Ocean plankton. Patterns and ecological drivers of ocean viral communities

Genome analysis of three novel lytic Vibrio coralliilyticus phages isolated from seawater, Okinawa, Japan

Three novel Vibrio phages were isolated from seawater in Okinawa. The Vibrio phage RYC infected Vibrio coralliilyticus SWA 07, while Vibrio phages CKB-S1 and CKB-S2 infected the coral pathogen V. coralliilyticus P1 (LMG 23696). The Vibrio phages CKB-S1 and CKB-S2 displayed head-tail structures whereas the Vibrio phage RYC showed a tailless non-enveloped capsid. All these Vibrio phages contained linear and double-stranded DNA. The whole genome sequencing revealed that Vibrio phage RYC has a larger genome size compared to Vibrio phages CKB-S1 and CKB-S2, and six tRNAs genes were found only in Vibrio phage RYC. Genome-wide comparison showed that Vibrio phage CKB-S1 was closely related, but was not identical, to Vibrio parahaemolyticus phages VP16T and VP16C. Meanwhile, the Vibrio phages RYC and CKB-S2 did not show high genome-wide similarity to any phages. These results suggest that the Vibrio phages CKB-S1, CKB-S2 and RYC are novel phages, which need further exploration, especially for their potential applications in phage therapy.

Comparative Omics and Trait Analyses of Marine Pseudoalteromonas Phages Advance the Phage OTU Concept

Viruses influence the ecology and evolutionary trajectory of microbial communities. Yet our understanding of their roles in ecosystems is limited by the paucity of model systems available for hypothesis generation and testing. Further, virology is limited by the lack of a broadly accepted conceptual framework to classify viral diversity into evolutionary and ecologically cohesive units. Here, we introduce genomes, structural proteomes, and quantitative host range data for eight Pseudoalteromonas phages isolated from Helgoland (North Sea, Germany) and use these data to advance a genome-based viral operational taxonomic unit (OTU) definition. These viruses represent five new genera and inform 498 unaffiliated or unannotated protein clusters (PCs) from global virus metagenomes. In a comparison of previously sequenced Pseudoalteromonas phage isolates (n = 7) and predicted prophages (n = 31), the eight phages are unique. They share a genus with only one other isolate, Pseudoalteromonas podophage RIO-1 (East Sea, South Korea) and two Pseudoalteromonas prophages. Mass-spectrometry of purified viral particles identified 12–20 structural proteins per phage. When combined with 3-D structural predictions, these data led to the functional characterization of five previously unidentified major capsid proteins. Protein functional predictions revealed mechanisms for hijacking host metabolism and resources. Further, they uncovered a hybrid sipho-myovirus that encodes genes for Mu-like infection rarely described in ocean systems. Finally, we used these data to evaluate a recently introduced definition for virus populations that requires members of the same population to have >95% average nucleotide identity across at least 80% of their genes. Using physiological traits and genomics, we proposed a conceptual model for a viral OTU definition that captures evolutionarily cohesive and ecologically distinct units. In this trait-based framework, sensitive hosts are considered viral niches, while host ranges and infection efficiencies are tracked as viral traits. Quantitative host range assays revealed conserved traits within virus OTUs that break down between OTUs, suggesting the defined units capture niche and fitness differentiation. Together these analyses provide a foundation for model system-based hypothesis testing that will improve our understanding of marine copiotrophs, as well as phage–host interactions on the ocean particles and aggregates where Pseudoalteromonas thrive.

Marine viruses discovered via metagenomics shed light on viral strategies throughout the oceans

Marine viruses are key drivers of host diversity, population dynamics and biogeochemical cycling and contribute to the daily flux of billions of tons of organic matter. Despite recent advancements in metagenomics, much of their biodiversity remains uncharacterized. Here we report a data set of 27,346 marine virome contigs that includes 44 complete genomes. These outnumber all currently known phage genomes in marine habitats and include members of previously uncharacterized lineages. We designed a new method for host prediction based on co-occurrence associations that reveals these viruses infect dominant members of the marine microbiome such as Prochlorococcus and Pelagibacter. A negative association between host abundance and the virus-to-host ratio supports the recently proposed Piggyback-the-Winner model of reduced phage lysis at higher host densities. An analysis of the abundance patterns of viruses throughout the oceans revealed how marine viral communities adapt to various seasonal, temperature and photic regimes according to targeted hosts and the diversity of auxiliary metabolic genes.

Aquarium Viromes: Viromes of Human-Managed Aquatic Systems

An aquarium ecosystem is home to many animal species providing conditions similar to native aquatic habitats but under highly controlled management. With a growing interest in understanding the interaction of microbiomes and resident animal health within aquarium environments, we undertook a metagenomic survey of viromes in seven aquarium systems with differing physicochemical and resident animal profiles. Our results show that a diverse array of viruses was represented in aquarium viromes, many of which were widespread in different aquarium systems (27 common viral families in all of the aquarium systems). Most viromes were dominated by DNA phages of the order Caudovirales as commonly found in other aquatic environments with average relative abundance greater than 64%. The composition and structure of aquarium viromes were associated with controlled system parameters, including nitrate, salinity, and temperature as well as resident animal profiles, indicating the close interaction of viromes with aquarium management practices. Furthermore, finding human associated viruses in a touch exhibit suggested that exposure of aquarium systems to human contact may lead to introduction of human cutaneous viruses into aquaria. This is consistent with the high abundance of skin microflora on the palms of healthy individuals and their detection in recreational waters, such as swimming pools. Lastly, assessment of antibiotic resistance genes (ARGs) in aquarium viromes revealed a unique signature of ARGs in different aquarium systems with trimethoprim being the most common. This is the first study to provide vital information on viromes and their unique relationships with management practices in a human-built and controlled aquarium environment.

Virus-host relationships of marine single-celled eukaryotes resolved from metatranscriptomics

Establishing virus–host relationships has historically relied on culture-dependent approaches. Here we report on the use of marine metatranscriptomics to probe virus–host relationships. Statistical co-occurrence analyses of dsDNA, ssRNA and dsRNA viral markers of polyadenylation-selected RNA sequences from microbial communities dominated by Aureococcus anophagefferens (Quantuck Bay, NY), and diatoms (Narragansett Bay, RI) show active infections by diverse giant viruses (NCLDVs) associated with algal and nonalgal hosts. Ongoing infections of A. anophagefferens by a known Mimiviridae (AaV) occur during bloom peak and decline. Bloom decline is also accompanied by increased activity of viruses other than AaV, including (+) ssRNA viruses. In Narragansett Bay, increased temporal resolution reveals active NCLDVs with both ‘boom-and-bust’ and ‘steady-state infection’-like ecologies that include known as well as novel virus–host interactions. Our approach offers a method for screening active viral infections and develops links between viruses and their potential hosts in situ. Our observations further demonstrate that previously unknown virus–host relationships in marine systems are abundant.

Viruses are partners in ecosystem ecology, yet their study has been primarily limited to laboratory models virus-host or derived from metagenomics. Here, Moniruzzaman et al. use metatranscriptomics to resolve interactions between giant viruses and single-celled eukaryotic hosts.

Single-virus genomics reveals hidden cosmopolitan and abundant viruses

Microbes drive ecosystems under constraints imposed by viruses. However, a lack of virus genome information hinders our ability to answer fundamental, biological questions concerning microbial communities. Here we apply single-virus genomics (SVGs) to assess whether portions of marine viral communities are missed by current techniques. The majority of the here-identified 44 viral single-amplified genomes (vSAGs) are more abundant in global ocean virome data sets than published metagenome-assembled viral genomes or isolates. This indicates that vSAGs likely best represent the dsDNA viral populations dominating the oceans. Species-specific recruitment patterns and virome simulation data suggest that vSAGs are highly microdiverse and that microdiversity hinders the metagenomic assembly, which could explain why their genomes have not been identified before. Altogether, SVGs enable the discovery of some of the likely most abundant and ecologically relevant marine viral species, such as vSAG 37-F6, which were overlooked by other methodologies.

