Genomic island variability facilitates Prochlorococcus-virus coexistence

The assembly and ecological roles of biofilms attached to plastic debris of Ashmore reef

Plastic pollution in the ocean is a global environmental hazard aggravated by poor management of plastic waste and growth of annual plastic consumption. Microbial communities colonizing the plastic's surface, the plastisphere, has gained global interest resulting in numerous efforts to characterize the plastisphere. However, there are insufficient studies deciphering the underlying metabolic processes governing the function of the plastisphere and the plastic they reside upon. Here, we collected plastic and seawater samples from Ashmore Reef in Australia to examine the planktonic microbes and plastic associated biofilm (PAB) to investigate the ecological impact, pathogenic potential, and plastic degradation capabilities of PAB in Ashmore Reef, as well as the role and impact of bacteriophages on PAB. Using high-throughput metagenomic sequencing, we demonstrated distinct microbial communities between seawater and PAB. Similar numbers of pathogenic bacteria were found in both sample types, yet plastic and seawater select for different pathogen populations. Virulence Factor analysis further illustrated stronger pathogenic potential in PAB, highlighting the pathogenicity of environmental PAB. Furthermore, functional analysis of Kyoto Encyclopedia of Genes and Genomes (KEGG) metabolic pathways revealed xenobiotic degradation and fatty acid degradation to be enriched in PABs. In addition, construction of metagenome-assembled genomes (MAG) and functional analysis further demonstrated the presence of a complete Polyethylene (PE) degradation pathway in multiple Proteobacteria MAGs, especially in Rhodobacteriaceae sp. Additionally, we identified viral population presence in PAB, revealing the key role of bacteriophages in shaping these communities within the PAB. Our result provides a comprehensive overview of the various ecological processes shaping microbial community on marine plastic debris.

Benefits and Challenges of Applying Bacteriophage Biocontrol in the Consumer Water Cycle

Bacteria (including disinfection- and antibiotic-resistant bacteria) are abundant in the consumer water cycle, where they may cause disease, and lead to biofouling and infrastructure damage in distributions systems, subsequently resulting in significant economic losses. Bacteriophages and their associated enzymes may then offer a biological control solution for application within the water sector. Lytic bacteriophages are of particular interest as biocontrol agents as their narrow host range can be exploited for the targeted removal of specific bacteria in a designated environment. Bacteriophages can also be used to improve processes such as wastewater treatment, while bacteriophage-derived enzymes can be applied to combat biofouling based on their effectiveness against preformed biofilms. However, the host range, environmental stability, bacteriophage resistance and biosafety risks are some of the factors that need to be considered prior to the large-scale application of these bacterial viruses. Characteristics of bacteriophages that highlight their potential as biocontrol agents are thus outlined in this review, as well as the potential application of bacteriophage biocontrol throughout the consumer water cycle. Additionally, the limitations of bacteriophage biocontrol and corresponding mitigation strategies are outlined, including the use of engineered bacteriophages for improved host ranges, environmental stability and the antimicrobial re-sensitisation of bacteria. Finally, the potential public and environmental risks associated with large-scale bacteriophage biocontrol application are considered, and alternative applications of bacteriophages to enhance the functioning of the consumer water cycle, including their use as water quality or treatment indicators and microbial source tracking markers, are discussed.

Cell-to-cell heterogeneity drives host-virus coexistence in a bloom-forming alga

Algal blooms drive global biogeochemical cycles of key nutrients and serve as hotspots for biological interactions in the ocean. The massive blooms of the cosmopolitan coccolithophore Emiliania huxleyi are often infected by the lytic E. huxleyi virus, which is a major mortality agent triggering bloom demise. This multi-annual "boom and bust" pattern of E. huxleyi blooms suggests that coexistence is essential for these host-virus dynamics. To investigate host-virus coexistence, we developed a new model system from an E. huxleyi culture that recovered from viral infection. The recovered population coexists with the virus, as host cells continue to divide in parallel to viral production. By applying single-molecule fluorescence in situ hybridization (smFISH) to quantify the fraction of infected cells, and assessing infection-specific lipid biomarkers, we identified a small subpopulation of cells that were infected and produced new virions, whereas most of the host population could resist infection. To further assess population heterogeneity, we generated clonal strain collections using single-cell sorting and subsequently phenotyped their susceptibility to E. huxleyi virus infection. This unraveled substantial cell-to-cell heterogeneity across a continuum of susceptibility to resistance, highlighting that infection outcome may vary depending on the individual cell. These results add a new dimension to our understanding of the complexity of host-virus interactions that are commonly assessed in bulk and described by binary definitions of resistance or susceptibility. We propose that phenotypic heterogeneity drives the host-virus coexistence and demonstrate how the coexistence with a lytic virus provides an ecological advantage for the host by killing competing strains.

Tradeoffs between phage resistance and nitrogen fixation drive the evolution of genes essential for cyanobacterial heterocyst functionality

Harmful blooms caused by diazotrophic (nitrogen-fixing) Cyanobacteria are becoming increasingly frequent and negatively impact aquatic environments worldwide. Cyanophages (viruses infecting Cyanobacteria) can potentially regulate cyanobacterial blooms, yet Cyanobacteria can rapidly acquire mutations that provide protection against phage infection. Here, we provide novel insights into cyanophage:Cyanobacteria interactions by characterizing the resistance to phages in two species of diazotrophic Cyanobacteria: Nostoc sp. and Cylindrospermopsis raciborskii. Our results demonstrate that phage resistance is associated with a fitness tradeoff by which resistant Cyanobacteria have reduced ability to fix nitrogen and/or to survive nitrogen starvation. Furthermore, we use whole-genome sequence analysis of 58 Nostoc-resistant strains to identify several mutations associated with phage resistance, including in cell surface-related genes and regulatory genes involved in the development and function of heterocysts (cells specialized in nitrogen fixation). Finally, we employ phylogenetic analyses to show that most of these resistance genes are accessory genes whose evolution is impacted by lateral gene transfer events. Together, these results further our understanding of the interplay between diazotrophic Cyanobacteria and their phages and suggest that a tradeoff between phage resistance and nitrogen fixation affects the evolution of cell surface-related genes and of genes involved in heterocyst differentiation and nitrogen fixation.

Clustered Regularly Interspaced Short Palindromic Repeat/CRISPR-Associated Protein and Its Utility All at Sea: Status, Challenges, and Prospects

Initially discovered over 35 years ago in the bacterium Escherichia coli as a defense system against invasion of viral (or other exogenous) DNA into the genome, CRISPR/Cas has ushered in a new era of functional genetics and served as a versatile genetic tool in all branches of life science. CRISPR/Cas has revolutionized the methodology of gene knockout with simplicity and rapidity, but it is also powerful for gene knock-in and gene modification. In the field of marine biology and ecology, this tool has been instrumental in the functional characterization of 'dark' genes and the documentation of the functional differentiation of gene paralogs. Powerful as it is, challenges exist that have hindered the advances in functional genetics in some important lineages. This review examines the status of applications of CRISPR/Cas in marine research and assesses the prospect of quickly expanding the deployment of this powerful tool to address the myriad fundamental marine biology and biological oceanography questions.

The Reemergence of Phycopathology: When Algal Biology Meets Ecology and Biosecurity

Viruses, bacteria, and eukaryotic symbionts interact with algae in a variety of ways to cause disease complexes, often shaping marine and freshwater ecosystems. The advent of phyconomy (a.k.a. seaweed agronomy) represents a need for a greater understanding of algal disease interactions, where underestimated cryptic diversity and lack of phycopathological basis are prospective constraints for algal domestication. Here, we highlight the limited yet increasing knowledge of algal pathogen biodiversity and the ecological interaction with their algal hosts. Finally, we discuss how ecology and cultivation experience contribute to and reinforce aquaculture practice, with the potential to reshape biosecurity policies of seaweed cultivation worldwide.

