bifunctional DNA primase-polymerase gene with both primase and polymerase activities. Read-mapping analysis showed that the CRP-901-type phages are widespread across the world's oceans and are most abundant in estuarine and polar waters. Their abundance is generally higher than other known roseophages and even higher than most pelagiphages in the polar region. In summary, this study has greatly expanded our understanding of the genetic diversity, evolution, and distribution of roseophages. Our analysis suggests that the CRP-901-type phage is an important and novel marine phage group that plays important roles in the physiology and ecology of roseobacters.

Heterologous production and characterization of a pyomelanin of Antarctic Pseudomonas sp. ANT_H4: a metabolite protecting against UV and free radicals, interacting with iron from minerals and exhibiting priming properties toward plant hairy roots

BACKGROUND

Antarctica has one of the most extreme environments in the world. This region is inhabited by specifically adapted microorganisms that produce various unique secondary metabolites (e.g. pigments) enabling their survival under the harsh environmental conditions. It was already shown that these natural, biologically active molecules may find application in various fields of biotechnology.

RESULTS

In this study, a cold-active brown-pigment-producing Pseudomonas sp. ANT_H4 strain was characterized. In-depth genomic analysis combined with the application of a fosmid expression system revealed two different pathways of melanin-like compounds biosynthesis by the ANT_H4 strain. The chromatographic behavior and Fourier-transform infrared spectroscopic analyses allowed for the identification of the extracted melanin-like compound as a pyomelanin. Furthermore, optimization of the production and thorough functional analyses of the pyomelanin were performed to test its usability in biotechnology. It was confirmed that ANT_H4-derived pyomelanin increases the sun protection factor, enables scavenging of free radicals, and interacts with the iron from minerals. Moreover, it was shown for the first time that pyomelanin exhibits priming properties toward Calendula officinalis hairy roots in in vitro cultures.

CONCLUSIONS

Results of the study indicate the significant biotechnological potential of ANT_H4-derived pyomelanin and open opportunities for future applications. Taking into account protective features of analyzed pyomelanin it may be potentially used in medical biotechnology and cosmetology. Especially interesting was showing that pyomelanin exhibits priming properties toward hairy roots, which creates a perspective for its usage for the development of novel and sustainable agrotechnical solutions.

Improving high-throughput techniques for bacteriophage discovery in multi-well plates

Bacteriophages (also called phages) are viruses of bacteria that have numerous applications in medicine, agriculture, ecology, and molecular biology. With the increasing interest in phages for their many uses, it is now especially important to make phage discovery more efficient and economical. Using the host Mycobacterium smegmatis mc²155, which is a model organism for phage discovery research and is closely related to important pathogens of humans and other animals, we investigated three procedures that are an integral part of phage discovery: enrichment of environmental samples, phage isolation and detection (which can also be used for host range determination), and phage purification. Enrichment in 6-well plates was successful with most environmental samples, and enrichment in 24- and 96-well plates was successful with some environmental samples, demonstrating that larger sample volumes are preferred when possible, but smaller sample volumes may be acceptable if the starting concentration of phages is sufficiently high. Measuring absorbance in multi-well plates was at least as sensitive as the traditional plaque assay for the detection of phages. We also demonstrated a technique for the purification of single phage types from mixed cultures in liquid medium. Multi-well techniques can be used as alternatives or complementary approaches to traditional methods of phage discovery and characterization depending on the needs of the researcher in terms of time, available resources, host species, phage-bacteria matches, and specific goals. In the future, these techniques could be applied to the discovery of phages of aquatic mycobacteria and other hosts for which few phages have currently been isolated.

Diversity and origins of bacterial and archaeal viruses on sinking particles reaching the abyssal ocean

Sinking particles and particle-associated microbes influence global biogeochemistry through particulate matter export from the surface to the deep ocean. Despite ongoing studies of particle-associated microbes, viruses in these habitats remain largely unexplored. Whether, where, and which viruses might contribute to particle production and export remain open to investigation. In this study, we analyzed 857 virus population genomes associated with sinking particles collected over three years in sediment traps moored at 4000 m in the North Pacific Subtropical Gyre. Particle-associated viruses here were linked to cellular hosts through matches to bacterial and archaeal metagenome-assembled genome (MAG)-encoded prophages or CRISPR spacers, identifying novel viruses infecting presumptive deep-sea bacteria such as Colwellia, Moritella, and Shewanella. We also identified lytic viruses whose abundances correlated with particulate carbon flux and/or were exported from the photic to abyssal ocean, including cyanophages. Our data are consistent with some of the predicted outcomes of the viral shuttle hypothesis, and further suggest that viral lysis of both autotrophic and heterotrophic prokaryotes may play a role in carbon export. Our analyses revealed the diversity and origins of prevalent viruses found on deep-sea sinking particles and identified prospective viral groups for future investigation into processes that govern particle export in the open ocean.

Expanding the Database of Signal-Anchor-Release Domain Endolysins Through Metagenomics

Endolysins are bacteriophage-derived lytic enzymes with antimicrobial activity. The action of endolysins against Gram-negative bacteria remains a challenge due to the physical protection of the outer membrane. However, recent research has demonstrated that signal-anchor-release (SAR) endolysins permeate the outer membrane of Gram-negative bacteria. This study investigates 2628 putative endolysin genes identified in 183,298 bacteriophage genomes. Previously, bioinformatic approaches resulted in a database of 66 SAR endolysins. This manuscript almost doubles the list with 53 additional SAR endolysin candidates. Forty-eight of the putative SAR endolysins described in this study contained one muramidase catalytic domain, and five included additional cell wall-binding domains at the C-terminus. For the moment, SAR domains are found in four protein families: glycoside hydrolase family 19 (GH19), glycoside hydrolase family 24 (GH24), glycoside hydrolase family 25 (GH25), and glycoside hydrolase family 108 (GH108). These SAR lysis are clustered in eight groups based on biochemical properties and domain presence/absence. Therefore, in this study, we expand the arsenal of endolysin candidates that might act against Gram-negative bacteria and develop a consult database for antimicrobial proteins derived from bacteriophages.