Putative archaeal viruses from the mesopelagic ocean

Oceanic viruses that infect bacteria, or phages, are known to modulate host diversity, metabolisms, and biogeochemical cycling, while the viruses that infect marine Archaea remain understudied despite the critical ecosystem roles played by their hosts. Here we introduce "MArVD", for Metagenomic Archaeal Virus Detector, an annotation tool designed to identify putative archaeal virus contigs in metagenomic datasets. MArVD is made publicly available through the online iVirus analytical platform. Benchmarking analysis of MArVD showed it to be >99% accurate and 100% sensitive in identifying the 127 known archaeal viruses among the 12,499 viruses in the VirSorter curated dataset. Application of MArVD to 10 viral metagenomes from two depth profiles in the Eastern Tropical North Pacific (ETNP) oxygen minimum zone revealed 43 new putative archaeal virus genomes and large genome fragments ranging in size from 10 to 31 kb. Network-based classifications, which were consistent with marker gene phylogenies where available, suggested that these putative archaeal virus contigs represented six novel candidate genera. Ecological analyses, via fragment recruitment and ordination, revealed that the diversity and relative abundances of these putative archaeal viruses were correlated with oxygen concentration and temperature along two OMZ-spanning depth profiles, presumably due to structuring of the host Archaea community. Peak viral diversity and abundances were found in surface waters, where Thermoplasmata 16S rRNA genes are prevalent, suggesting these archaea as hosts in the surface habitats. Together these findings provide a baseline for identifying archaeal viruses in sequence datasets, and an initial picture of the ecology of such viruses in non-extreme environments.

A Novel Roseosiphophage Isolated from the Oligotrophic South China Sea

The Roseobacter clade is abundant and widespread in marine environments and plays an important role in oceanic biogeochemical cycling. In this present study, a lytic siphophage (labeled vB_DshS-R5C) infecting the strain type of Dinoroseobacter shibae named DFL12^T, which is part of the Roseobacter clade, was isolated from the oligotrophic South China Sea. Phage R5C showed a narrow host range, short latent period and low burst size. The genome length of phage R5C was 77, 874 bp with a G+C content of 61.5%. Genomic comparisons detected no genome matches in the GenBank database and phylogenetic analysis based on DNA polymerase I revealed phylogenetic features that were distinct to other phages, suggesting the novelty of R5C. Several auxiliary metabolic genes (e.g., phoH gene, heat shock protein and queuosine biosynthesis genes) were identified in the R5C genome that may be beneficial to the host and/or offer a competitive advantage for the phage. Among siphophages infecting the Roseobacter clade (roseosiphophages), four gene transfer agent-like genes were commonly located with close proximity to structural genes, suggesting that their function may be related to the tail of siphoviruses. The isolation and characterization of R5C demonstrated the high genomic and physiological diversity of roseophages as well as improved our understanding of host-phage interactions and the ecology of the marine Roseobacter.

Genomic exploration of individual giant ocean viruses

Viruses are major pathogens in all biological systems. Virus propagation and downstream analysis remains a challenge, particularly in the ocean where the majority of their microbial hosts remain recalcitrant to current culturing techniques. We used a cultivation-independent approach to isolate and sequence individual viruses. The protocol uses high-speed fluorescence-activated virus sorting flow cytometry, multiple displacement amplification (MDA), and downstream genomic sequencing. We focused on 'giant viruses' that are readily distinguishable by flow cytometry. From a single-milliliter sample of seawater collected from off the dock at Boothbay Harbor, ME, USA, we sorted almost 700 single virus particles, and subsequently focused on a detailed genome analysis of 12. A wide diversity of viruses was identified that included Iridoviridae, extended Mimiviridae and even a taxonomically novel (unresolved) giant virus. We discovered a viral metacaspase homolog in one of our sorted virus particles and discussed its implications in rewiring host metabolism to enhance infection. In addition, we demonstrated that viral metacaspases are widespread in the ocean. We also discovered a virus that contains both a reverse transcriptase and a transposase; although highly speculative, we suggest such a genetic complement would potentially allow this virus to exploit a latency propagation mechanism. Application of single virus genomics provides a powerful opportunity to circumvent cultivation of viruses, moving directly to genomic investigation of naturally occurring viruses, with the assurance that the sequence data is virus-specific, non-chimeric and contains no cellular contamination.

vConTACT: an iVirus tool to classify double-stranded DNA viruses that infect Archaea and Bacteria

Taxonomic classification of archaeal and bacterial viruses is challenging, yet also fundamental for developing a predictive understanding of microbial ecosystems. Recent identification of hundreds of thousands of new viral genomes and genome fragments, whose hosts remain unknown, requires a paradigm shift away from traditional classification approaches and towards the use of genomes for taxonomy. Here we revisited the use of genomes and their protein content as a means for developing a viral taxonomy for bacterial and archaeal viruses. A network-based analytic was evaluated and benchmarked against authority-accepted taxonomic assignments and found to be largely concordant. Exceptions were manually examined and found to represent areas of viral genome 'sequence space' that are under-sampled or prone to excessive genetic exchange. While both cases are poorly resolved by genome-based taxonomic approaches, the former will improve as viral sequence space is better sampled and the latter are uncommon. Finally, given the largely robust taxonomic capabilities of this approach, we sought to enable researchers to easily and systematically classify new viruses. Thus, we established a tool, vConTACT, as an app at iVirus, where it operates as a fast, highly scalable, user-friendly app within the free and powerful CyVerse cyberinfrastructure.

Novel Abundant Oceanic Viruses of Uncultured Marine Group II Euryarchaeota

Marine group II Euryarchaeota (MG-II) are among the most abundant microbes in oceanic surface waters [1, 2, 3, 4]. So far, however, representatives of MG-II have not been cultivated, and no viruses infecting these organisms have been described. Here, we present complete genomes for three distinct groups of viruses assembled from metagenomic sequence datasets highly enriched for MG-II. These novel viruses, which we denote magroviruses, possess double-stranded DNA genomes of 65 to 100 kilobases in size that encode a structural module characteristic of head-tailed viruses and, unusually for archaeal and bacterial viruses, a nearly complete replication apparatus of apparent archaeal origin. The newly identified magroviruses are widespread and abundant and therefore are likely to be major ecological agents.

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A novel viral group, magroviruses, likely infects marine group II archaea

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Magroviruses are highly abundant in oceanic surface waters worldwide

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Magroviruses have linear, double-stranded DNA genomes of about 100 kilobases

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Magroviruses encode a near complete replication apparatus of apparent archaeal origin

Intriguing Interaction of Bacteriophage-Host Association: An Understanding in the Era of Omics

Innovations in next-generation sequencing technology have introduced new avenues in microbial studies through “omics” approaches. This technology has considerably augmented the knowledge of the microbial world without isolation prior to their identification. With an enormous volume of bacterial “omics” data, considerable attempts have been recently invested to improve an insight into virosphere. The interplay between bacteriophages and their host has created a significant influence on the biogeochemical cycles, microbial diversity, and bacterial population regulation. This review highlights various concepts such as genomics, transcriptomics, proteomics, and metabolomics to infer the phylogenetic affiliation and function of bacteriophages and their impact on diverse microbial communities. Omics technologies illuminate the role of bacteriophage in an environment, the influences of phage proteins on the bacterial host and provide information about the genes important for interaction with bacteria. These investigations will reveal some of bio-molecules and biomarkers of the novel phage which demand to be unveiled.