Coexistence of Dominant Marine Phytoplankton Sustained by Nutrient Specialization

Prochlorococcus and Synechococcus are the two dominant picocyanobacteria in the low-nutrient surface waters of the subtropical ocean, but the basis for their coexistence has not been quantitatively demonstrated. Here, we combine in situ microcosm experiments and an ecological model to show that this coexistence can be sustained by specialization in the uptake of distinct nitrogen (N) substrates at low-level concentrations that prevail in subtropical environments. In field incubations, the response of both Prochlorococcus and Synechococcus to nanomolar N amendments demonstrates N limitation of growth in both populations. However, Prochlorococcus showed a higher affinity to ammonium, whereas Synechococcus was more adapted to nitrate uptake. A simple ecological model demonstrates that the differential nutrient preference inferred from field experiments with these genera may sustain their coexistence. It also predicts that as the supply of NO3- decreases, as expected under climate warming, the dominant genera should undergo a nonlinear shift from Synechococcus to Prochlorococcus, a pattern that is supported by subtropical field observations. Our study suggests that the evolution of differential nutrient affinities is an important mechanism for sustaining the coexistence of genera and that climate change is likely to shift the relative abundance of the dominant plankton genera in the largest biomes in the ocean. IMPORTANCE Our manuscript addresses the following fundamental question in microbial ecology: how do different plankton using the same essential nutrients coexist? Prochlorococcus and Synechococcus are the two dominant picocyanobacteria in the low-nutrient surface waters of the subtropical ocean, which support a significant amount of marine primary production. The geographical distributions of these two organisms are largely overlapping, but the basis for their coexistence in these biomes remains unclear. In this study, we combined in situ microcosm experiments and an ecosystem model to show that the coexistence of these two organisms can arise from specialization in the uptake of distinct nitrogen substrates; Prochlorococcus prefers ammonium, whereas Synechococcus prefers nitrate when these nutrients exist at low concentrations. Our framework can be used for simulating and predicting the coexistence in the future ocean and may provide hints toward understanding other similar types of coexistence.

Sequencing the genomes of LPP-1, the first isolated cyanophage, and its relative LPP-2 reveal different integration mechanisms in closely related phages

In the early 1960s, the first cyanophage was isolated. The description of this phage, named LPP-1, led to the extensive investigation of various cyanophages and to the study of their interactions with their cyanobacterial hosts towards controlling blooms. Here, the genomes of LPP-1 and its putative relative, LPP-2 were sequenced. Sequencing these genomes revealed that LPP-1 and LPP-2 are members of a group of short-tailed cyanophages, which are distantly related to the T7-like cyanophages. Most of the phages in this group have the ability to lysogenize their hosts. Their ability to switch between lytic and lysogenic infection may explain the formation of cyanobacterial blooms despite the persistence of their phages. This lysogenic capacity of the LPP-1-like phages occurs despite the lack of an obvious integrase gene within their genomes. Interestingly, we show that LPP-2 integrates into the host genome through an integration site in high proximity to a recombination endonuclease that may have integrase activity. Further understanding of cyanobacterial-phage relationships may provide insight into their population dynamics and suggest novel approaches for control of destructive cyanobacterial blooms.

Bacterial defences: mechanisms, evolution and antimicrobial resistance

Throughout their evolutionary history, bacteria have faced diverse threats from other microorganisms, including competing bacteria, bacteriophages and predators. In response to these threats, they have evolved sophisticated defence mechanisms that today also protect bacteria against antibiotics and other therapies. In this Review, we explore the protective strategies of bacteria, including the mechanisms, evolution and clinical implications of these ancient defences. We also review the countermeasures that attackers have evolved to overcome bacterial defences. We argue that understanding how bacteria defend themselves in nature is important for the development of new therapies and for minimizing resistance evolution.

From natural to artificial cyanophages: Current progress and application prospects

The over proliferation of harmful cyanobacteria and their cyanotoxins resulted in damaged aquatic ecosystem, polluted drinking water and threatened human health. Cyanophages are a kind of viruses that exclusively infect cyanobacteria, which is considered as a potential strategy to deal with cyanobacterial blooms. Nevertheless, the infecting host range and/or lysis efficiency of natural cyanophages is limited, rising the necessity of constructing non-natural cyanophages via artificial modification, design and synthesis to expand their host range and/or efficiency. The paper firstly reviewed representative cyanophages such as P60 with a short latent period of 1.5 h and S-CBS1 having a burst size up to 200 PFU/cell. To explore the in-silico design principles, we critically summarized the interactions between cyanophages and the hosts, indicating modifying the recognized receptors, enhancing the adsorption ability, changing the lysogeny and excluding the defense of hosts are important for artificial cyanophages. The research progress of synthesizing artificial cyanophages were summarized subsequently, raising the importance of developing genetic manipulation technologies and their rescue strategies in the future. Meanwhile, Large-scale preparation of cyanophages for bloom control is a big challenge. The application prospects of artificial cyanophages besides cyanobacteria bloom control like adaptive evolution and phage therapy were discussed at last. The review will promote the design, synthesis and application of cyanophages for cyanobacteria blooms, which may provide new insights for the related water pollution control and ensuring hydrosphere security.

Phage-bacterial evolutionary interactions: experimental models and complications

Phage treatment of bacterial infections has offered some hope even as the crisis of antimicrobial resistance continues to be on the rise. However, bacterial resistance to phage is another looming challenge capable of undermining the effectiveness of phage therapy. Moreover, the consideration of including phage therapy in modern medicine calls for more careful research around every aspect of phage study. In an attempt to adequately prepare for the events of phage resistance, many studies have attempted to experimentally evolve phage resistance in different bacterial strains, as well as train phages to evolve counter-infectivity of resistant bacterial mutants, in view of answering such questions as coevolutionary dynamics between phage and bacteria, mechanisms of phage resistance, fitness costs of phage resistance on bacteria, etc. In this review, we summarised many such studies and by careful examination, highlighted critical issues to the outcome of phage therapy. We also discuss the insufficiency of many of these in vitro studies to represent actual disease conditions during phage application, alongside other complications that exist in phage-bacterial evolutionary interactions. Conclusively, we present the exploitation of phage-bacterial interactions for successful infection managements, as well as some future perspectives to direct phage research.

Diverse Marine T4-like Cyanophage Communities Are Primarily Comprised of Low-Abundance Species Including Species with Distinct Seasonal, Persistent, Occasional, or Sporadic Dynamics

Cyanophages exert important top-down controls on their cyanobacteria hosts; however, concurrent analysis of both phage and host populations is needed to better assess phage-host interaction models. We analyzed picocyanobacteria Prochlorococcus and Synechococcus and T4-like cyanophage communities in Pacific Ocean surface waters using five years of monthly viral and cellular fraction metagenomes. Cyanophage communities contained thousands of mostly low-abundance (<2% relative abundance) species with varying temporal dynamics, categorized as seasonally recurring or non-seasonal and occurring persistently, occasionally, or sporadically (detected in ≥85%, 15-85%, or <15% of samples, respectively). Viromes contained mostly seasonal and persistent phages (~40% each), while cellular fraction metagenomes had mostly sporadic species (~50%), reflecting that these sample sets capture different steps of the infection cycle-virions from prior infections or within currently infected cells, respectively. Two groups of seasonal phages correlated to Synechococcus or Prochlorococcus were abundant in spring/summer or fall/winter, respectively. Cyanophages likely have a strong influence on the host community structure, as their communities explained up to 32% of host community variation. These results support how both seasonally recurrent and apparent stochastic processes, likely determined by host availability and different host-range strategies among phages, are critical to phage-host interactions and dynamics, consistent with both the Kill-the-Winner and the Bank models.