Newly identified HMO-2011-type phages reveal genomic diversity and biogeographic distributions of this marine viral group

Viruses play critical roles in influencing biogeochemical cycles and adjusting host mortality, population structure, physiology, and evolution in the ocean. Marine viral communities are composed of numerous genetically distinct subfamily/genus-level viral groups. Among currently identified viral groups, the HMO-2011-type group is known to be dominant and broadly distributed. However, only four HMO-2011-type cultivated representatives that infect marine SAR116 and Roseobacter strains have been reported to date, and the genetic diversity, potential hosts, and ecology of this group remain poorly elucidated. Here, we present the genomes of seven HMO-2011-type phages that were isolated using four Roseobacter strains and one SAR11 strain, as well as additional 207 HMO-2011-type metagenomic viral genomes (MVGs) identified from various marine viromes. Phylogenomic and shared-gene analyses revealed that the HMO-2011-type group is a subfamily-level group comprising at least 10 discernible genus-level subgroups. Moreover, >2000 HMO-2011-type DNA polymerase sequences were identified, and the DNA polymerase phylogeny also revealed that the HMO-2011-type group contains diverse subgroups and is globally distributed. Metagenomic read-mapping results further showed that most HMO-2011-type phages are prevalent in global oceans and display distinct geographic distributions, with the distribution of most HMO-2011-type phages being associated with temperature. Lastly, we found that members in subgroup IX, represented by pelagiphage HTVC033P, were among the most abundant HMO-2011-type phages, which implies that SAR11 bacteria are crucial hosts for this viral group. In summary, our findings substantially expand current knowledge regarding the phylogenetic diversity, evolution, and distribution of HMO-2011-type phages, highlighting HMO-2011-type phages as major ecological agents that can infect certain key bacterial groups.

Quantifying and Cataloguing Unknown Sequences within Human Microbiomes

Advances in genome sequencing technologies and lower costs have enabled the exploration of a multitude of known and novel environments and microbiomes. This has led to an exponential growth in the raw sequence data that are deposited in online repositories. Metagenomic and metatranscriptomic data sets are typically analysed with regard to a specific biological question. However, it is widely acknowledged that these data sets are comprised of a proportion of sequences that bear no similarity to any currently known biological sequence, and this so-called "dark matter" is often excluded from downstream analyses. In this study, a systematic framework was developed to assemble, identify, and measure the proportion of unknown sequences present in distinct human microbiomes. This framework was applied to 40 distinct studies, comprising 963 samples, and covering 10 different human microbiomes including fecal, oral, lung, skin, and circulatory system microbiomes. We found that while the human microbiome is one of the most extensively studied, on average 2% of assembled sequences have not yet been taxonomically defined. However, this proportion varied extensively among different microbiomes and was as high as 25% for skin and oral microbiomes that have more interactions with the environment. A rate of taxonomic characterization of 1.64% of unknown sequences being characterized per month was calculated from these taxonomically unknown sequences discovered in this study. A cross-study comparison led to the identification of similar unknown sequences in different samples and/or microbiomes. Both our computational framework and the novel unknown sequences produced are publicly available for future cross-referencing. Our approach led to the discovery of several novel viral genomes that bear no similarity to sequences in the public databases. Some of these are widespread as they have been found in different microbiomes and studies. Hence, our study illustrates how the systematic characterization of unknown sequences can help the discovery of novel microbes, and we call on the research community to systematically collate and share the unknown sequences from metagenomic studies to increase the rate at which the unknown sequence space can be classified.

Novel Viral DNA Polymerases From Metagenomes Suggest Genomic Sources of Strand-Displacing Biochemical Phenotypes

Viruses are the most abundant and diverse biological entities on the planet and constitute a significant proportion of Earth's genetic diversity. Most of this diversity is not represented by isolated viral-host systems and has only been observed through sequencing of viral metagenomes (viromes) from environmental samples. Viromes provide snapshots of viral genetic potential, and a wealth of information on viral community ecology. These data also provide opportunities for exploring the biochemistry of novel viral enzymes. The in vitro biochemical characteristics of novel viral DNA polymerases were explored, testing hypothesized differences in polymerase biochemistry according to protein sequence phylogeny. Forty-eight viral DNA Polymerase I (PolA) proteins from estuarine viromes, hot spring metagenomes, and reference viruses, encompassing a broad representation of currently known diversity, were synthesized, expressed, and purified. Novel functionality was shown in multiple PolAs. Intriguingly, some of the estuarine viral polymerases demonstrated moderate to strong innate DNA strand displacement activity at high enzyme concentration. Strand-displacing polymerases have important technological applications where isothermal reactions are desirable. Bioinformatic investigation of genes neighboring these strand displacing polymerases found associations with SNF2 helicase-associated proteins. The specific function of SNF2 family enzymes is unknown for prokaryotes and viruses. In eukaryotes, SNF2 enzymes have chromatin remodeling functions but do not separate nucleic acid strands. This suggests the strand separation function may be fulfilled by the DNA polymerase for viruses carrying SNF2 helicase-associated proteins. Biochemical data elucidated from this study expands understanding of the biology and ecological behavior of unknown viruses. Moreover, given the numerous biotechnological applications of viral DNA polymerases, novel viral polymerases discovered within viromes may be a rich source of biological material for further in vitro DNA amplification advancements.

Highly diverse flavobacterial phages isolated from North Sea spring blooms

It is generally recognized that phages are a mortality factor for their bacterial hosts. This could be particularly true in spring phytoplankton blooms, which are known to be closely followed by a highly specialized bacterial community. We hypothesized that phages modulate these dense heterotrophic bacteria successions following phytoplankton blooms. In this study, we focused on Flavobacteriia, because they are main responders during these blooms and have an important role in the degradation of polysaccharides. A cultivation-based approach was used, obtaining 44 lytic flavobacterial phages (flavophages), representing twelve new species from two viral realms. Taxonomic analysis allowed us to delineate ten new phage genera and ten new families, from which nine and four, respectively, had no previously cultivated representatives. Genomic analysis predicted various life styles and genomic replication strategies. A likely eukaryote-associated host habitat was reflected in the gene content of some of the flavophages. Detection in cellular metagenomes and by direct-plating showed that part of these phages were actively replicating in the environment during the 2018 spring bloom. Furthermore, CRISPR/Cas spacers and re-isolation during two consecutive years suggested that, at least part of the new flavophages are stable components of the microbial community in the North Sea. Together, our results indicate that these diverse flavophages have the potential to modulate their respective host populations.