Nucleic and Amino Acid Sequences Support Structure-Based Viral Classification

Viral capsids ensure viral genome integrity by protecting the enclosed nucleic acids. Interactions between the genome and capsid and between individual capsid proteins (i.e., capsid architecture) are intimate and are expected to be characterized by strong evolutionary conservation. For this reason, a capsid structure-based viral classification has been proposed as a way to bring order to the viral universe. The seeming lack of sufficient sequence similarity to reproduce this classification has made it difficult to reject structural convergence as the basis for the classification. We reinvestigate whether the structure-based classification for viral coat proteins making icosahedral virus capsids is in fact supported by previously undetected sequence similarity. Since codon choices can influence nascent protein folding cotranslationally, we searched for both amino acid and nucleotide sequence similarity. To demonstrate the sensitivity of the approach, we identify a candidate gene for the pandoravirus capsid protein. We show that the structure-based classification is strongly supported by amino acid and also nucleotide sequence similarities, suggesting that the similarities are due to common descent. The correspondence between structure-based and sequence-based analyses of the same proteins shown here allow them to be used in future analyses of the relationship between linear sequence information and macromolecular function, as well as between linear sequence and protein folds.

IMPORTANCE Viral capsids protect nucleic acid genomes, which in turn encode capsid proteins. This tight coupling of protein shell and nucleic acids, together with strong functional constraints on capsid protein folding and architecture, leads to the hypothesis that capsid protein-coding nucleotide sequences may retain signatures of ancient viral evolution. We have been able to show that this is indeed the case, using the major capsid proteins of viruses forming icosahedral capsids. Importantly, we detected similarity at the nucleotide level between capsid protein-coding regions from viruses infecting cells belonging to all three domains of life, reproducing a previously established structure-based classification of icosahedral viral capsids.

A Pelagic Microbiome (Viruses to Protists) from a Small Cup of Seawater

The aquatic microbiome is composed of a multi-phylotype community of microbes, ranging from the numerically dominant viruses to the phylogenetically diverse unicellular phytoplankton. They influence key biogeochemical processes and form the base of marine food webs, becoming food for secondary consumers. Due to recent advances in next-generation sequencing, this previously overlooked component of our hydrosphere is starting to reveal its true diversity and biological complexity. We report here that 250 mL of seawater is sufficient to provide a comprehensive description of the microbial diversity in an oceanic environment. We found that there was a dominance of the order Caudovirales (59%), with the family Myoviridae being the most prevalent. The families Phycodnaviridae and Mimiviridae made up the remainder of pelagic double-stranded DNA (dsDNA) virome. Consistent with this analysis, the Cyanobacteria dominate (52%) the prokaryotic diversity. While the dinoflagellates and their endosymbionts, the superphylum Alveolata dominates (92%) the microbial eukaryotic diversity. A total of 834 prokaryotic, 346 eukaryotic and 254 unique virus phylotypes were recorded in this relatively small sample of water. We also provide evidence, through a metagenomic-barcoding comparative analysis, that viruses are the likely source of microbial environmental DNA (meDNA). This study opens the door to a more integrated approach to oceanographic sampling and data analysis.

Prediction of virus-host infectious association by supervised learning methods

Background

The study of virus-host infectious association is important for understanding the functions and dynamics of microbial communities. Both cellular and fractionated viral metagenomic data generate a large number of viral contigs with missing host information. Although relative simple methods based on the similarity between the word frequency vectors of viruses and bacterial hosts have been developed to study virus-host associations, the problem is significantly understudied. We hypothesize that machine learning methods based on word frequencies can be efficiently used to study virus-host infectious associations.

Methods

We investigate four different representations of word frequencies of viral sequences including the relative word frequency and three normalized word frequencies by subtracting the number of expected from the observed word counts. We also study five machine learning methods including logistic regression, support vector machine, random forest, Gaussian naive Bayes and Bernoulli naive Bayes for separating infectious from non-infectious viruses for nine bacterial host genera with at least 45 infecting viruses. Area under the receiver operating characteristic curve (AUC) is used to compare the performance of different machine learning method and feature combinations. We then evaluate the performance of the best method for the identification of the hosts of contigs in metagenomic studies. We also develop a maximum likelihood method to estimate the fraction of true infectious viruses for a given host in viral tagging experiments.

Results

Based on nine bacterial host genera with at least 45 infectious viruses, we show that random forest together with the relative word frequency vector performs the best in identifying viruses infecting particular hosts. For all the nine host genera, the AUC is over 0.85 and for five of them, the AUC is higher than 0.98 when the word size is 6 indicating the high accuracy of using machine learning approaches for identifying viruses infecting particular hosts. We also show that our method can predict the hosts of viral contigs of length at least 1kbps in metagenomic studies with high accuracy. The random forest together with word frequency vector outperforms current available methods based on Manhattan and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$d_{2}^{*}$\end{document}d2∗ dissimilarity measures. Based on word frequencies, we estimate that about 95% of the identified T4-like viruses in viral tagging experiment infect Synechococcus, while only about 29% of the identified non-T4-like viruses and 30% of the contigs in the study potentially infect Synechococcus.

Conclusions

The random forest machine learning method together with the relative word frequencies as features of viruses can be used to predict viruses and viral contigs for specific bacterial hosts. The maximum likelihood approach can be used to estimate the fraction of true infectious associated viruses in viral tagging experiments.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-017-1473-7) contains supplementary material, which is available to authorized users.

Environmental Viral Genomes Shed New Light on Virus-Host Interactions in the Ocean

Viruses are diverse and play significant ecological roles in marine ecosystems. However, our knowledge of genome-level diversity in viruses is biased toward those isolated from few culturable hosts. Here, we determined 1,352 nonredundant complete viral genomes from marine environments. Lifting the uncertainty that clouds short incomplete sequences, whole-genome-wide analysis suggests that these environmental genomes represent hundreds of putative novel viral genera. Predicted hosts include dominant groups of marine bacteria and archaea with no isolated viruses to date. Some of the viral genomes encode many functionally related enzymes, suggesting a strong selection pressure on these marine viruses to control cellular metabolisms by accumulating genes.

Metagenomics has revealed the existence of numerous uncharacterized viral lineages, which are referred to as viral “dark matter.” However, our knowledge regarding viral genomes is biased toward culturable viruses. In this study, we analyzed 1,600 (1,352 nonredundant) complete double-stranded DNA viral genomes (10 to 211 kb) assembled from 52 marine viromes. Together with 244 previously reported uncultured viral genomes, a genome-wide comparison delineated 617 genus-level operational taxonomic units (OTUs) for these environmental viral genomes (EVGs). Of these, 600 OTUs contained no representatives from known viruses, thus putatively corresponding to novel viral genera. Predicted hosts of the EVGs included major groups of marine prokaryotes, such as marine group II Euryarchaeota and SAR86, from which no viruses have been isolated to date, as well as Flavobacteriaceae and SAR116. Our analysis indicates that marine cyanophages are already well represented in genome databases and that one of the EVGs likely represents a new cyanophage lineage. Several EVGs encode many enzymes that appear to function for an efficient utilization of iron-sulfur clusters or to enhance host survival. This suggests that there is a selection pressure on these marine viruses to accumulate genes for specific viral propagation strategies. Finally, we revealed that EVGs contribute to a 4-fold increase in the recruitment of photic-zone viromes compared with the use of current reference viral genomes.

IMPORTANCE Viruses are diverse and play significant ecological roles in marine ecosystems. However, our knowledge of genome-level diversity in viruses is biased toward those isolated from few culturable hosts. Here, we determined 1,352 nonredundant complete viral genomes from marine environments. Lifting the uncertainty that clouds short incomplete sequences, whole-genome-wide analysis suggests that these environmental genomes represent hundreds of putative novel viral genera. Predicted hosts include dominant groups of marine bacteria and archaea with no isolated viruses to date. Some of the viral genomes encode many functionally related enzymes, suggesting a strong selection pressure on these marine viruses to control cellular metabolisms by accumulating genes.