Molecular evidence of parallel evolution in a cyanophage

Antagonistic interactions between bacteriophage (phage) and its bacterial host drives the continual selection for resistance and counter-defence. To date, much remains unknown about the genomic evolution that occurs as part of the underlying mechanisms. Such is the case for the marine cyanobacteria Synechococcus and viruses (cyanophages) that infect them. Here, we monitored host and phage abundances, alongside genomic changes to the phage populations, in a 500-day (~55 bacterial generations) infection experiment between Synechococcus sp. WH7803 and the T4-type cyanophage S-PM2d, run parallel in three replicate chemostats (plus one control chemostat). Flow cytometric count of total abundances revealed relatively similar host-phage population dynamics across the chemostats, starting with a cycle of host population collapse and recovery that led to phases of host-phage coexistence. Whole-genome analysis of the S-PM2d populations detected an assemblage of strongly selected and repeatable genomic changes, and therefore parallel evolution in the phage populations, early in the experiment (sampled on day 39). These consisted mostly of non-synonymous single-nucleotide-polymorphisms and a few instances of indel, altogether affecting 18 open-reading-frames, the majority of which were predicted to encode virion structures including those involved in phage adsorption onto host (i.e., baseplate wedge, short tail fibre, adhesin component). Mutations that emerged later (sampled on day 500), on the other hand, were found at a larger range of frequencies, with many lacking repeatability across the chemostats. This is indicative of some degree of between-population divergence in the phage evolutionary trajectory over time. A few of the early and late mutations were detected within putative auxiliary metabolic genes, but these generally occurred in only one or two of the chemostats. Less repeatable mutations may have higher fitness costs, thus drawing our attention onto the role of trade-offs in modulating the trajectory of a host-phage coevolution.

Abundant and cosmopolitan lineage of cyanopodoviruses lacking a DNA polymerase gene

Cyanopodoviruses affect the mortality and population dynamics of the unicellular picocyanobacteria Prochlorococcus and Synechococcus, the dominant primary producers in the oceans. Known cyanopodoviruses all contain the DNA polymerase gene (DNA pol) that is important for phage DNA replication and widely used in field quantification and diversity studies. However, we isolated 18 cyanopodoviruses without identifiable DNA pol. They form a new MPP-C clade that was separated from the existing MPP-A, MPP-B, and P-RSP2 clades. The MPP-C phages have the smallest genomes (37.3-37.9 kb) among sequenced cyanophages, and show longer latent periods than the MPP-B phages. Metagenomic reads of both clades are highly abundant in surface waters, but the MPP-C phages show higher relative abundance in surface waters than in deeper waters, while MPP-B phages have higher relative abundance in deeper waters. Our study reveals that cyanophages with distinct genomic contents and infection kinetics can exhibit different depth profiles in the oceans.

Functional ecology of bacteriophages in the environment

Bacteriophages are as ubiquitous as their bacterial hosts and often more abundant. Understanding how bacteriophages control their bacterial host populations requires a number of different approaches. Bacteriophages can control bacterial populations through lysis, drive evolution of bacterial immunity systems through infection, provide a conduit for horizontal gene transfer and alter host metabolism by carriage of auxiliary metabolic genes. Understanding and quantifying how bacteriophages drive these processes, requires both technological developments to take measurements in situ, and laboratory-based studies to understand mechanisms. Technological advances have allowed quantification of the number of infected cells in situ, revealing far-lower levels than expected. Understanding how observations in laboratory conditions relate to what occurs in the environment, and experimental confirmation of the predicted function of phage genes from observations in environmental omics data, remains challenging.

Viral infection in the ocean-A journey across scales

Viruses are the most abundant biological entity in the ocean and infect a wide range of microbial life across bacteria, archaea, and eukaryotes. In this essay, we take a journey across several orders of magnitude in the scales of biological organization, time, and space of host-virus interactions in the ocean, aiming to shed light on their ecological relevance. We start from viruses infecting microbial host cells by delivering their genetic material in seconds across nanometer-size membranes, which highjack their host's metabolism in a few minutes to hours, leading to a profound transcriptomic and metabolic rewiring. The outcome of lytic infection leads to a release of virions and signaling molecules that can reach neighboring cells a few millimeters away, resulting in a population whose heterogeneous infection level impacts the surrounding community for days. These population dynamics can leave unique metabolic and biogeochemical fingerprints across scales of kilometers and over several decades. One of the biggest challenges in marine microbiology is to assess the impact of viruses across these scales, from the single cell to the ecosystem level. Here, we argue that the advent of new methodologies and conceptual frameworks represents an exciting time to pursue these efforts and propose a set of important challenges for the field. A better understanding of host-virus interactions across scales will inform models of global ocean ecosystem function in different climate change scenarios.

Novel integrative elements and genomic plasticity in ocean ecosystems

Horizontal gene transfer accelerates microbial evolution. The marine picocyanobacterium Prochlorococcus exhibits high genomic plasticity, yet the underlying mechanisms are elusive. Here, we report a novel family of DNA transposons-"tycheposons"-some of which are viral satellites while others carry cargo, such as nutrient-acquisition genes, which shape the genetic variability in this globally abundant genus. Tycheposons share distinctive mobile-lifecycle-linked hallmark genes, including a deep-branching site-specific tyrosine recombinase. Their excision and integration at tRNA genes appear to drive the remodeling of genomic islands-key reservoirs for flexible genes in bacteria. In a selection experiment, tycheposons harboring a nitrate assimilation cassette were dynamically gained and lost, thereby promoting chromosomal rearrangements and host adaptation. Vesicles and phage particles harvested from seawater are enriched in tycheposons, providing a means for their dispersal in the wild. Similar elements are found in microbes co-occurring with Prochlorococcus, suggesting a common mechanism for microbial diversification in the vast oligotrophic oceans.

Multistep diversification in spatiotemporal bacterial-phage coevolution

The evolutionary arms race between phages and bacteria, where bacteria evolve resistance to phages and phages retaliate with resistance-countering mutations, is a major driving force of molecular innovation and genetic diversification. Yet attempting to reproduce such ongoing retaliation dynamics in the lab has been challenging; laboratory coevolution experiments of phage and bacteria are typically performed in well-mixed environments and often lead to rapid stagnation with little genetic variability. Here, co-culturing motile E. coli with the lytic bacteriophage T7 on swimming plates, we observe complex spatiotemporal dynamics with multiple genetically diversifying adaptive cycles. Systematically quantifying over 10,000 resistance-infectivity phenotypes between evolved bacteria and phage isolates, we observe diversification into multiple coexisting ecotypes showing a complex interaction network with both host-range expansion and host-switch tradeoffs. Whole-genome sequencing of these evolved phage and bacterial isolates revealed a rich set of adaptive mutations in multiple genetic pathways including in genes not previously linked with phage-bacteria interactions. Synthetically reconstructing these new mutations, we discover phage-general and phage-specific resistance phenotypes as well as a strong synergy with the more classically known phage-resistance mutations. These results highlight the importance of spatial structure and migration for driving phage-bacteria coevolution, providing a concrete system for revealing new molecular mechanisms across diverse phage-bacterial systems.

Population structure of an Antarctic aquatic cyanobacterium

BACKGROUND

Ace Lake is a marine-derived, stratified lake in the Vestfold Hills of East Antarctica with an upper oxic and lower anoxic zone. Cyanobacteria are known to reside throughout the water column. A Synechococcus-like species becomes the most abundant member in the upper sunlit waters during summer while persisting annually even in the absence of sunlight and at depth in the anoxic zone. Here, we analysed ~ 300 Gb of Ace Lake metagenome data including 59 Synechococcus-like metagenome-assembled genomes (MAGs) to determine depth-related variation in cyanobacterial population structure. Metagenome data were also analysed to investigate viruses associated with this cyanobacterium and the host's capacity to defend against or evade viruses.

RESULTS

A single Synechococcus-like species was found to exist in Ace Lake, Candidatus Regnicoccus frigidus sp. nov., consisting of one phylotype more abundant in the oxic zone and a second phylotype prevalent in the oxic-anoxic interface and surrounding depths. An important aspect of genomic variation pertained to nitrogen utilisation, with the capacity to perform cyanide assimilation and asparagine synthesis reflecting the depth distribution of available sources of nitrogen. Both specialist (host specific) and generalist (broad host range) viruses were identified with a predicted ability to infect Ca. Regnicoccus frigidus. Host-virus interactions were characterised by a depth-dependent distribution of virus type (e.g. highest abundance of specialist viruses in the oxic zone) and host phylotype capacity to defend against (e.g. restriction-modification, retron and BREX systems) and evade viruses (cell surface proteins and cell wall biosynthesis and modification enzymes).