Genomic Characterization of a Novel Freshwater Cyanophage Reveals a New Lineage of Cyanopodovirus

Cyanobacteria are one of the dominant autotrophs in tropical freshwater communities, yet phages infecting them remain poorly characterized. Here we present the characterization of cyanophage S-SRP02, isolated from a tropical freshwater lake in Singapore, which infects Synechococcus sp. Strain SR-C1 isolated from the same lake. S-SRP02 represents a new evolutionary lineage of cyanophage. Out of 47 open reading frames (ORFs), only 20 ORFs share homology with genes encoding proteins of known function. There is lack of auxiliary metabolic genes which was commonly found as core genes in marine cyanopodoviruses. S-SRP02 also harbors unique structural genes highly divergent from other cultured phages. Phylogenetic analysis and viral proteomic tree further demonstrate the divergence of S-SRP02 from other sequenced phage isolates. Nonetheless, S-SRP02 shares synteny with phage genes of uncultured phages obtained from the Mediterranean Sea deep chlorophyll maximum fosmids, indicating the ecological importance of S-SRP02 and its related viruses. This is further supported by metagenomic mapping of environmental viral metagenomic reads onto the S-SRP02 genome.

Bacteriophages in Biological Wastewater Treatment Systems: Occurrence, Characterization, and Function

Phage bacteria interactions can affect structure, dynamics, and function of microbial communities. In the context of biological wastewater treatment (BWT), the presence of phages can alter the efficiency of the treatment process and influence the quality of the treated effluent. The active role of phages in BWT has been demonstrated, but many questions remain unanswered regarding the diversity of phages in these engineered environments, the dynamics of infection, the determination of bacterial hosts, and the impact of their activity in full-scale processes. A deeper understanding of the phage ecology in BWT can lead the improvement of process monitoring and control, promote higher influent quality, and potentiate the use of phages as biocontrol agents. In this review, we highlight suitable methods for studying phages in wastewater adapted from other research fields, provide a critical overview on the current state of knowledge on the effect of phages on structure and function of BWT bacterial communities, and highlight gaps, opportunities, and priority questions to be addressed in future research.

Unexpected myriad of co-occurring viral strains and species in one of the most abundant and microdiverse viruses on Earth

Viral genetic microdiversity drives adaptation, pathogenicity, and speciation and has critical consequences for the viral-host arms race occurring at the strain and species levels, which ultimately impact microbial community structure and biogeochemical cycles. Despite the fact that most efforts have focused on viral macrodiversity, little is known about the microdiversity of ecologically important viruses on Earth. Recently, single-virus genomics discovered the putatively most abundant ocean virus in temperate and tropical waters: the uncultured dsDNA virus vSAG 37-F6 infecting Pelagibacter, the most abundant marine bacteria. In this study, we report the cooccurrence of up to ≈1,500 different viral strains (>95% nucleotide identity) and ≈30 related species (80-95% nucleotide identity) in a single oceanic sample. Viral microdiversity was maintained over space and time, and most alleles were the result of synonymous mutations without any apparent adaptive benefits to cope with host translation codon bias and efficiency. Gene flow analysis used to delimitate species according to the biological species concept (BSC) revealed the impact of recombination in shaping vSAG 37-F6 virus and Pelagibacter speciation. Data demonstrated that this large viral microdiversity somehow mirrors the host species diversity since ≈50% of the 926 analyzed Pelagibacter genomes were found to belong to independent BSC species that do not significantly engage in gene flow with one another. The host range of this evolutionarily successful virus revealed that a single viral species can infect multiple Pelagibacter BSC species, indicating that this virus crosses not only formal BSC barriers but also biomes since viral ancestors are found in freshwater.

Genomic Characterization of Two Novel RCA Phages Reveals New Insights into the Diversity and Evolution of Marine Viruses

Viruses are the most abundant living entities in marine ecosystems, playing critical roles in altering the structure and function of microbial communities and driving ocean biogeochemistry. Phages that infect Roseobacter clade-affiliated (RCA) cluster strains are an important component of marine viral communities. Here, we characterize the genome sequences of two new RCA phages, CRP-9 and CRP-13, which infect RCA strain FZCC0023. Genomic analysis reveals that CRP-9 and CRP-13 represent a novel evolutionary lineage of marine phages. They both have a DNA replication module most similar to those in Cobavirus group phages. In contrast, their morphogenesis and packaging modules are distinct from those in cobaviruses but homologous to those in HMO-2011-type phages. The genomic architecture of CRP-9 and CRP-13 suggests a genomic recombination event between distinct phage groups. Metagenomic data sets were examined for metagenome-assembled viral genomes (MAVGs) with similar recombinant genome architectures. Fifteen CRP-9-type MAVGs were identified from marine viromes. Additionally, 158 MAVGs were identified containing HMO-2011-type morphogenesis and packaging modules with other types of DNA replication genes, providing more evidence that recombination between different phage groups is a major driver of phage evolution. Altogether, this study significantly expands the understanding of diversity and evolution of marine roseophages. Meanwhile, the analysis of these novel RCA phages and MAVGs highlights the critical role of recombination in shaping phage diversity. These phage sequences are valuable resources for inferring the evolutionary connection of distinct phage groups. IMPORTANCE Diversity and evolution of phages that infect the relatively slow-growing but dominant Roseobacter lineages are largely unknown. In this study, RCA phages CRP-9 and CRP-13 have been isolated on a Roseobacter RCA strain and shown to have a unique genomic architecture, which appears to be the result of a recombination event. CRP-9 and CRP-13 have a DNA replication module most similar to those in Cobavirus group phages and morphogenesis and packaging modules most similar to those in HMO-2011-type phages. HMO-2011-type morphogenesis and packaging modules are found in combination with distinct types of DNA replication genes, suggesting compatibility with various DNA replication modules. Altogether, this study contributes toward a better understanding of marine viral diversity and evolution.