Advances in the application of high-throughput sequencing in invertebrate virology

Over the last decade, advances in high-throughput sequencing technologies have revolutionised biological research, making it possible for DNA/RNA sequencing of any organism of interest to be undertaken. Sequencing approaches are now routinely used in the detection and characterisation of (novel) viruses, investigation of host-pathogen interactions, and effective development of disease treatment strategies. For the sequencing and identification of viruses of interest, metagenomics approaches using infected host tissue are frequently used, as it is not always possible to culture and isolate these pathogens. High-throughput sequencing can also be used to investigate host-pathogen interactions by investigating (temporal) transcriptomic responses of both the host and virus, potentially leading to the discovery of novel opportunities for treatment and drug targets. In addition, viruses in environmental samples (e.g. water or soil samples) can be identified using eDNA/metagenomics approaches. The promise that recent developments in sequencing brings to the field of invertebrate virology are not devoid of technical challenges, including the need for better laboratory and bioinformatics strategies to sequence and assemble virus genomes within complex tissue or environmental samples, and the difficulties associated with the annotation of the large number of novel viruses being discovered.

Viruses infecting marine molluscs

Although a wide range of viruses have been reported in marine molluscs, most of these reports rely on ultrastructural examination and few of these viruses have been fully characterized. The lack of marine mollusc cell lines restricts virus isolation capacities and subsequent characterization works. Our current knowledge is mostly restricted to viruses affecting farmed species such as oysters Crassostrea gigas, abalone Haliotis diversicolor supertexta or the scallop Chlamys farreri. Molecular approaches which are needed to identify virus affiliation have been carried out for a small number of viruses, most of them belonging to the Herpesviridae and birnaviridae families. These last years, the use of New Generation Sequencing approach has allowed increasing the number of sequenced viral genomes and has improved our capacity to investigate the diversity of viruses infecting marine molluscs. This new information has in turn allowed designing more efficient diagnostic tools. Moreover, the development of experimental infection protocols has answered some questions regarding the pathogenesis of these viruses and their interactions with their hosts. Control and management of viral diseases in molluscs mostly involve active surveillance, implementation of effective bio security measures and development of breeding programs. However factors triggering pathogen development and the life cycle and status of the viruses outside their mollusc hosts still need further investigations.

Global occurrence and heterogeneity of the Roseobacter-clade species Ruegeria mobilis

Tropodithietic acid (TDA)-producing Ruegeria mobilis strains of the Roseobacter clade have primarily been isolated from marine aquaculture and have probiotic potential due to inhibition of fish pathogens. We hypothesized that TDA producers with additional novel features are present in the oceanic environment. We isolated 42 TDA-producing R. mobilis strains during a global marine research cruise. While highly similar on the 16S ribosomal RNA gene level (99-100% identity), the strains separated into four sub-clusters in a multilocus sequence analysis. They were further differentiated to the strain level by average nucleotide identity using pairwise genome comparison. The four sub-clusters could not be associated with a specific environmental niche, however, correlated with the pattern of sub-typing using co-isolated phages, the number of prophages in the genomes and the distribution in ocean provinces. Major genomic differences within the sub-clusters include prophages and toxin-antitoxin systems. In general, the genome of R. mobilis revealed adaptation to a particle-associated life style and querying TARA ocean data confirmed that R. mobilis is more abundant in the particle-associated fraction than in the free-living fraction occurring in 40% and 6% of the samples, respectively. Our data and the TARA data, although lacking sufficient data from the polar regions, demonstrate that R. mobilis is a globally distributed marine bacterial species found primarily in the upper open oceans. It has preserved key phenotypic behaviors such as the production of TDA, but contains diverse sub-clusters, which could provide new capabilities for utilization in aquaculture.

Seasonal Dynamics and Metagenomic Characterization of Marine Viruses in Goseong Bay, Korea

Viruses are the most abundant biological entities in the oceans, and account for a significant amount of the genetic diversity of marine ecosystems. However, there is little detailed information about the biodiversity of viruses in marine environments. Rapid advances in metagenomics have enabled the identification of previously unknown marine viruses. We performed metagenomic profiling of seawater samples collected at 6 sites in Goseong Bay (South Sea, Korea) during the spring, summer, autumn, and winter of 2014. The results indicated the presence of highly diverse virus communities. The DNA libraries from samples collected during four seasons were sequenced using Illumina HiSeq 2000. The number of viral reads was 136,850 during March, 70,651 during June, 66,165 during September, and 111,778 during December. Species identification indicated that Pelagibacter phage HTVC010P, Ostreococcus lucimarinus OIV5 and OIV1, and Roseobacter phage SIO1 were the most common species in all samples. For viruses with at least 10 reads, there were 204 species during March, 189 during June, 170 during September, and 173 during December. Analysis of virus families indicated that the Myoviridae was the most common during all four seasons, and viruses in the Polyomaviridae were only present during March. Viruses in the Iridoviridae were only present during three seasons. Additionally, viruses in the Iridoviridae, Herpesviridae, and Poxviridae, which may affect fish and marine animals, appeared during different seasons. These results suggest that seasonal changes in temperature contribute to the dynamic structure of the viral community in the study area. The information presented here will be useful for comparative analyses with other marine viral communities.

Microbiota Transfer Therapy alters gut ecosystem and improves gastrointestinal and autism symptoms: an open-label study

BACKGROUND

Autism spectrum disorders (ASD) are complex neurobiological disorders that impair social interactions and communication and lead to restricted, repetitive, and stereotyped patterns of behavior, interests, and activities. The causes of these disorders remain poorly understood, but gut microbiota, the 1013 bacteria in the human intestines, have been implicated because children with ASD often suffer gastrointestinal (GI) problems that correlate with ASD severity. Several previous studies have reported abnormal gut bacteria in children with ASD. The gut microbiome-ASD connection has been tested in a mouse model of ASD, where the microbiome was mechanistically linked to abnormal metabolites and behavior. Similarly, a study of children with ASD found that oral non-absorbable antibiotic treatment improved GI and ASD symptoms, albeit temporarily. Here, a small open-label clinical trial evaluated the impact of Microbiota Transfer Therapy (MTT) on gut microbiota composition and GI and ASD symptoms of 18 ASD-diagnosed children.

RESULTS

MTT involved a 2-week antibiotic treatment, a bowel cleanse, and then an extended fecal microbiota transplant (FMT) using a high initial dose followed by daily and lower maintenance doses for 7-8 weeks. The Gastrointestinal Symptom Rating Scale revealed an approximately 80% reduction of GI symptoms at the end of treatment, including significant improvements in symptoms of constipation, diarrhea, indigestion, and abdominal pain. Improvements persisted 8 weeks after treatment. Similarly, clinical assessments showed that behavioral ASD symptoms improved significantly and remained improved 8 weeks after treatment ended. Bacterial and phagedeep sequencing analyses revealed successful partial engraftment of donor microbiota and beneficial changes in the gut environment. Specifically, overall bacterial diversity and the abundance of Bifidobacterium, Prevotella, and Desulfovibrio among other taxa increased following MTT, and these changes persisted after treatment stopped (followed for 8 weeks).

CONCLUSIONS

This exploratory, extended-duration treatment protocol thus appears to be a promising approach to alter the gut microbiome and virome and improve GI and behavioral symptoms of ASD. Improvements in GI symptoms, ASD symptoms, and the microbiome all persisted for at least 8 weeks after treatment ended, suggesting a long-term impact.