CONCLUSION

In Ace Lake, specific environmental factors such as the seasonal availability of sunlight affects microbial abundances and the associated processes that the microbial community performs. Here, we find that the population structure for Ca. Regnicoccus frigidus has evolved differently to the other dominant phototroph in the lake, Candidatus Chlorobium antarcticum. The geography (i.e. Antarctica), limnology (e.g. stratification) and abiotic (e.g. sunlight) and biotic (e.g. microbial interactions) factors determine the types of niches that develop in the lake. While the lake community has become increasingly well studied, metagenome-based studies are revealing that niche adaptation can take many paths; these paths need to be determined in order to make reasonable predictions about the consequences of future ecosystem perturbations. Video Abstract.

Widespread yet persistent low abundance of TIM5-like cyanophages in the oceans

Ocean ecosystems are inhabited by a diverse set of viruses that impact microbial mortality and evolution. However, the distribution and abundances of specific viral lineages, particularly those from the large bank of rare viruses, remains largely unknown. Here, we assessed the diversity and abundance of the TIM5-like cyanophages. The sequencing of three new TIM5-like cyanophage genomes and environmental amplicons of a signature gene from the Red Sea revealed highly conserved gene content and sequence similarity. We adapted the polony method, a solid-phase polymerase chain reaction assay, to quantify TIM5-like cyanophages during three 2000 km expeditions in the Pacific Ocean and four annual cycles in the Red Sea. TIM5-like cyanophages were widespread, detected at all latitudes and seasons surveyed throughout the photic zone. Yet they were generally rare, ranging between <100 and 4000 viruses·ml^-1 . Occasional peaks in abundance of 10- to 100-fold were observed, reaching 71,000 viruses·ml^-1 . These peaks were ephemeral and seasonally variable in the Red Sea. Infection levels, quantified during one such peak, were very low. These characteristics of low diversity and abundance, as well as variable outbreaks, distinguishes the TIM5-like lineage from other major cyanophage lineages and illuminates that rare virus lineages can be persistent and widespread in the oceans.

A Unique Set of Auxiliary Metabolic Genes Found in an Isolated Cyanophage Sheds New Light on Marine Phage-Host Interactions

Cyanophages, viruses that infect cyanobacteria, are abundant and widely distributed in aquatic ecosystems, playing important roles in regulating the abundance, activity, diversity, and evolution of cyanobacteria. A T4-like cyanophage, S-SCSM1, infecting Synechococcus and Prochlorococcus strains of different ecotypes, was isolated from the South China Sea in this study. For the first time, a mannose-6-phosphate isomerase (MPI) gene was identified in the cultured cyanophage. At least 11 phylogenetic clusters of cyanophage MPIs were retrieved and identified from the marine metagenomic data sets, indicating that cyanophage MPIs in the marine environment are extremely diverse. The existence of 24 genes encoding 2-oxoglutarate (2OG)-Fe(II) oxygenase superfamily proteins in the S-SCSM1 genome emphasizes their potential importance and diverse functions in reprogramming host metabolism during phage infection. Novel cell wall synthesis and modification genes found in the S-SCSM1 genome indicate that diverse phenotypic modifications imposed by phages on cyanobacterial hosts remain to be discovered. Two noncoding RNAs of cis-regulatory elements in the S-SCSM1 genome were predicted to be associated with host exopolysaccharide metabolism and photosynthesis. The isolation and genomic characterization of cyanophage S-SCSM1 provide more information on the genetic diversity of cyanophages and phage-host interactions in the marine environment. IMPORTANCE Cyanophages play important ecological roles in aquatic ecosystems. Genomic and proteomic characterizations of the T4-like cyanophage S-SCSM1 indicate that novel and diverse viral genes and phage-host interactions in the marine environment remain unexplored. The first identified mannose-6-phosphate isomerase (MPI) gene from a cultured cyanophage was found in the S-SCSM1 genome, although MPIs were previously found in viral metagenomes at high frequencies similar to those of the cyanophage photosynthetic gene psbA. The presence of 24 genes encoding 2-oxoglutarate (2OG)-Fe(II) oxygenase superfamily proteins, novel cell wall synthesis and modification genes, a nonbleaching protein A gene, and 2 noncoding RNAs of cis-regulatory elements in the S-SCSM1 genome as well as the presence of a virion-associated regulatory protein indicate the diverse functions that cyanophages have in reprogramming the metabolism and modifying the phenotypes of hosts during infection.

Cyanophages from a less virulent clade dominate over their sister clade in global oceans

Environmental virus communities are highly diverse. However, the infection physiology underlying the evolution of diverse phage lineages and their ecological consequences are largely unknown. T7-like cyanophages are abundant in nature and infect the marine unicellular cyanobacteria, Synechococcus and Prochlorococcus, important primary producers in the oceans. Viruses belonging to this genus are divided into two distinct phylogenetic clades: clade A and clade B. These viruses have narrow host-ranges with clade A phages primarily infecting Synechococcus genotypes, while clade B phages are more diverse and can infect either Synechococcus or Prochlorococcus genotypes. Here we investigated infection properties (life history traits) and environmental abundances of these two clades of T7-like cyanophages. We show that clade A cyanophages have more rapid infection dynamics, larger burst sizes and greater virulence than clade B cyanophages. However, clade B cyanophages were at least 10-fold more abundant in all seasons, and infected more cyanobacteria, than clade A cyanophages in the Red Sea. Models predicted that steady-state cyanophage abundances, infection frequency, and virus-induced mortality, peak at intermediate virulence values. Our findings indicate that differences in infection properties are reflected in virus phylogeny at the clade level. They further indicate that infection properties, together with differences in subclade diversity and host repertoire, have important ecological consequences with the less aggressive, more diverse virus clade having greater ecological impacts.

Reversion is most likely under high mutation supply when compensatory mutations do not fully restore fitness costs

The dynamics of adaptation, reversion, and compensation have been central topics in microbial evolution, and several studies have attempted to resolve the population genetics underlying how these dynamics occur. However, questions remain regarding how certain features-the evolution of mutators and whether compensatory mutations alleviate costs fully or partially-may influence the evolutionary dynamics of compensation and reversion. In this study, we attempt to explain findings from experimental evolution by utilizing computational and theoretical approaches toward a more refined understanding of how mutation rate and the fitness effects of compensatory mutations influence adaptive dynamics. We find that high mutation rates increase the probability of reversion toward the wild type when compensation is only partial. However, the existence of even a single fully compensatory mutation is associated with a dramatically decreased probability of reversion to the wild type. These findings help to explain specific results from experimental evolution, where compensation was observed in nonmutator strains, but reversion (sometimes with compensation) was observed in mutator strains, indicating that real-world compensatory mutations are often unable to fully alleviate the costs associated with adaptation. Our findings emphasize the potential role of the supply and quality of mutations in crafting the dynamics of adaptation and reversal, with implications for theoretical population genetics and for biomedical contexts like the evolution of antibiotic resistance.

Greater Phage Genotypic Diversity Constrains Arms-Race Coevolution

Antagonistic coevolution between hosts and parasites, the reciprocal evolution of host resistance and parasite infectivity, has important implications in ecology and evolution. The dynamics of coevolution—notably whether host or parasite has an evolutionary advantage—is greatly affected by the relative amount of genetic variation in host resistance and parasite infectivity traits. While studies have manipulated genetic diversity during coevolution, such as by increasing mutation rates, it is unclear how starting genetic diversity affects host–parasite coevolution. Here, we (co)evolved the bacterium Pseudomonas fluorescens SBW25 and two bacteriophage genotypes of its lytic phage SBW25ɸ2 in isolation (one phage genotype) and together (two phage genotypes). Bacterial populations rapidly evolved phage resistance, and phage reciprocally increased their infectivity in response. When phage populations were evolved with bacteria in isolation, bacterial resistance and phage infectivity increased through time, indicative of arms-race coevolution. In contrast, when both phage genotypes were together, bacteria did not increase their resistance in response to increasing phage infectivity. This was likely due to bacteria being unable to evolve resistance to both phage via the same mutations. These results suggest that increasing initial parasite genotypic diversity can give parasites an evolutionary advantage that arrests long-term coevolution. This study has important implications for the applied use of phage in phage therapy and in understanding host–parasite dynamics in broader ecological and evolutionary theory.