Pseudomonas Phage MD8: Genetic Mosaicism and Challenges of Taxonomic Classification of Lambdoid Bacteriophages

Pseudomonas phage MD8 is a temperate phage isolated from the freshwater lake Baikal. The organisation of the MD8 genome resembles the genomes of lambdoid bacteriophages. However, MD8 gene and protein sequences have little in common with classified representatives of lambda-like phages. Analysis of phage genomes revealed a group of other Pseudomonas phages related to phage MD8 and the genomic layout of MD8-like phages indicated extensive gene exchange involving even the most conservative proteins and leading to a high degree of genomic mosaicism. Multiple horizontal transfers and mosaicism of the genome of MD8, related phages and other λ-like phages raise questions about the principles of taxonomic classification of the representatives of this voluminous phage group. Comparison and analysis of various bioinformatic approaches applied to λ-like phage genomes demonstrated different efficiency and contradictory results in the estimation of genomic similarity and relatedness. However, we were able to make suggestions for the possible origin of the MD8 genome and the basic principles for the taxonomic classification of lambdoid phages. The group comprising 26 MD8-related phages was proposed to classify as two close genera belonging to a big family of λ-like phages.

Enhanced Recovery of Microbial Genes and Genomes From a Marine Water Column Using Long-Read Metagenomics

Third-generation sequencing has penetrated little in metagenomics due to the high error rate and dependence for assembly on short-read designed bioinformatics. However, second-generation sequencing metagenomics (mostly Illumina) suffers from limitations, particularly in the assembly of microbes with high microdiversity and retrieval of the flexible (adaptive) fraction of prokaryotic genomes. Here, we have used a third-generation technique to study the metagenome of a well-known marine sample from the mixed epipelagic water column of the winter Mediterranean. We have compared PacBio Sequel II with the classical approach using Illumina Nextseq short reads followed by assembly to study the metagenome. Long reads allow for efficient direct retrieval of complete genes avoiding the bias of the assembly step. Besides, the application of long reads on metagenomic assembly allows for the reconstruction of much more complete metagenome-assembled genomes (MAGs), particularly from microbes with high microdiversity such as Pelagibacterales. The flexible genome of reconstructed MAGs was much more complete containing many adaptive genes (some with biotechnological potential). PacBio Sequel II CCS appears particularly suitable for cellular metagenomics due to its low error rate. For most applications of metagenomics, from community structure analysis to ecosystem functioning, long reads should be applied whenever possible. Specifically, for in silico screening of biotechnologically useful genes, or population genomics, long-read metagenomics appears presently as a very fruitful approach and can be analyzed from raw reads before a computationally demanding (and potentially artifactual) assembly step.

Expanding our understanding of marine viral diversity through metagenomic analyses of biofilms

Recent metagenomics surveys have provided insights into the marine virosphere. However, these surveys have focused solely on viruses in seawater, neglecting those associated with biofilms. By analyzing 1.75 terabases of biofilm metagenomic data, 3974 viral sequences were identified from eight locations around the world. Over 90% of these viral sequences were not found in previously reported datasets. Comparisons between biofilm and seawater metagenomes identified viruses that are endemic to the biofilm niche. Analysis of viral sequences integrated within biofilm-derived microbial genomes revealed potential functional genes for trimeric autotransporter adhesin and polysaccharide metabolism, which may contribute to biofilm formation by the bacterial hosts. However, more than 70% of the genes could not be annotated. These findings show marine biofilms to be a reservoir of novel viruses and have enhanced our understanding of natural virus-bacteria ecosystems.

Illuminating the Virosphere Through Global Metagenomics

Viruses are the most abundant biological entity on Earth, infect cellular organisms from all domains of life, and are central players in the global biosphere. Over the last century, the discovery and characterization of viruses have progressed steadily alongside much of modern biology. In terms of outright numbers of novel viruses discovered, however, the last few years have been by far the most transformative for the field. Advances in methods for identifying viral sequences in genomic and metagenomic datasets, coupled to the exponential growth of environmental sequencing, have greatly expanded the catalog of known viruses and fueled the tremendous growth of viral sequence databases. Development and implementation of new standards, along with careful study of the newly discovered viruses, have transformed and will continue to transform our understanding of microbial evolution, ecology, and biogeochemical cycles, leading to new biotechnological innovations across many diverse fields, including environmental, agricultural, and biomedical sciences.

Genomic diversity, life strategies and ecology of marine HTVC010P-type pelagiphages

SAR11 bacteria dominate ocean surface bacterioplankton communities, and play an important role in marine carbon and nutrient cycling. The biology and ecology of SAR11 are impacted by SAR11 phages (pelagiphages) that are highly diverse and abundant in the ocean. Among the currently known pelagiphages, HTVC010P represents an extremely abundant but under-studied phage group in the ocean. In this study, we have isolated seven new HTVC010P-type pelagiphages, and recovered 77 nearly full-length HTVC010P-type metagenomic viral genomes from marine metagenomes. Comparative genomic and phylogenomic analyses showed that HTVC010P-type pelagiphages display genome synteny and can be clustered into two major subgroups, with subgroup I consisting of strictly lytic phages and subgroup II mostly consisting of phages with potential lysogenic life cycles. All but one member of the subgroup II contain an integrase gene. Site-specific integration of subgroup II HTVC010P-type pelagiphage was either verified experimentally or identified by in silico genomic sequence analyses, which revealed that various SAR11 tRNA genes can serve as the integration sites of HTVC010P-type pelagiphages. Moreover, HTVC010P-type pelagiphage integration was confirmed by the detection of several Global Ocean Survey (GOS) fragments that contain hybrid phage–host integration sites. Metagenomic recruitment analysis revealed that these HTVC010P-type phages were globally distributed and most lytic subgroup I members exhibited higher relative abundance. Altogether, this study significantly expands our knowledge about the genetic diversity, life strategies and ecology of HTVC010P-type pelagiphages.