TRIAL REGISTRATION

This trial was registered on the ClinicalTrials.gov, with the registration number  NCT02504554.

Virus-host interactions and their roles in coral reef health and disease

Coral reefs occur in nutrient-poor shallow waters, constitute biodiversity and productivity hotspots, and are threatened by anthropogenic disturbance. This Review provides an introduction to coral reef virology and emphasizes the links between viruses, coral mortality and reef ecosystem decline. We describe the distinctive benthic-associated and water-column- associated viromes that are unique to coral reefs, which have received less attention than viruses in open-ocean systems. We hypothesize that viruses of bacteria and eukaryotes dynamically interact with their hosts in the water column and with scleractinian (stony) corals to influence microbial community dynamics, coral bleaching and disease, and reef biogeochemical cycling. Last, we outline how marine viruses are an integral part of the reef system and suggest that the influence of viruses on reef function is an essential component of these globally important environments.

Consensus statement: Virus taxonomy in the age of metagenomics

The number and diversity of viral sequences that are identified in metagenomic data far exceeds that of experimentally characterized virus isolates. In a recent workshop, a panel of experts discussed the proposal that, with appropriate quality control, viruses that are known only from metagenomic data can, and should be, incorporated into the official classification scheme of the International Committee on Taxonomy of Viruses (ICTV). Although a taxonomy that is based on metagenomic sequence data alone represents a substantial departure from the traditional reliance on phenotypic properties, the development of a robust framework for sequence-based virus taxonomy is indispensable for the comprehensive characterization of the global virome. In this Consensus Statement article, we consider the rationale for why metagenomic sequence data should, and how it can, be incorporated into the ICTV taxonomy, and present proposals that have been endorsed by the Executive Committee of the ICTV.

Environmental Viral Metagenomics Analyses in Aquaculture: Applications in Epidemiology and Disease Control

Studies on the epidemiology of viral diseases in aquaculture have for a long time depended on isolation of viruses from infected aquatic organisms. The role of aquatic environments in the epidemiology of viral diseases in aquaculture has not been extensively expounded mainly because of the lack of appropriate tools for environmental studies on aquatic viruses. However, the upcoming of metagenomics analyses opens great avenues in which environmental samples can be used to study the epidemiology of viral diseases outside their host species. Hence, in this review I have shown that epidemiological factors that influence the composition of viruses in different aquatic environments include ecological factors, anthropogenic activities and stocking densities of cultured organisms based on environmental metagenomics studies carried out this far. Ballast water transportation and global trade of aquatic organisms are the most common virus dispersal process identified this far. In terms of disease control for outdoor aquaculture systems, baseline data on viruses found in different environments intended for aquaculture use can be obtained to enable the design of effective disease control strategies. And as such, high-risk areas having a high specter of pathogenic viruses can be identified as an early warning system. As for the control of viral diseases for indoor recirculation aquaculture systems (RAS), the most effective disinfection methods able to eliminate pathogenic viruses from water used in RAS can be identified. Overall, the synopsis I have put forth in this review shows that environmental samples can be used to study the epidemiology of viral diseases in aquaculture using viral metagenomics analysis as an overture for the design of rational disease control strategies.

Stable core virome despite variable microbiome after fecal transfer

We recently described the 4.5-year time course of the enteric bacterial microbiota and virome of a patient cured from recurrent Clostridium difficile infection (rCDI) by fecal microbiota transplantation (FMT). Here, we extended the virome analyses and found the patient's phage population to exhibit highly donor-similar characteristics following FMT, which remained stable for the whole period tested (up to 7 months). Moreover, the detected viral populations of donor and patient exhibited comparable diversity and richness. These findings were unexpected since enteric viromes are normally highly variable, assumed to influence the bacterial host community and change with environmental conditions. In contrast to the virome, the bacterial microbiota varied indeed for more than 7 months with ongoing dysbiosis before it reached donor similarity. Our findings that are based on sequence information and protein domain analysis seem to suggest that stable phage properties correlate with successful FMT better than the changing bacterial communities. We speculate that we here preferentially detected a stable core virome, which dominated over a variable flexible virome that may have been too heterogeneous for experimental detection or was underrepresented in the databases. It will be interesting to analyze whether the enteric virome allows predictions for the clinical outcome of FMT for rCDI and other diseases such as inflammatory bowel disease or obesity.

Distributions and relationships of virio- and picoplankton in the epi-, meso- and bathypelagic zones of the Western Pacific Ocean

Virio- and picoplankton mediate important biogeochemical processes and the environmental factors that regulate their dynamics, and the virus-host interactions are incompletely known, especially in the deep sea. Here we report on their distributions and relationships with environmental factors at 21 stations covering a latitudinal range (2-23° N) in the Western Pacific Ocean. This region is characterized by a complex western boundary current system. Synechococcus, autotrophic picoeukaryotes, heterotrophic prokaryotes and virus-like particles (VLPs) were high (<2.4 × 10²-6.3 × 10⁴, <34-2.8 × 10³, 3.9 × 10⁴-1.3 × 10⁶ cells mL^-1 and 5.1 × 10⁵-2.7 × 10⁷ mL^-1, respectively), and Prochlorococcus were low (<2.3 × 10²-1.0 × 10⁵ cells mL^-1) in the Luzon Strait and the four most southerly stations, where upwelling occurs. Covariations in the abundances of VLPs with heterotrophic and autotrophic picoplankton, and their correlation (i.e. r² = 0.63 and 0.52, respectively) suggested a strong host dependence in the epi- and mesopelagic zones. In the bathypelagic zone, only abiotic factors significantly influenced VLPs abundance variation (r² = 0.12). This study shows that the dynamics of virio- and picoplankton in this Western Pacific are controlled by suite of complex and depth-dependent relationship among physical and biological factors that in turn link the physical hydrography of the western boundary current system with microbial-mediated biogeochemical processes.

Alignment-free $d_2^*$ oligonucleotide frequency dissimilarity measure improves prediction of hosts from metagenomically-derived viral sequences

Viruses and their host genomes often share similar oligonucleotide frequency (ONF) patterns, which can be used to predict the host of a given virus by finding the host with the greatest ONF similarity. We comprehensively compared 11 ONF metrics using several k-mer lengths for predicting host taxonomy from among ∼32 000 prokaryotic genomes for 1427 virus isolate genomes whose true hosts are known. The background-subtracting measure [Formula: see text] at k = 6 gave the highest host prediction accuracy (33%, genus level) with reasonable computational times. Requiring a maximum dissimilarity score for making predictions (thresholding) and taking the consensus of the 30 most similar hosts further improved accuracy. Using a previous dataset of 820 bacteriophage and 2699 bacterial genomes, [Formula: see text] host prediction accuracies with thresholding and consensus methods (genus-level: 64%) exceeded previous Euclidian distance ONF (32%) or homology-based (22-62%) methods. When applied to metagenomically-assembled marine SUP05 viruses and the human gut virus crAssphage, [Formula: see text]-based predictions overlapped (i.e. some same, some different) with the previously inferred hosts of these viruses. The extent of overlap improved when only using host genomes or metagenomic contigs from the same habitat or samples as the query viruses. The [Formula: see text] ONF method will greatly improve the characterization of novel, metagenomic viruses.

Aquatic viral metagenomics: Lights and shadows

Viruses are the most abundant biological entities on Earth, exceeding bacteria in most of the ecosystems. Specially in oceans, viruses are thought to be the major planktonic predators shaping microorganism communities and controlling ocean biological capacity. Plankton lysis by viruses plays an important role in ocean nutrient and energy cycles. Viral metagenomics has emerged as a powerful tool to uncover viral diversity in aquatic ecosystems through the use of Next Generation Sequencing. However, many of the commonly used viral sample preparation steps have several important biases that must be considered to avoid a misinterpretation of the results. In addition to biases caused by the purification of virus particles, viral DNA/RNA amplification and the preparation of genomic libraries could also introduce biases, and a detailed knowledge about such protocols is required. In this review, the main steps in the viral metagenomic workflow are described paying special attention to the potential biases introduced by each one.