Genetic engineering of marine cyanophages reveals integration but not lysogeny in T7-like cyanophages

Marine cyanobacteria of the genera Synechococcus and Prochlorococcus are the most abundant photosynthetic organisms on earth, spanning vast regions of the oceans and contributing significantly to global primary production. Their viruses (cyanophages) greatly influence cyanobacterial ecology and evolution. Although many cyanophage genomes have been sequenced, insight into the functional role of cyanophage genes is limited by the lack of a cyanophage genetic engineering system. Here, we describe a simple, generalizable method for genetic engineering of cyanophages from multiple families, that we named REEP for REcombination, Enrichment and PCR screening. This method enables direct investigation of key cyanophage genes, and its simplicity makes it adaptable to other ecologically relevant host-virus systems. T7-like cyanophages often carry integrase genes and attachment sites, yet exhibit lytic infection dynamics. Here, using REEP, we investigated their ability to integrate and maintain a lysogenic life cycle. We found that these cyanophages integrate into the host genome and that the integrase and attachment site are required for integration. However, stable lysogens did not form. The frequency of integration was found to be low in both lab cultures and the oceans. These findings suggest that T7-like cyanophage integration is transient and is not part of a classical lysogenic cycle.

Microbial defenses against mobile genetic elements and viruses: Who defends whom from what?

Prokaryotes have numerous mobile genetic elements (MGEs) that mediate horizontal gene transfer (HGT) between cells. These elements can be costly, even deadly, and cells use numerous defense systems to filter, control, or inactivate them. Recent studies have shown that prophages, conjugative elements, their parasites (phage satellites and mobilizable elements), and other poorly described MGEs encode defense systems homologous to those of bacteria. These constitute a significant fraction of the repertoire of cellular defense genes. As components of MGEs, these defense systems have presumably evolved to provide them, not the cell, adaptive functions. While the interests of the host and MGEs are aligned when they face a common threat such as an infection by a virulent phage, defensive functions carried by MGEs might also play more selfish roles to fend off other antagonistic MGEs or to ensure their maintenance in the cell. MGEs are eventually lost from the surviving host genomes by mutational processes and their defense systems can be co-opted when they provide an advantage to the cell. The abundance of defense systems in MGEs thus sheds new light on the role, effect, and fate of the so-called "cellular defense systems," whereby they are not only merely microbial defensive weapons in a 2-partner arms race, but also tools of intragenomic conflict between multiple genetic elements with divergent interests that shape cell fate and gene flow at the population level.

Genomic population structure of Helicobacter pylori Shanghai isolates and identification of genomic features uniquely linked with pathogenicity

Severe Helicobacter pylori-linked gastric disorders are especially prevalent in the East Asia region. The ability of H. pylori to cause different clinical outcomes is thought to be associated with unique sets of its genetic features. However, only few genetic features have been definitively linked to specific gastrointestinal pathologies. Genome heterogeneity of clinical H. pylori strains from patients with four different gastric disorders was studied to explore the population structure and molecular genomic features and their association with pathogenicity. Population analysis showed that 92.9% of the Shanghai H. pylori isolates were clustered in the East Asia group. Among 2,866 genes detected in all genomes, 1,146 genes formed the core genome, whereas 209 unique genes were detected in individual disease groups. The unique genes of peptic ulcer and gastric cancer groups represented the inorganic ion transport and metabolism function gene clusters. Sixteen virulence genes were detected with statistically different detection rates among the four disease groups. Furthermore, 127 clustered regularly interspaced short palindromic repeats were found with significantly different rates in the four disease groups. A total of 337 putative genomic islands were identified, and three genomic islands were individually found in more than 10% of strains. The genomic islands included several metabolism-associated genes and many genes with unknown function. In total, 88 sequence types were detected among the 112 Shanghai H. pylori isolates. Our study provides an essential milestone in the mapping of specific genomic features and their functions to identify factors needed to induce specific gastric disorders in H. pylori.

The genome of the diatom Chaetoceros tenuissimus carries an ancient integrated fragment of an extant virus

Diatoms are one of the most prominent oceanic primary producers and are now recognized to be distributed throughout the world. They maintain their population despite predators, infections, and unfavourable environmental conditions. One of the smallest diatoms, Chaetoceros tenuissimus, can coexist with infectious viruses during blooms. To further understand this relationship, we sequenced the C. tenuissimus strain NIES-3715 genome. A gene fragment of a replication-associated gene from the infectious ssDNA virus (designated endogenous virus-like fragment, EVLF) was found to be integrated into each 41 Mb of haploid assembly. In addition, the EVLF was transcriptionally active and conserved in nine other C. tenuissimus strains from different geographical areas, although the primary structures of their proteins varied. The phylogenetic tree further suggested that the EVLF was acquired by the ancestor of C. tenuissimus. Additionally, retrotransposon genes possessing a reverse transcriptase function were more abundant in C. tenuissimus than in Thalassiosira pseudonana and Phaeodactylum tricornutum. Moreover, a target site duplication, a hallmark for long interspersed nuclear element retrotransposons, flanked the EVLF. Therefore, the EVLF was likely integrated by a retrotransposon during viral infection. The present study provides further insights into the diatom-virus evolutionary relationship.

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.

Rapid evolutionary turnover of mobile genetic elements drives bacterial resistance to phages

[Figure: see text].

Individual bacteria in structured environments rely on phenotypic resistance to phage

Bacteriophages represent an avenue to overcome the current antibiotic resistance crisis, but evolution of genetic resistance to phages remains a concern. In vitro, bacteria evolve genetic resistance, preventing phage adsorption or degrading phage DNA. In natural environments, evolved resistance is lower possibly because the spatial heterogeneity within biofilms, microcolonies, or wall populations favours phenotypic survival to lytic phages. However, it is also possible that the persistence of genetically sensitive bacteria is due to less efficient phage amplification in natural environments, the existence of refuges where bacteria can hide, and a reduced spread of resistant genotypes. Here, we monitor the interactions between individual planktonic bacteria in isolation in ephemeral refuges and bacteriophage by tracking the survival of individual cells. We find that in these transient spatial refuges, phenotypic resistance due to reduced expression of the phage receptor is a key determinant of bacterial survival. This survival strategy is in contrast with the emergence of genetic resistance in the absence of ephemeral refuges in well-mixed environments. Predictions generated via a mathematical modelling framework to track bacterial response to phages reveal that the presence of spatial refuges leads to fundamentally different population dynamics that should be considered in order to predict and manipulate the evolutionary and ecological dynamics of bacteria–phage interactions in naturally structured environments.

Bacteriophages represent a promising avenue to overcome the current antibiotic resistance crisis, but evolution of phage resistance remains a concern. This study shows that in the presence of spatial refuges, genetic resistance to phage is less of a problem than commonly assumed, but the persistence of genetically susceptible bacteria suggests that eradicating bacterial pathogens from structured environments may require combined phage-antibiotic therapies.

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.