Ancient saltern metagenomics: tracking changes in microbes and their viruses from the underground to the surface

Microbial communities in hypersaline underground waters derive from ancient organisms trapped within the evaporitic salt crystals and are part of the poorly known subterranean biosphere. Here, we characterized the viral and prokaryotic assemblages present in the hypersaline springs that dissolve Triassic-Keuper evaporite rocks and feed the Añana Salt Valley (Araba/Alava, Basque Country, Spain). Four underground water samples (around 23% total salinity) with different levels of exposure to the open air were analysed by means of microscopy and metagenomics. Cells and viruses in the spring water had lower concentrations than what are normally found in hypersaline environments and seemed to be mostly inactive. Upon exposure to the open air, there was an increase in activity of both cells and viruses as well as a selection of phylotypes. The underground water was inhabited by a rich community harbouring a diverse set of genes coding for retinal binding proteins. A total of 35 viral contigs from 15 to 104 kb, representing partial or total viral genomes, were assembled and their evolutionary changes through the spring system were followed by SNP analysis and metagenomic island tracking. Overall, both the viral and the prokaryotic assemblages changed quickly upon exposure to the open air conditions.

Ecogenomics of Groundwater Phages Suggests Niche Differentiation Linked to Specific Environmental Tolerance

Viruses are ubiquitous microbiome components, shaping ecosystems via strain-specific predation, horizontal gene transfer and redistribution of nutrients through host lysis. Viral impacts are important in groundwater ecosystems, where microbes drive many nutrient fluxes and metabolic processes; however, little is known about the diversity of viruses in these environments. We analyzed four groundwater plasmidomes (the entire plasmid content of an environment) and identified 200 viral sequences, which clustered into 41 genus-level viral clusters (approximately equivalent to viral genera) including 9 known and 32 putative new genera. We used publicly available bacterial whole-genome sequences (WGS) and WGS from 261 bacterial isolates from this groundwater environment to identify potential viral hosts. We linked 76 of the 200 viral sequences to a range of bacterial phyla, the majority associated with Proteobacteria, followed by Firmicutes, Bacteroidetes, and Actinobacteria. The publicly available WGS enabled mapping bacterial hosts to several viral sequences. The WGS of groundwater isolates increased the depth of host prediction by allowing host identification at the strain level. The latter included 4 viruses that were almost entirely (>99% query coverage, >99% identity) identified as integrated in the genomes of Pseudomonas, Acidovorax, and Castellaniella strains, resulting in high-confidence host assignments. Lastly, 21 of these viruses carried putative auxiliary metabolite genes for metal and antibiotic resistance, which might drive their infection cycles and/or provide selective advantage to infected hosts. Exploring the groundwater virome provides a necessary foundation for integration of viruses into ecosystem models where they are key players in microbial adaption to environmental stress. IMPORTANCE To our knowledge, this is the first study to identify the bacteriophage distribution in a groundwater ecosystem shedding light on their prevalence and distribution across metal-contaminated and background sites. Our study is uniquely based on selective sequencing of solely the extrachromosomal elements of a microbiome followed by analysis for viral signatures, thus establishing a more focused approach for phage identifications. Using this method, we detected several novel phage genera along with those previously established. Our approach of using the whole-genome sequences of hundreds of bacterial isolates from the same site enabled us to make host assignments with high confidence, several at strain levels. Certain phage genes suggest that they provide an environment-specific selective advantage to their bacterial hosts. Our study lays the foundation for future research on directed phage isolations using specific bacterial host strains to further characterize groundwater phages, their life cycles, and their effects on groundwater microbiome and biogeochemistry.

Marine Actinomycetes, New Sources of Biotechnological Products

The Actinomycetales order is one of great genetic and functional diversity, including diversity in the production of secondary metabolites which have uses in medical, environmental rehabilitation, and industrial applications. Secondary metabolites produced by actinomycete species are an abundant source of antibiotics, antitumor agents, anthelmintics, and antifungals. These actinomycete-derived medicines are in circulation as current treatments, but actinomycetes are also being explored as potential sources of new compounds to combat multidrug resistance in pathogenic bacteria. Actinomycetes as a potential to solve environmental concerns is another area of recent investigation, particularly their utility in the bioremediation of pesticides, toxic metals, radioactive wastes, and biofouling. Other applications include biofuels, detergents, and food preservatives/additives. Exploring other unique properties of actinomycetes will allow for a deeper understanding of this interesting taxonomic group. Combined with genetic engineering, microbial experimental evolution, and other enhancement techniques, it is reasonable to assume that the use of marine actinomycetes will continue to increase. Novel products will begin to be developed for diverse applied research purposes, including zymology and enology. This paper outlines the current knowledge of actinomycete usage in applied research, focusing on marine isolates and providing direction for future research.

Expanding standards in viromics: in silico evaluation of dsDNA viral genome identification, classification, and auxiliary metabolic gene curation

BACKGROUND

Viruses influence global patterns of microbial diversity and nutrient cycles. Though viral metagenomics (viromics), specifically targeting dsDNA viruses, has been critical for revealing viral roles across diverse ecosystems, its analyses differ in many ways from those used for microbes. To date, viromics benchmarking has covered read pre-processing, assembly, relative abundance, read mapping thresholds and diversity estimation, but other steps would benefit from benchmarking and standardization. Here we use in silico-generated datasets and an extensive literature survey to evaluate and highlight how dataset composition (i.e., viromes vs bulk metagenomes) and assembly fragmentation impact (i) viral contig identification tool, (ii) virus taxonomic classification, and (iii) identification and curation of auxiliary metabolic genes (AMGs).

RESULTS

The in silico benchmarking of five commonly used virus identification tools show that gene-content-based tools consistently performed well for long (≥3 kbp) contigs, while k-mer- and blast-based tools were uniquely able to detect viruses from short (≤3 kbp) contigs. Notably, however, the performance increase of k-mer- and blast-based tools for short contigs was obtained at the cost of increased false positives (sometimes up to ∼5% for virome and ∼75% bulk samples), particularly when eukaryotic or mobile genetic element sequences were included in the test datasets. For viral classification, variously sized genome fragments were assessed using gene-sharing network analytics to quantify drop-offs in taxonomic assignments, which revealed correct assignations ranging from ∼95% (whole genomes) down to ∼80% (3 kbp sized genome fragments). A similar trend was also observed for other viral classification tools such as VPF-class, ViPTree and VIRIDIC, suggesting that caution is warranted when classifying short genome fragments and not full genomes. Finally, we highlight how fragmented assemblies can lead to erroneous identification of AMGs and outline a best-practices workflow to curate candidate AMGs in viral genomes assembled from metagenomes.