Genomic differentiation among wild cyanophages despite widespread horizontal gene transfer

Background

Genetic recombination is a driving force in genome evolution. Among viruses it has a dual role. For genomes with higher fitness, it maintains genome integrity in the face of high mutation rates. Conversely, for genomes with lower fitness, it provides immediate access to sequence space that cannot be reached by mutation alone. Understanding how recombination impacts the cohesion and dissolution of individual whole genomes within viral sequence space is poorly understood across double-stranded DNA bacteriophages (a.k.a phages) due to the challenges of obtaining appropriately scaled genomic datasets.

Results

Here we explore the role of recombination in both maintaining and differentiating whole genomes of 142 wild double-stranded DNA marine cyanophages. Phylogenomic analysis across the 51 core genes revealed ten lineages, six of which were well represented. These phylogenomic lineages represent discrete genotypic populations based on comparisons of intra- and inter- lineage shared gene content, genome-wide average nucleotide identity, as well as detected gaps in the distribution of pairwise differences between genomes. McDonald-Kreitman selection tests identified putative niche-differentiating genes under positive selection that differed across the six well-represented genotypic populations and that may have driven initial divergence. Concurrent with patterns of recombination of discrete populations, recombination analyses of both genic and intergenic regions largely revealed decreased genetic exchange across individual genomes between relative to within populations.

Conclusions

These findings suggest that discrete double-stranded DNA marine cyanophage populations occur in nature and are maintained by patterns of recombination akin to those observed in bacteria, archaea and in sexual eukaryotes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3286-x) contains supplementary material, which is available to authorized users.

IMG/VR: a database of cultured and uncultured DNA Viruses and retroviruses

Viruses represent the most abundant life forms on the planet. Recent experimental and computational improvements have led to a dramatic increase in the number of viral genome sequences identified primarily from metagenomic samples. As a result of the expanding catalog of metagenomic viral sequences, there exists a need for a comprehensive computational platform integrating all these sequences with associated metadata and analytical tools. Here we present IMG/VR (https://img.jgi.doe.gov/vr/), the largest publicly available database of 3908 isolate reference DNA viruses with 264 413 computationally identified viral contigs from >6000 ecologically diverse metagenomic samples. Approximately half of the viral contigs are grouped into genetically distinct quasi-species clusters. Microbial hosts are predicted for 20 000 viral sequences, revealing nine microbial phyla previously unreported to be infected by viruses. Viral sequences can be queried using a variety of associated metadata, including habitat type and geographic location of the samples, or taxonomic classification according to hallmark viral genes. IMG/VR has a user-friendly interface that allows users to interrogate all integrated data and interact by comparing with external sequences, thus serving as an essential resource in the viral genomics community.

A bioinformatics approach reveals seven nearly-complete RNA-virus genomes in bivalve RNA-seq data

Viral metagenomics (viromics) can provide a great contribution in expanding the knowledge of viruses and the relationship with their hosts. Viromic studies on marine organisms are still at a very early stage and only little efforts have been spent in the identification of viruses associated to marine invertebrates to date, leaving the complexity of marine viromes associated to bivalve hosts almost completely unexplored. However, the potential use of viromic approaches in the management of viral diseases affecting aquacultured species has been recently evidenced by the flourishing of studies on the Ostreid herpesvirus type-1, which has been associated with bivalve mortality events. Herein we discuss an effective pipeline to retrieve and reconstruct nearly complete and previously unreported viral genomes from existing host RNA-seq data. As a case study, we report the identification of seven RNA-virus genomes within the frame of a highly diversified viral community that characterizes both Crassostrea gigas and Mytilus galloprovincialis samples collected from the lagoon of Goro (Italy).

Ecogenomics and potential biogeochemical impacts of globally abundant ocean viruses

Ocean microbes drive biogeochemical cycling on a global scale. However, this cycling is constrained by viruses that affect community composition, metabolic activity, and evolutionary trajectories. Owing to challenges with the sampling and cultivation of viruses, genome-level viral diversity remains poorly described and grossly understudied, with less than 1% of observed surface-ocean viruses known. Here we assemble complete genomes and large genomic fragments from both surface- and deep-ocean viruses sampled during the Tara Oceans and Malaspina research expeditions, and analyse the resulting 'global ocean virome' dataset to present a global map of abundant, double-stranded DNA viruses complete with genomic and ecological contexts. A total of 15,222 epipelagic and mesopelagic viral populations were identified, comprising 867 viral clusters (defined as approximately genus-level groups). This roughly triples the number of known ocean viral populations and doubles the number of candidate bacterial and archaeal virus genera, providing a near-complete sampling of epipelagic communities at both the population and viral-cluster level. We found that 38 of the 867 viral clusters were locally or globally abundant, together accounting for nearly half of the viral populations in any global ocean virome sample. While two-thirds of these clusters represent newly described viruses lacking any cultivated representative, most could be computationally linked to dominant, ecologically relevant microbial hosts. Moreover, we identified 243 viral-encoded auxiliary metabolic genes, of which only 95 were previously known. Deeper analyses of four of these auxiliary metabolic genes (dsrC, soxYZ, P-II (also known as glnB) and amoC) revealed that abundant viruses may directly manipulate sulfur and nitrogen cycling throughout the epipelagic ocean. This viral catalog and functional analyses provide a necessary foundation for the meaningful integration of viruses into ecosystem models where they act as key players in nutrient cycling and trophic networks.

Microbial metabolisms in a 2.5-km-deep ecosystem created by hydraulic fracturing in shales

Hydraulic fracturing is the industry standard for extracting hydrocarbons from shale formations. Attention has been paid to the economic benefits and environmental impacts of this process, yet the biogeochemical changes induced in the deep subsurface are poorly understood. Recent single-gene investigations revealed that halotolerant microbial communities were enriched after hydraulic fracturing. Here, the reconstruction of 31 unique genomes coupled to metabolite data from the Marcellus and Utica shales revealed that many of the persisting organisms play roles in methylamine cycling, ultimately supporting methanogenesis in the deep biosphere. Fermentation of injected chemical additives also sustains long-term microbial persistence, while thiosulfate reduction could produce sulfide, contributing to reservoir souring and infrastructure corrosion. Extensive links between viruses and microbial hosts demonstrate active viral predation, which may contribute to the release of labile cellular constituents into the extracellular environment. Our analyses show that hydraulic fracturing provides the organismal and chemical inputs for colonization and persistence in the deep terrestrial subsurface.

A novel roseobacter phage possesses features of podoviruses, siphoviruses, prophages and gene transfer agents

Bacteria in the Roseobacter lineage have been studied extensively due to their significant biogeochemical roles in the marine ecosystem. However, our knowledge on bacteriophage which infects the Roseobacter clade is still very limited. Here, we report a new bacteriophage, phage DSS3Φ8, which infects marine roseobacter Ruegeria pomeroyi DSS-3. DSS3Φ8 is a lytic siphovirus. Genomic analysis showed that DSS3Φ8 is most closely related to a group of siphoviruses, CbK-like phages, which infect freshwater bacterium Caulobacter crescentus. DSS3Φ8 contains a smaller capsid and has a reduced genome size (146 kb) compared to the CbK-like phages (205-279 kb). DSS3Φ8 contains the DNA polymerase gene which is closely related to T7-like podoviruses. DSS3Φ8 also contains the integrase and repressor genes, indicating its potential to involve in lysogenic cycle. In addition, four GTA (gene transfer agent) genes were identified in the DSS3Φ8 genome. Genomic analysis suggests that DSS3Φ8 is a highly mosaic phage that inherits the genetic features from siphoviruses, podoviruses, prophages and GTAs. This is the first report of CbK-like phages infecting marine bacteria. We believe phage isolation is still a powerful tool that can lead to discovery of new phages and help interpret the overwhelming unknown sequences in the viral metagenomics.