Interactions between bacterial and phage communities in natural environments

We commonly acknowledge that bacterial viruses (phages) shape the composition and evolution of bacterial communities in nature and therefore have important roles in ecosystem functioning. This view stems from studies in the 1990s to the first decade of the twenty-first century that revealed high viral abundance, high viral diversity and virus-induced microbial death in aquatic ecosystems as well as an association between collapses in bacterial density and peaks in phage abundance. The recent surge in metagenomic analyses has provided deeper insight into the abundance, genomic diversity and spatio-temporal dynamics of phages in a wide variety of ecosystems, ranging from deep oceans to soil and the mammalian digestive tract. However, the causes and consequences of variations in phage community compositions remain poorly understood. In this Review, we explore current knowledge of the composition and evolution of phage communities, as well as their roles in controlling the population and evolutionary dynamics of bacterial communities. We discuss the need for greater ecological realism in laboratory studies to capture the complexity of microbial communities that thrive in natural environments.

Dynamics of Baltic Sea phages driven by environmental changes

Phage predation constitutes a major mortality factor for bacteria in aquatic ecosystems, and thus, directly impacts nutrient cycling and microbial community dynamics. Yet, the population dynamics of specific phages across time scales from days to months remain largely unexplored, which limits our understanding of their influence on microbial succession. To investigate temporal changes in diversity and abundance of phages infecting particular host strains, we isolated 121 phage strains that infected three bacterial hosts during a Baltic Sea mesocosm experiment. Genome analysis revealed a novel Flavobacterium phage genus harboring gene sets putatively coding for synthesis of modified nucleotides and glycosylation of bacterial cell surface components. Another novel phage genus revealed a microdiversity of phage species that was largely maintained during the experiment and across mesocosms amended with different nutrients. In contrast to the newly described Flavobacterium phages, phages isolated from a Rheinheimera strain were highly similar to previously isolated genotypes, pointing to genomic consistency in this population. In the mesocosm experiment, the investigated phages were mainly detected after a phytoplankton bloom peak. This concurred with recurrent detection of the phages in the Baltic Proper during summer months, suggesting an influence on the succession of heterotrophic bacteria associated with phytoplankton blooms.

Genome-informed approach to identify genetic determinants of Flavobacterium psychrophilum phage susceptibility

The fish pathogen Flavobacterium psychrophilum infects farmed salmonids worldwide, and application of bacteriophages has been suggested for controlling disease outbreaks in aquaculture. Successful application of phages requires detailed knowledge about the variability in phage susceptibility of the host communities. In this study, we analysed the genetic diversity of F. psychrophilum hosts and phages from the Baltic Sea area to identify genetic determinants of phage-host interaction patterns. A host range analysis of 103 phages tested against 177 F. psychrophilum strains (18 231 phage-host interactions) identified nine phage clusters, infecting from 10% to 91% of the strain collection. The core genome-based comparison of 35 F. psychrophilum isolates revealed an extremely low overall genomic diversity (>99.5% similarity). However, a small subset of 16 ORFs, including genes involved in the type IX secretion system (T9SS), gliding motility and hypothetical cell-surface related proteins, exhibited a highly elevated genetic diversity. These specific genetic variations were linked to variability in phage infection patterns obtained from experimental studies, indicating that these genes are key determinants of phage susceptibility. These findings provide novel insights on the molecular mechanisms determining phage susceptibility in F. psychrophilum and emphasizes the importance of phages as drivers of core genomic diversity in this pathogen.

A thermal trade-off between viral production and degradation drives virus-phytoplankton population dynamics

Marine viruses interact with microbial hosts in dynamic environments shaped by variation in abiotic factors, including temperature. However, the impacts of temperature on viral infection of phytoplankton are not well understood. Here we coupled mathematical modelling with experiments to explore the effect of temperature on virus-phytoplankton interactions. Our model shows the negative consequences of high temperatures on infection and suggests a temperature-dependent threshold between viral production and degradation. Modelling long-term dynamics in environments with different average temperatures revealed the potential for long-term host-virus coexistence, epidemic free or habitat loss states. We generalised our model to variation in global sea surface temperatures corresponding to present and future seas and show that climate change may differentially influence virus-host dynamics depending on the virus-host pair. Temperature-dependent changes in the infectivity of virus particles may lead to shifts in virus-host habitats in warmer oceans, analogous to projected changes in the habitats of macro-, microorganisms and pathogens.

Viruses of freshwater bloom-forming cyanobacteria: genomic features, infection strategies and coexistence with the host

Freshwater bloom-forming cyanobacteria densely grow in the aquatic environments, leading to an increase in the viral-contact rate. They possess numerous antiviral genes, as well as cell differentiation- and physiological performance-related genes, owing to genome expansion. Their genomic features and unique lifestyles suggest that they coexist with cyanoviruses in ways different from marine cyanobacteria. Furthermore, genome contents of isolated freshwater bloom-forming cyanobacterial viruses have little in common with those of marine cyanoviruses studied to date. They lack the marine cyanoviral hallmark genes that sustain photosynthetic activity and redirect host metabolism to viral reproduction; therefore, they are predicted to share metabolisms and precursor pools with host cyanobacteria to ensure efficient viral reproduction and avoid nutrient deficiencies and antiviral response. Additionally, cyanovirus-cyanobacteria coexistence strategies may change as bloom density increases. Diverse genotypic populations of cyanoviruses and hosts coexist and fluctuate under high viral-contact rate conditions, leading to their rapid coevolution through antiviral responses. The ancestral and newly evolved genotypes coexist, thereby expanding the diversity levels of host and viral populations. Bottleneck events occurring due to season-related decreases in bloom-forming species abundance provide each genotype within cyanobacterial population an equal chance to increase in prevalence during the next bloom and enhance further diversification.

A drift-barrier model drives the genomic landscape of a structured bacterial population

Bacterial populations differentiate over time and space to form distinct genetic units. The mechanisms governing this diversification are presumed to result from the ecological context of living units to adapt to specific niches. Recently, a model assuming the acquisition of advantageous genes among populations rather than whole genome sweeps has emerged to explain population differentiation. However, the characteristics of these exchanged, or flexible, genes and whether their evolution is driven by adaptive or neutral processes remain controversial. By analysing the flexible genome of single-amplified genomes of co-occurring populations of the marine Prochlorococcus HLII ecotype, we highlight that genomic compartments - rather than population units - are characterized by different evolutionary trajectories. The dynamics of gene fluxes vary across genomic compartments and therefore the effectiveness of selection depends on the fluctuation of the effective population size along the genome. Taken together, these results support the drift-barrier model of bacterial evolution.

Quantification of Lysogeny Caused by Phage Coinfections in Microbial Communities from Biophysical Principles

The association of temperate phages and bacterial hosts during lysogeny manipulates microbial dynamics from the oceans to the human gut. Lysogeny is well studied in laboratory models, but its environmental drivers remain unclear. Here, we quantified the probability of lysogenization caused by phage coinfections, a well-known trigger of lysogeny, in marine and gut microbial environments. Coinfections were quantified by developing a biophysical model that incorporated the traits of viral and bacterial communities. Lysogenization via coinfection was more frequent in highly productive environments like the gut, due to higher microbial densities and higher phage adsorption rates. At low cell densities, lysogenization occurred in bacteria with long duplication times. These results bridge the molecular understanding of lysogeny with the ecology of complex microbial communities.

Temperate phages can associate with their bacterial host to form a lysogen, often modifying the phenotype of the host. Lysogens are dominant in the microbially dense environment of the mammalian gut. This observation contrasts with the long-standing hypothesis of lysogeny being favored at low microbial densities, such as in oligotrophic marine environments. Here, we hypothesized that phage coinfections—a well-understood molecular mechanism of lysogenization—increase at high microbial abundances. To test this hypothesis, we developed a biophysical model of coinfection for marine and gut microbiomes. The model stochastically sampled ranges of phage and bacterial concentrations, adsorption rates, lysogenic commitment times, and community diversity from each environment. In 90% of the sampled marine communities, less than 10% of the bacteria were predicted to be lysogenized via coinfection. In contrast, 25% of the sampled gut communities displayed more than 25% of lysogenization. The probability of lysogenization in the gut was a consequence of the higher densities and higher adsorption rates. These results suggest that, on average, coinfections can form two trillion lysogens in the human gut every day. In marine microbiomes, which were characterized by lower densities and phage adsorption rates, lysogeny via coinfection was still possible for communities with long lysogenic commitment times. Our study indicates that different physical factors causing coinfections can reconcile the traditional view of lysogeny at poor host growth (long commitment times) and the recent Piggyback-the-Winner framework proposing that lysogeny is favored in rich environments (high densities and adsorption rates).