CONCLUSION

Together, these benchmarking experiments and annotation guidelines should aid researchers seeking to best detect, classify, and characterize the myriad viruses 'hidden' in diverse sequence datasets.

Global overview and major challenges of host prediction methods for uncultivated phages

Bacterial communities play critical roles across all of Earth's biomes, affecting human health and global ecosystem functioning. They do so under strong constraints exerted by viruses, that is, bacteriophages or 'phages'. Phages can reshape bacterial communities' structure, influence long-term evolution of bacterial populations, and alter host cell metabolism during infection. Metagenomics approaches, that is, shotgun sequencing of environmental DNA or RNA, recently enabled large-scale exploration of phage genomic diversity, yielding several millions of phage genomes now to be further analyzed and characterized. One major challenge however is the lack of direct host information for these phages. Several methods and tools have been proposed to bioinformatically predict the potential host(s) of uncultivated phages based only on genome sequence information. Here we review these different approaches and highlight their distinct strengths and limitations. We also outline complementary experimental assays which are being proposed to validate and refine these bioinformatic predictions.

Efficient dilution-to-extinction isolation of novel virus-host model systems for fastidious heterotrophic bacteria

Microbes and their associated viruses are key drivers of biogeochemical processes in marine and soil biomes. While viruses of phototrophic cyanobacteria are well-represented in model systems, challenges of isolating marine microbial heterotrophs and their viruses have hampered experimental approaches to quantify the importance of viruses in nutrient recycling. A resurgence in cultivation efforts has improved the availability of fastidious bacteria for hypothesis testing, but this has not been matched by similar efforts to cultivate their associated bacteriophages. Here, we describe a high-throughput method for isolating important virus-host systems for fastidious heterotrophic bacteria that couples advances in culturing of hosts with sequential enrichment and isolation of associated phages. Applied to six monthly samples from the Western English Channel, we first isolated one new member of the globally dominant bacterial SAR11 clade and three new members of the methylotrophic bacterial clade OM43. We used these as bait to isolate 117 new phages, including the first known siphophage-infecting SAR11, and the first isolated phage for OM43. Genomic analyses of 13 novel viruses revealed representatives of three new viral genera, and infection assays showed that the viruses infecting SAR11 have ecotype-specific host ranges. Similar to the abundant human-associated phage ɸCrAss001, infection dynamics within the majority of isolates suggested either prevalent lysogeny or chronic infection, despite a lack of associated genes, or host phenotypic bistability with lysis putatively maintained within a susceptible subpopulation. Broader representation of important virus-host systems in culture collections and genomic databases will improve both our understanding of virus-host interactions, and accuracy of computational approaches to evaluate ecological patterns from metagenomic data.

Seasonal dynamics of natural Ostreococcus viral infection at the single cell level using VirusFISH

Ostreococcus is a cosmopolitan marine genus of phytoplankton found in mesotrophic and oligotrophic waters, and the smallest free-living eukaryotes known to date, with a cell diameter close to 1 μm. Ostreococcus has been extensively studied as a model system to investigate viral-host dynamics in culture, yet the impact of viruses in naturally occurring populations is largely unknown. Here, we used Virus Fluorescence in situ Hybridization (VirusFISH) to visualize and quantify viral-host dynamics in natural populations of Ostreococcus during a seasonal cycle in the central Cantabrian Sea (Southern Bay of Biscay). Ostreococcus were predominantly found during summer and autumn at surface and 50 m depth, in coastal, mid-shelf and shelf waters, representing up to 21% of the picoeukaryotic communities. Viral infection was only detected in surface waters, and its impact was variable but highest from May to July and November to December, when up to half of the population was infected. Metatranscriptomic data available from the mid-shelf station unveiled that the Ostreococcus population was dominated by the species O. lucimarinus. This work represents a proof of concept that the VirusFISH technique can be used to quantify the impact of viruses on targeted populations of key microbes from complex natural communities.

The microbial rare biosphere: current concepts, methods and ecological principles

Our ability to describe the highly diverse pool of low abundance populations present in natural microbial communities is increasing at an unprecedented pace. Yet we currently lack an integrative view of the key taxa, functions and metabolic activity which make-up this communal pool, usually referred to as the 'rare biosphere', across the domains of life. In this context, this review examines the microbial rare biosphere in its broader sense, providing an historical perspective on representative studies which enabled to bridge the concept from macroecology to microbial ecology. It then addresses our current knowledge of the prokaryotic rare biosphere, and covers emerging insights into the ecology, taxonomy and evolution of low abundance microeukaryotic, viral and host-associated communities. We also review recent methodological advances and provide a synthetic overview on how the rare biosphere fits into different conceptual models used to explain microbial community assembly mechanisms, composition and function.

Isolation and characterization of bacteriophages from the human skin microbiome that infect Staphylococcus epidermidis

Phage therapy might be a useful approach for the treatment of nosocomial infections; however, only few lytic phages suitable for this application are available for the opportunistic pathogen, Staphylococcus epidermidis. In the current study, we developed an efficient method to isolate bacteriophages present within the human skin microbiome, by using niche-specific S. epidermidis as the host for phage propagation. Staphylococcus epidermidis was identified on the forehead of 92% of human subjects tested. These isolates were then used to propagate phages present in the same skin sample. Plaques were observable on bacterial lawns in 46% of the cases where S. epidermidis was isolated. A total of eight phage genomes were genetically characterized, including the previously described phage 456. A total of six phage sequences were unique, and spanned each of the major staphylococcal phage families; Siphoviridae (n = 3), Podoviridae (n = 1) and Myoviridae (n = 2). One of the myoviruses (vB_SepM_BE06) was identified on the skin of three different humans. Comparative analysis identified novel genes including a putative N-acetylmuramoyl-L-alanine amidase gene. The host-range of each unique phage was characterized using a panel of diverse staphylococcal strains (n = 78). None of the newly isolated phages infected more than 52% of the S. epidermidis strains tested (n = 44), and non-S. epidermidis strains where rarely infected, highlighting the narrow host-range of the phages. One of the phages (vB_SepM_BE04) was capable of killing staphylococcal cells within biofilms formed on polyurethane catheters. Uncovering a richer diversity of available phages will likely improve our understanding of S. epidermidis-phage interactions, which will be important for future therapy.