Genomes of Abundant and Widespread Viruses from the Deep Ocean

The deep sea is a massive, largely oligotrophic ecosystem, stretched over nearly 65% of the planet’s surface. Deep-sea planktonic communities are almost completely dependent upon organic carbon sinking from the productive surface, forming a vital component of global biogeochemical cycles. However, despite their importance, viruses from the deep ocean remain largely unknown. Here, we describe the first complete genomes of deep-sea viruses assembled from metagenomic fosmid libraries. “Candidatus Pelagibacter” (SAR11) phage HTVC010P and Puniceispirillum phage HMO-2011 are considered the most abundant cultured marine viruses known to date. Remarkably, some of the viruses described here recruited as many reads from deep waters as these viruses do in the photic zone, and, considering the gigantic scale of the bathypelagic habitat, these genomes provide information about what could be some of the most abundant viruses in the world at large. Their role in the viral shunt in the global ocean could be very significant. Despite the challenges encountered in inferring the identity of their hosts, we identified one virus predicted to infect members of the globally distributed SAR11 cluster. We also identified a number of putative proviruses from diverse taxa, including deltaproteobacteria, bacteroidetes, SAR11, and gammaproteobacteria. Moreover, our findings also indicate that lysogeny is the preferred mode of existence for deep-sea viruses inhabiting an energy-limited environment, in sharp contrast to the predominantly lytic lifestyle of their photic-zone counterparts. Some of the viruses show a widespread distribution, supporting the tenet “everything is everywhere” for the deep-ocean virome.

The deep sea is among the largest known habitats and a critical cog in biogeochemical cycling but remains underexplored in its microbiology. Even more than is the case for its prokaryotic community, our knowledge of its viral component has remained limited by the paucity of information provided by studies dependent upon short sequence fragments. In this work, we attempt to fill this existing gap by using a combination of classical fosmid libraries with next-generation sequencing and assembly to recover long viral genomic fragments. We have sequenced ca. 6,000 fosmids from two metagenomics libraries made from prokaryotic biomass from the deep Mediterranean Sea and recovered twenty-eight complete viral genomes, all of them novel and quite distinct from all previously described viral genomes. They are preferentially found in deeper waters and are widely distributed all over the oceans. To our knowledge, this is the first report on complete and cosmopolitan viral genomes from the bathypelagic habitat.

iVirus: facilitating new insights in viral ecology with software and community data sets imbedded in a cyberinfrastructure

Microbes affect nutrient and energy transformations throughout the world's ecosystems, yet they do so under viral constraints. In complex communities, viral metagenome (virome) sequencing is transforming our ability to quantify viral diversity and impacts. Although some bottlenecks, for example, few reference genomes and nonquantitative viromics, have been overcome, the void of centralized data sets and specialized tools now prevents viromics from being broadly applied to answer fundamental ecological questions. Here we present iVirus, a community resource that leverages the CyVerse cyberinfrastructure to provide access to viromic tools and data sets. The iVirus Data Commons contains both raw and processed data from 1866 samples and 73 projects derived from global ocean expeditions, as well as existing and legacy public repositories. Through the CyVerse Discovery Environment, users can interrogate these data sets using existing analytical tools (software applications known as ‘Apps') for assembly, open reading frame prediction and annotation, as well as several new Apps specifically developed for analyzing viromes. Because Apps are web based and powered by CyVerse supercomputing resources, they enable scalable analyses for a broad user base. Finally, a use-case scenario documents how to apply these advances toward new data. This growing iVirus resource should help researchers utilize viromics as yet another tool to elucidate viral roles in nature.

Contributions of P2- and P22-like prophages to understanding the enormous diversity and abundance of tailed bacteriophages

We identified 9371 tailed phage prophages of 20 known types in reported complete genome sequences of 3298 bacteria in the Salmonella genus. These include 4758 P2 type and 744 P22 type prophages. The latter prophage types were found in the genome sequences of 127 and 24 bacterial host genera, increasing the known host ranges of phages in these groups by 114 and 20 genera, respectively. These prophage nucleotide sequences displayed much more diversity than was previously known from the 48 P2 and 24 P22 type authentic phages whose genomes have been sequenced. More detailed analysis of these prophage sequences indicated that major capsid protein (MCP) gene exchange between tailed phage clusters or types is extremely rare and that P22 prophage-encoded tailspikes correspond perfectly with their hosts' surface polysaccharide structure; thus, MCP and tailspike sequences accurately predict tailed phage type (and thus lifestyle) and host cell surface polysaccharide structure, respectively.

Large-scale maps of variable infection efficiencies in aquatic Bacteroidetes phage-host model systems

Microbes drive ecosystem functioning and their viruses modulate these impacts through mortality, gene transfer and metabolic reprogramming. Despite the importance of virus-host interactions and likely variable infection efficiencies of individual phages across hosts, such variability is seldom quantified. Here, we quantify infection efficiencies of 38 phages against 19 host strains in aquatic Cellulophaga (Bacteroidetes) phage-host model systems. Binary data revealed that some phages infected only one strain while others infected 17, whereas quantitative data revealed that efficiency of infection could vary 10 orders of magnitude, even among phages within one population. This provides a baseline for understanding and modeling intrapopulation host range variation. Genera specific host ranges were also informative. For example, the Cellulophaga Microviridae, showed a markedly broader intra-species host range than previously observed in Escherichia coli systems. Further, one phage genus, Cba41, was examined to investigate nonheritable changes in plating efficiency and burst size that depended on which host strain it most recently infected. While consistent with host modification of phage DNA, no differences in nucleotide sequence or DNA modifications were detected, leaving the observation repeatable, but the mechanism unresolved. Overall, this study highlights the importance of quantitatively considering replication variations in studies of phage-host interactions.

Biogeographic Variation in Host Range Phenotypes and Taxonomic Composition of Marine Cyanophage Isolates

Despite the important role of phages in marine systems, little is understood about how their diversity is distributed in space. Biogeographic patterns of marine phages may be difficult to detect due to their vast genetic diversity, which may not be accurately represented by conserved marker genes. To investigate the spatial biogeographic structure of marine phages, we isolated over 400 cyanophages on Synechococcus host strain WH7803 at three coastal locations in the United States (Rhode Island, Washington, and southern California). Approximately 90% of the cyanophage isolates were myoviruses, while the other 10% were podoviruses. The diversity of isolates was further characterized in two ways: (i) taxonomically, using conserved marker genes and (ii) phenotypically, by testing isolates for their ability to infect a suite of hosts, or their "host range." Because host range is a highly variable trait even among closely related isolates, we hypothesized that host range phenotypes of cyanophage isolates would vary more strongly among locations than would taxonomic composition. Instead, we found evidence for strong biogeographic variation both in taxonomic composition and host range phenotypes, with little taxonomic overlap among the three coastal regions. For both taxonomic composition and host range phenotypes, cyanophage communities from California and Rhode Island were the most dissimilar, while Washington communities exhibited similarity to each of the other two locations. These results suggest that selection imposed by spatial variation in host dynamics influence the biogeographic distribution of cyanophages.