IMPORTANCE The association of temperate phages and bacterial hosts during lysogeny manipulates microbial dynamics from the oceans to the human gut. Lysogeny is well studied in laboratory models, but its environmental drivers remain unclear. Here, we quantified the probability of lysogenization caused by phage coinfections, a well-known trigger of lysogeny, in marine and gut microbial environments. Coinfections were quantified by developing a biophysical model that incorporated the traits of viral and bacterial communities. Lysogenization via coinfection was more frequent in highly productive environments like the gut, due to higher microbial densities and higher phage adsorption rates. At low cell densities, lysogenization occurred in bacteria with long duplication times. These results bridge the molecular understanding of lysogeny with the ecology of complex microbial communities.

Evolutionary Mechanisms of Long-Term Genome Diversification Associated With Niche Partitioning in Marine Picocyanobacteria

Marine picocyanobacteria of the genera Prochlorococcus and Synechococcus are the most abundant photosynthetic organisms on Earth, an ecological success thought to be linked to the differential partitioning of distinct ecotypes into specific ecological niches. However, the underlying processes that governed the diversification of these microorganisms and the appearance of niche-related phenotypic traits are just starting to be elucidated. Here, by comparing 81 genomes, including 34 new Synechococcus, we explored the evolutionary processes that shaped the genomic diversity of picocyanobacteria. Time-calibration of a core-protein tree showed that gene gain/loss occurred at an unexpectedly low rate between the different lineages, with for instance 5.6 genes gained per million years (My) for the major Synechococcus lineage (sub-cluster 5.1), among which only 0.71/My have been fixed in the long term. Gene content comparisons revealed a number of candidates involved in nutrient adaptation, a large proportion of which are located in genomic islands shared between either closely or more distantly related strains, as identified using an original network construction approach. Interestingly, strains representative of the different ecotypes co-occurring in phosphorus-depleted waters (Synechococcus clades III, WPC1, and sub-cluster 5.3) were shown to display different adaptation strategies to this limitation. In contrast, we found few genes potentially involved in adaptation to temperature when comparing cold and warm thermotypes. Indeed, comparison of core protein sequences highlighted variants specific to cold thermotypes, notably involved in carotenoid biosynthesis and the oxidative stress response, revealing that long-term adaptation to thermal niches relies on amino acid substitutions rather than on gene content variation. Altogether, this study not only deciphers the respective roles of gene gains/losses and sequence variation but also uncovers numerous gene candidates likely involved in niche partitioning of two key members of the marine phytoplankton.

A single-cell polony method reveals low levels of infected Prochlorococcus in oligotrophic waters despite high cyanophage abundances

Long-term stability of picocyanobacteria in the open oceans is maintained by a balance between synchronous division and death on daily timescales. Viruses are considered a major source of microbial mortality, however, current methods to measure infection have significant methodological limitations. Here we describe a method that pairs flow-cytometric sorting with a PCR-based polony technique to simultaneously screen thousands of taxonomically resolved individual cells for intracellular virus DNA, enabling sensitive, high-throughput, and direct quantification of infection by different virus lineages. Under controlled conditions with picocyanobacteria-cyanophage models, the method detected infection throughout the lytic cycle and discriminated between varying infection levels. In North Pacific subtropical surface waters, the method revealed that only a small percentage of Prochlorococcus (0.35–1.6%) were infected, predominantly by T4-like cyanophages, and that infection oscillated 2-fold in phase with the diel cycle. This corresponds to 0.35–4.8% of Prochlorococcus mortality daily. Cyanophages were 2–4-fold more abundant than Prochlorococcus, indicating that most encounters did not result in infection and suggesting infection is mitigated via host resistance, reduced phage infectivity and inefficient adsorption. This method will enable quantification of infection for key microbial taxa across oceanic regimes and will help determine the extent that viruses shape microbial communities and ecosystem level processes.

Genomic and fossil windows into the secret lives of the most ancient fungi

Fungi have crucial roles in modern ecosystems as decomposers and pathogens, and they engage in various mutualistic associations with other organisms, especially plants. They have a lengthy geological history, and there is an emerging understanding of their impact on the evolution of Earth systems on a large scale. In this Review, we focus on the roles of fungi in the establishment and early evolution of land and freshwater ecosystems. Today, questions of evolution over deep time are informed by discoveries of new fossils and evolutionary analysis of new genomes. Inferences can be drawn from evolutionary analysis by comparing the genes and genomes of fungi with the biochemistry and development of their plant and algal hosts. We then contrast this emerging picture against evidence from the fossil record to develop a new, integrated perspective on the origin and early evolution of fungi.

Flexible genes establish widespread bacteriophage pan-genomes in cryoconite hole ecosystems

Bacteriophage genomes rapidly evolve via mutation and horizontal gene transfer to counter evolving bacterial host defenses; such arms race dynamics should lead to divergence between phages from similar, geographically isolated ecosystems. However, near-identical phage genomes can reoccur over large geographical distances and several years apart, conversely suggesting many are stably maintained. Here, we show that phages with near-identical core genomes in distant, discrete aquatic ecosystems maintain diversity by possession of numerous flexible gene modules, where homologous genes present in the pan-genome interchange to create new phage variants. By repeatedly reconstructing the core and flexible regions of phage genomes from different metagenomes, we show a pool of homologous gene variants co-exist for each module in each location, however, the dominant variant shuffles independently in each module. These results suggest that in a natural community, recombination is the largest contributor to phage diversity, allowing a variety of host recognition receptors and genes to counter bacterial defenses to co-exist for each phage.

Bacteriophages and their hosts are involved in a constant evolutionary arms race that should lead to divergence between phage genes over time. Here, the authors recruit metagenomic reads to virus reference genomes and genome fragments in samples from cryoconite holes and show that phages with near-identical core genomes maintain diversity by possession of numerous flexible gene modules, where homologous genes present in the pan-genome interchange to create new phage variants.

Broadscale phage therapy is unlikely to select for widespread evolution of bacterial resistance to virus infection

Multi-drug resistant bacterial pathogens are alarmingly on the rise, signaling that the golden age of antibiotics may be over. Phage therapy is a classic approach that often employs strictly lytic bacteriophages (bacteria-specific viruses that kill cells) to combat infections. Recent success in using phages in patient treatment stimulates greater interest in phage therapy among Western physicians. But there is concern that widespread use of phage therapy would eventually lead to global spread of phage-resistant bacteria and widespread failure of the approach. Here, we argue that various mechanisms of horizontal genetic transfer (HGT) have largely contributed to broad acquisition of antibiotic resistance in bacterial populations and species, whereas similar evolution of broad resistance to therapeutic phages is unlikely. The tendency for phages to infect only particular bacterial genotypes limits their broad use in therapy, in turn reducing the likelihood that bacteria could acquire beneficial resistance genes from distant relatives via HGT. We additionally consider whether HGT of clustered regularly interspaced short palindromic repeats (CRISPR) immunity would thwart generalized use of phages in therapy, and argue that phage-specific CRISPR spacer regions from one taxon are unlikely to provide adaptive value if horizontally-transferred to other taxa. For these reasons, we conclude that broadscale phage therapy efforts are unlikely to produce widespread selection for evolution of bacterial resistance.