Genomic Characterization and Distribution Pattern of a Novel Marine OM43 Phage

Bacteriophages have a significant impact on the structure and function of marine microbial communities. Phages of some major bacterial lineages have recently been shown to dominate the marine viral communities. However, phages that infect many important bacterial clades still remained unexplored. Members of the marine OM43 clade are methylotrophs that play important roles in C1 metabolism. OM43 phages (phages that infect the OM43 bacteria) represent an understudied viral group with only one known isolate. In this study, we describe the genomic characterization and biogeography of an OM43 phage that infects the strain HTCC2181, designated MEP301. MEP301 has a genome size of 34,774 bp. We found that MEP301 is genetically distinct from other known phage isolates and only displays significant sequence similarity with some metagenomic viral genomes (MVGs). A total of 12 MEP301-type MVGs were identified from metagenomic datasets. Comparative genomic and phylogenetic analyses revealed that MEP301-type phages can be separated into two subgroups (subgroup I and subgroup II). We also performed a metagenomic recruitment analysis to determine the relative abundance of reads mapped to these MEP301-type phages, which suggested that subgroup I MEP301-type phages are present predominantly in the cold upper waters with lower salinity. Notably, subgroup II phages have an inverse different distribution pattern, implying that they may infect hosts from a distinct OM43 subcluster. Our study has expanded the knowledge about the genomic diversity of marine OM43 phages and identified a new phage group that is widespread in the ocean.

The communities and functional profiles of virioplankton along a salinity gradient in a subtropical estuary

Viruses are the major drivers shaping microorganismal communities, and impact marine biogeochemical cycling. They are affected by various environmental parameters, such as salinity. Although the spatiotemporal distribution and dynamics of virioplankton have been extensively studied in saline environments, few detailed studies of community structure and function of viruses along salinity gradients have been conducted. Here, we used the 16S and 18S rRNA gene amplicon and metagenomic sequencing from a subtropical estuary (Pearl River Estuary, PRE; located in Shenzhen, Guangdong Province, China) to explore how viral community composition and function vary along a salinity gradient. Results showed that the detected viruses were mainly bacteriophages. The double-stranded DNA viruses were the most abundant (especially Siphoviridae, Myoviridae, Mimiviridae, Phycodnaviridae, and Podoviridae), followed by a small number of single-stranded DNA (Circoviridae) and RNA (Retroviridae) viruses. Viral biodiversity significantly declined and community structure varied greatly along the salinity gradient. The salinity, ammonium and dissolved oxygen were dominated factors influencing the community composition of viruses. Association network analysis showed that viruses had a negative effect on multiple host taxa (prokaryotic and eukaryotic species). Metagenomic data revealed that the main viral functional potential was involved in organic matter metabolism by carbohydrate-active enzymes (CAZymes). Deeper comparative functional analyses showed that viruses in the low-salinity environment had more carbohydrate-binding module and glycosidase hydrolases activities than those under high-salinity conditions. However, an opposite pattern was observed for carbohydrate esterases. These results suggest that virus-encoded CAZyme genes may alter the bacterial metabolism in estuaries. Overall, our results demonstrate that there is a spatial heterogeneity in the composition and function of virioplankton along a salinity gradient. This study enhances our understanding of viral distribution and their contribution to regulating carbon degradation throughout environments with varying salinities in subtropical estuaries.

Draft Genome Sequences of Pelagimyophage Mosig EXVC030M and Pelagipodophage Lederberg EXVC029P, Isolated from Devil's Hole, Bermuda

We present the genomes of two isolated bacteriophages infecting Pelagibacter ubique HTCC1062. Pelagibacter phage Mosig EXVC030M (Myoviridae) and Pelagibacter phage Lederberg EXVC029P (Podoviridae) were isolated by dilution-to-extinction culturing from the oxygen minimum zone at Devil’s Hole (Harrington Sound, Bermuda).

We present the genomes of two isolated bacteriophages infecting Pelagibacter ubique HTCC1062. Pelagibacter phage Mosig EXVC030M (Myoviridae) and Pelagibacter phage Lederberg EXVC029P (Podoviridae) were isolated by dilution-to-extinction culturing from the oxygen minimum zone at Devil’s Hole (Harrington Sound, Bermuda).

Benchmarking of single-virus genomics: a new tool for uncovering the virosphere

Metagenomics and single-cell genomics have enabled the discovery of relevant uncultured microbes. Recently, single-virus genomics (SVG), although still in an incipient stage, has opened new avenues in viral ecology by allowing the sequencing of one single virus at a time. The investigation of methodological alternatives and optimization of existing procedures for SVG is paramount to deliver high-quality genomic data. We report a sequencing dataset of viral single-amplified genomes (vSAGs) from cultured and uncultured viruses obtained by applying different conditions in each SVG step, from viral preservation and novel whole-genome amplification (WGA) to sequencing platforms and genome assembly. Sequencing data showed that cryopreservation and mild fixation were compatible with WGA, although fresh samples delivered better genome quality data. The novel TruPrime WGA, based on primase-polymerase features, and WGA-X employing a thermostable phi29 polymerase, were proven to be with sufficient sensitivity in SVG. The Oxford Nanopore (ON) sequencing platform did not provide a significant improvement of vSAG assembly compared to Illumina alone. Finally, the SPAdes assembler performed the best. Overall, our results represent a valuable genomic dataset that will help to standardized and advance new tools in viral ecology.

Genomes of the "Candidatus Actinomarinales" Order: Highly Streamlined Marine Epipelagic Actinobacteria

Microbiology is in a new age in which sequence databases are primary sources of information about many microbes. However, in-depth analysis of environmental genomes thus retrieved is essential to substantiate the new knowledge.