Assessing viral taxonomic composition in benthic marine ecosystems: reliability and efficiency of different bioinformatic tools for viral metagenomic analyses

In benthic deep-sea ecosystems, which represent the largest biome on Earth, viruses have a recognised key ecological role, but their diversity is still largely unknown. Identifying the taxonomic composition of viruses is crucial for understanding virus-host interactions, their role in food web functioning and evolutionary processes. Here, we compared the performance of various bioinformatic tools (BLAST, MG-RAST, NBC, VMGAP, MetaVir, VIROME) for analysing the viral taxonomic composition in simulated viromes and viral metagenomes from different benthic deep-sea ecosystems. The analyses of simulated viromes indicate that all the BLAST tools, followed by MetaVir and VMGAP, are more reliable in the affiliation of viral sequences and strains. When analysing the environmental viromes, tBLASTx, MetaVir, VMGAP and VIROME showed a similar efficiency of sequence annotation; however, MetaVir and tBLASTx identified a higher number of viral strains. These latter tools also identified a wider range of viral families than the others, providing a wider view of viral taxonomic diversity in benthic deep-sea ecosystems. Our findings highlight strengths and weaknesses of available bioinformatic tools for investigating the taxonomic diversity of viruses in benthic ecosystems in order to improve our comprehension of viral diversity in the oceans and its relationships with host diversity and ecosystem functioning.

Marine phage genomics: the tip of the iceberg

Marine viruses are the most abundant biological entity in the oceans, the majority of which infect bacteria and are known as bacteriophages. Yet, the bulk of bacteriophages form part of the vast uncultured dark matter of the microbial biosphere. In spite of the paucity of cultured marine bacteriophages, it is known that marine bacteriophages have major impacts on microbial population structure and the biogeochemical cycling of key elements. Despite the ecological relevance of marine bacteriophages, there are relatively few isolates with complete genome sequences. This minireview focuses on knowledge gathered from these genomes put in the context of viral metagenomic data and highlights key advances in the field, particularly focusing on genome structure and auxiliary metabolic genes.

Only a tiny fraction of marine phages have been discovered, yet are known to have important roles in the ocean.

Freshwater Metaviromics and Bacteriophages: A Current Assessment of the State of the Art in Relation to Bioinformatic Challenges

Advances in bioinformatics and sequencing technologies have allowed for the analysis of complex microbial communities at an unprecedented rate. While much focus is often placed on the cellular members of these communities, viruses play a pivotal role, particularly bacteria-infecting viruses (bacteriophages); phages mediate global biogeochemical processes and drive microbial evolution through bacterial grazing and horizontal gene transfer. Despite their importance and ubiquity in nature, very little is known about the diversity and structure of viral communities. Though the need for culture-based methods for viral identification has been somewhat circumvented through metagenomic techniques, the analysis of metaviromic data is marred with many unique issues. In this review, we examine the current bioinformatic approaches for metavirome analyses and the inherent challenges facing the field as illustrated by the ongoing efforts in the exploration of freshwater phage populations.

HoloVir: A Workflow for Investigating the Diversity and Function of Viruses in Invertebrate Holobionts

Abundant bioinformatics resources are available for the study of complex microbial metagenomes, however their utility in viral metagenomics is limited. HoloVir is a robust and flexible data analysis pipeline that provides an optimized and validated workflow for taxonomic and functional characterization of viral metagenomes derived from invertebrate holobionts. Simulated viral metagenomes comprising varying levels of viral diversity and abundance were used to determine the optimal assembly and gene prediction strategy, and multiple sequence assembly methods and gene prediction tools were tested in order to optimize our analysis workflow. HoloVir performs pairwise comparisons of single read and predicted gene datasets against the viral RefSeq database to assign taxonomy and additional comparison to phage-specific and cellular markers is undertaken to support the taxonomic assignments and identify potential cellular contamination. Broad functional classification of the predicted genes is provided by assignment of COG microbial functional category classifications using EggNOG and higher resolution functional analysis is achieved by searching for enrichment of specific Swiss-Prot keywords within the viral metagenome. Application of HoloVir to viral metagenomes from the coral Pocillopora damicornis and the sponge Rhopaloeides odorabile demonstrated that HoloVir provides a valuable tool to characterize holobiont viral communities across species, environments, or experiments.

Assessing Illumina technology for the high-throughput sequencing of bacteriophage genomes

Bacteriophages are the most abundant biological entities on the planet, playing crucial roles in the shaping of bacterial populations. Phages have smaller genomes than their bacterial hosts, yet there are currently fewer fully sequenced phage than bacterial genomes. We assessed the suitability of Illumina technology for high-throughput sequencing and subsequent assembly of phage genomes. In silico datasets reveal that 30× coverage is sufficient to correctly assemble the complete genome of ~98.5% of known phages, with experimental data confirming that the majority of phage genomes can be assembled at 30× coverage. Furthermore, in silico data demonstrate it is possible to co-sequence multiple phages from different hosts, without introducing assembly errors.

Optimization of viral resuspension methods for carbon-rich soils along a permafrost thaw gradient

Permafrost stores approximately 50% of global soil carbon (C) in a frozen form; it is thawing rapidly under climate change, and little is known about viral communities in these soils or their roles in C cycling. In permafrost soils, microorganisms contribute significantly to C cycling, and characterizing them has recently been shown to improve prediction of ecosystem function. In other ecosystems, viruses have broad ecosystem and community impacts ranging from host cell mortality and organic matter cycling to horizontal gene transfer and reprogramming of core microbial metabolisms. Here we developed an optimized protocol to extract viruses from three types of high organic-matter peatland soils across a permafrost thaw gradient (palsa, moss-dominated bog, and sedge-dominated fen). Three separate experiments were used to evaluate the impact of chemical buffers, physical dispersion, storage conditions, and concentration and purification methods on viral yields. The most successful protocol, amended potassium citrate buffer with bead-beating or vortexing and BSA, yielded on average as much as 2-fold more virus-like particles (VLPs) g⁻¹ of soil than other methods tested. All method combinations yielded VLPs g⁻¹ of soil on the 10⁸ order of magnitude across all three soil types. The different storage and concentration methods did not yield significantly more VLPs g⁻¹ of soil among the soil types. This research provides much-needed guidelines for resuspending viruses from soils, specifically carbon-rich soils, paving the way for incorporating viruses into soil ecology studies.

Co-occurrence Analysis of Microbial Taxa in the Atlantic Ocean Reveals High Connectivity in the Free-Living Bacterioplankton

We determined the taxonomic composition of the bacterioplankton of the epipelagic zone of the Atlantic Ocean along a latitudinal transect (51°S–47°N) using Illumina sequencing of the V5-V6 region of the 16S rRNA gene and inferred co-occurrence networks. Bacterioplankon community composition was distinct for Longhurstian provinces and water depth. Free-living microbial communities (between 0.22 and 3 μm) were dominated by highly abundant and ubiquitous taxa with streamlined genomes (e.g., SAR11, SAR86, OM1, Prochlorococcus) and could clearly be separated from particle-associated communities which were dominated by Bacteroidetes, Planktomycetes, Verrucomicrobia, and Roseobacters. From a total of 369 different communities we then inferred co-occurrence networks for each size fraction and depth layer of the plankton between bacteria and between bacteria and phototrophic micro-eukaryotes. The inferred networks showed a reduction of edges in the deepest layer of the photic zone. Networks comprised of free-living bacteria had a larger amount of connections per OTU when compared to the particle associated communities throughout the water column. Negative correlations accounted for roughly one third of the total edges in the free-living communities at all depths, while they decreased with depth in the particle associated communities where they amounted for roughly 10% of the total in the last part of the epipelagic zone. Co-occurrence networks of bacteria with phototrophic micro-eukaryotes were not taxon-specific, and dominated by mutual exclusion (~60%). The data show a high degree of specialization to micro-environments in the water column and highlight the importance of interdependencies particularly between free-living bacteria in the upper layers of the epipelagic zone.