“Candidatus Actinomarinales” was defined as a subclass of exclusively marine Actinobacteria with small cells and genomes. We have collected all the available genomes in databases to assess the diversity included in this group and analyzed it by comparative genomics. We have found the equivalent of five genera and 18 genomospecies. They have genome reduction parameters equal to those of freshwater actinobacterial “Candidatus Nanopelagicales” or marine alphaproteobacterial Pelagibacterales. Genome recruitment shows that they are found only in the photic zone and mainly in surface waters, with only one genus that is found preferentially at or below the deep chlorophyll maximum. “Ca. Actinomarinales” show a highly conserved core genome (80% of the gene families conserved for the whole order) with a saturation of genomic diversity of the flexible genome at the genomospecies level. We found only a flexible genomic island preserved throughout the order; it is related to the sugar decoration of the envelope and uses several tRNAs as hot spots to increase its genomic diversity. Populations had a discrete level of sequence diversity similar to other marine microbes but drastically different from the much higher levels found for Pelagibacterales. Genomic analysis suggests that they are all aerobic photoheterotrophs with one type 1 rhodopsin and a heliorhodopsin. Like other actinobacteria, they possess the F420 coenzyme biosynthesis pathway, and its lower reduction potential could provide access to an increased range of redox chemical transformations. Last, sequence analysis revealed the first “Ca. Actinomarinales” phages, including a prophage, with metaviromic islands related to sialic acid cleavage.

IMPORTANCE Microbiology is in a new age in which sequence databases are primary sources of information about many microbes. However, in-depth analysis of environmental genomes thus retrieved is essential to substantiate the new knowledge. Here, we study 182 genomes belonging to the only known exclusively marine pelagic group of the phylum Actinobacteria. The aquatic branch of this phylum is largely known from environmental sequencing studies (single-amplified genomes [SAGs] and metagenome-assembled genomes [MAGs]), and we have collected and analyzed the available information present in databases about the “Ca. Actinomarinales.” They are among the most streamlined microbes to live in the epipelagic zone of the ocean, and their study is critical to obtain a proper view of the diversity of Actinobacteria and their role in aquatic ecosystems.

Reconstructing Genomes of Carbon Monoxide Oxidisers in Volcanic Deposits Including Members of the Class Ktedonobacteria

Microorganisms can potentially colonise volcanic rocks using the chemical energy in reduced gases such as methane, hydrogen (H₂) and carbon monoxide (CO). In this study, we analysed soil metagenomes from Chilean volcanic soils, representing three different successional stages with ages of 380, 269 and 63 years, respectively. A total of 19 metagenome-assembled genomes (MAGs) were retrieved from all stages with a higher number observed in the youngest soil (1640: 2 MAGs, 1751: 1 MAG, 1957: 16 MAGs). Genomic similarity indices showed that several MAGs had amino-acid identity (AAI) values >50% to the phyla Actinobacteria, Acidobacteria, Gemmatimonadetes, Proteobacteria and Chloroflexi. Three MAGs from the youngest site (1957) belonged to the class Ktedonobacteria (Chloroflexi). Complete cellular functions of all the MAGs were characterised, including carbon fixation, terpenoid backbone biosynthesis, formate oxidation and CO oxidation. All 19 environmental genomes contained at least one gene encoding a putative carbon monoxide dehydrogenase (CODH). Three MAGs had form I coxL operon (encoding the large subunit CO-dehydrogenase). One of these MAGs (MAG-1957-2.1, Ktedonobacterales) was highly abundant in the youngest soil. MAG-1957-2.1 also contained genes encoding a [NiFe]-hydrogenase and hyp genes encoding accessory enzymes and proteins. Little is known about the Ktedonobacterales through cultivated isolates, but some species can utilise H₂ and CO for growth. Our results strongly suggest that the remote volcanic sites in Chile represent a natural habitat for Ktedonobacteria and they may use reduced gases for growth.

Origin and Evolution of Studiervirinae Bacteriophages Infecting Pectobacterium: Horizontal Transfer Assists Adaptation to New Niches

Black leg and soft rot are devastating diseases causing up to 50% loss of potential potato yield. The search for, and characterization of, bacterial viruses (bacteriophages) suitable for the control of these diseases is currently a sought-after task for agricultural microbiology. Isolated lytic Pectobacterium bacteriophages Q19, PP47 and PP81 possess a similar broad host range but differ in their genomic properties. The genomic features of characterized phages have been described and compared to other Studiervirinae bacteriophages. Thorough phylogenetic analysis has clarified the taxonomy of the phages and their positioning relative to other genera of the Autographiviridae family. Pectobacterium phage Q19 seems to represent a new genus not described previously. The genomes of the phages are generally similar to the genome of phage T7 of the Teseptimavirus genus but possess a number of specific features. Examination of the structure of the genes and proteins of the phages, including the tail spike protein, underlines the important role of horizontal gene exchange in the evolution of these phages, assisting their adaptation to Pectobacterium hosts. The results provide the basis for the development of bacteriophage-based biocontrol of potato soft rot as an alternative to the use of antibiotics.

Culturing novel and abundant pelagiphages in the ocean

Viruses play a key role in biogeochemical cycling and host mortality, metabolism, physiology and evolution in the ocean. Viruses that infect the globally abundant SAR11 bacteria (pelagiphages) were reported to be an important component of the marine viral communities. Our current knowledge of pelagiphages is based on a few studies and therefore is limited. In this study, 10 new pelagiphages were isolated and genomically characterized. These pelagiphages represent the first cultivated representatives of four viral lineages only found in metagenomic sequencing datasets previously. Many abundant environmental viral sequences, i.e., single-virus vSAG 37-F6 and several Global Ocean Viromes (GOV) viral populations, are now further confirmed with these pelagiphages. Viromic read mapping reveals that these new pelagiphages are globally distributed in the ocean and can be detected throughout the water column. Remarkably, isolation of these pelagiphages contributed up to 12% of all viromic reads annotated in the analysed viromes. Altogether, this study has greatly broadened our understanding of pelagiphages regarding their morphology, genetic diversity, infection strategies, and distribution pattern. The availability of these newly isolated pelagiphages and their genome sequences will allow us to further explore their infectivities and ecological strategies.