Four New Genes of Cyanobacterium Synechococcus elongatus PCC 7942 Are Responsible for Sensitivity to 2-Nonanone

Microbial volatile organic compounds (VOCs) are cell metabolites that affect many physiological functions of prokaryotic and eukaryotic organisms. Earlier we have demonstrated the inhibitory effects of soil bacteria volatiles, including ketones, on cyanobacteria. Cyanobacteria are very sensitive to ketone action. To investigate the possible molecular mechanisms of the ketone 2-nonanone influence on cyanobacterium Synechococcus elongatus PCC 7942, we applied a genetic approach. After Tn5-692 transposon mutagenesis, several 2-nonanone resistant mutants have been selected. Four different mutant strains were used for identification of the impaired genes (Synpcc7942_1362, Synpcc7942_0351, Synpcc7942_0732, Synpcc7942_0726) that encode correspondingly: 1) a murein-peptide ligase Mpl that is involved in the biogenesis of cyanobacteria cell wall; 2) a putative ABC transport system substrate-binding proteins MlaD, which participates in ABC transport system that maintains lipid asymmetry in the gram-negative outer membrane by aberrantly localized phospholipids transport from outer to inner membranes of bacterial cells; 3) a conserved hypothetical protein that is encoding by gene belonging to phage gene cluster in Synechococcus elongatus PCC 7942 genome; 4) a protein containing the VRR-NUC (virus-type replication-repair nuclease) domain present in restriction-modification enzymes involved in replication and DNA repair. The obtained results demonstrated that 2-nonanone may have different targets in Synechococcus elongatus PCC 7942 cells. Among them are proteins involved in the biogenesis and functioning of the cyanobacteria cell wall (Synpcc7942_1362, Synpcc7942_0351, Synpcc7942_0732) and protein participating in stress response at DNA restriction-modification level (Synpcc7942_0726). This paper is the first report about the genes that encode protein products, which can be affected by 2-nonanone.

Adaptations Accumulated under Prolonged Resource Exhaustion Are Highly Transient

Many nonsporulating bacterial species can survive for years within exhausted growth media in a state termed long-term stationary phase (LTSP). We have been carrying out evolutionary experiments aimed at elucidating the dynamics of genetic adaptation under LTSP. We showed that Escherichia coli adapts to prolonged resource exhaustion through the highly convergent acquisition of mutations. In the most striking example of such convergent adaptation, we observed that across all independently evolving LTSP populations, over 90% of E. coli cells carry mutations to one of three specific sites of the RNA polymerase core enzyme (RNAPC). These LTSP adaptations reduce the ability of the cells carrying them to grow once fresh resources are again provided. Here, we examine how LTSP populations recover from costs associated with their adaptation once resources are again provided to them. We demonstrate that due to the ability of LTSP populations to maintain high levels of standing genetic variation during adaptation, costly adaptations are very rapidly purged from the population once they are provided with fresh resources. We further demonstrate that recovery from costs acquired during adaptation under LTSP occurs more rapidly than would be possible if LTSP adaptations had fixed during the time populations spent under resource exhaustion. Finally, we previously reported that under LTSP, some clones develop a mutator phenotype, greatly increasing their mutation accumulation rates. Here, we show that the mechanisms by which populations recover from costs associated with fixed adaptations may depend on mutator status.IMPORTANCE Many bacterial species can survive for decades under starvation, following the exhaustion of external growth resources. We have previously shown that bacteria genetically adapt under these conditions in a manner that reduces their ability to grow once resources again become available. Here, we study how populations that have been subject to very prolonged resource exhaustion recover from costs associated with their adaptation. We demonstrate that rapid adaptations acquired under prolonged starvation tend to be highly transient, rapidly reducing in frequency once bacteria are no longer starved. Our results shed light on the longer-term consequences of bacterial survival under prolonged starvation. More generally, these results may also be applicable to understanding longer-term consequences of rapid adaptation to additional conditions as well.

Quantification of T4-Like and T7-Like Cyanophages Using the Polony Method Show They Are Significant Members of the Virioplankton in the North Pacific Subtropical Gyre

The North Pacific Subtropical Gyre (NPSG) is one of the largest biomes on Earth, with the cyanobacterium Prochlorococcus being the most abundant primary producer year-round. Viruses that infect cyanobacteria (cyanophages) influence cyanobacterial mortality, diversity and evolution. Two major cyanophage families are the T4-like cyanomyoviruses and T7-like cyanopodoviruses, yet their abundances and distribution patterns remain unknown due to difficulty in quantifying their populations. To address this limitation, we previously adapted the polony method (for PCR colony) to quantify T7-like cyanophages and applied it to spring populations in the Red Sea. Here, we further adapted the method for the quantification of T4-like cyanophages and analyzed the abundances of T4-like and T7-like cyanophage populations in the photic zone of the NPSG in summer 2015 and spring 2016. Combined, the peak abundances of these two cyanophage families reached 2.8 × 10⁶ and 1.1 × 10⁶ cyanophages ⋅ ml^–1 in the summer and spring, respectively. They constituted between 3 and 16% of total virus-like particles (VLPs), comprising a substantial component of the virioplankton in the NPSG. While both cyanophage families were highly abundant, the T4-like cyanophages were generally 1.3–4.4 fold more so. In summer, cyanophages had similar and reproducible distribution patterns with depth. Abundances were relatively low in the upper mixed layer and increased to form a pronounced subsurface peak at 100 m (1.9 × 10⁶ and 9.1 × 10⁵ phages ⋅ ml^–1 for the T4-like and T7-like cyanophages, respectively), coincident with the maximum in Prochlorococcus populations. Less vertical structure in cyanophage abundances was apparent in the spring profile, despite a subsurface peak in Prochlorococcus numbers. In the summer upper mixed layer, cyanophages constituted a smaller proportion of VLPs than below it and cyanophage to cyanobacteria ratios were considerably lower (1.3–2.8) than those of VLPs to bacteria (8.1–21.2). Differences in abundances between the two families and their contribution to VLPs with depth suggest differences in cyanophage production and/or decay processes relative to other members of the virioplankton in the upper mixed layer. These findings highlight the importance of quantifying distinct populations within the virioplankton to gain accurate understanding of their distribution patterns.

Bacteriophage-mediated manipulation of the gut microbiome – promises and presents limitations

Gut microbiome (GM) composition and function are linked to human health and disease, and routes for manipulating the GM have become an area of intense research. Due to its high treatment efficacy, the use of fecal microbiota transplantation (FMT) is generally accepted as a promising experimental treatment for patients suffering from GM imbalances (dysbiosis), e.g. caused by recurrent Clostridioides difficile infections (rCDI). Mounting evidence suggests that bacteriophages (phages) play a key role in successful FMT treatment by restoring the dysbiotic bacterial GM. As a refinement to FMT, removing the bacterial component of donor feces by sterile filtration, also referred to as fecal virome transplantation (FVT), decreases the risk of invasive infections caused by bacteria. However, eukaryotic viruses and prophage-encoded virulence factors remain a safety issue. Recent in vivo studies show how cascading effects are initiated when phage communities are transferred to the gut by e.g. FVT, which leads to changes in the GM composition, host metabolome, and improve host health such as alleviating symptoms of obesity and type-2-diabetes (T2D). In this review, we discuss the promises and limitations of FVT along with the perspectives of using FVT to treat various diseases associated with GM dysbiosis.

Honey bees harbor a diverse gut virome engaging in nested strain-level interactions with the microbiota

The honey bee gut microbiota influences bee health and has become an important model to study the ecology and evolution of microbiota-host interactions. Yet, little is known about the phage community associated with the bee gut, despite its potential to modulate bacterial diversity or to govern important symbiotic functions. Here we analyzed two metagenomes derived from virus-like particles, analyzed the prevalence of the identified phages across 73 bacterial metagenomes from individual bees, and tested the host range of isolated phages. Our results show that the honey bee gut virome is composed of at least 118 distinct clusters corresponding to both temperate and lytic phages and representing novel genera with a large repertoire of unknown gene functions. We find that the phage community is prevalent in honey bees across space and time and targets the core members of the bee gut microbiota. The large number and high genetic diversity of the viral clusters seems to mirror the high extent of strain-level diversity in the bee gut microbiota. We isolated eight lytic phages that target the core microbiota member Bifidobacterium asteroides, but that exhibited different host ranges at the strain level, resulting in a nested interaction network of coexisting phages and bacterial strains. Collectively, our results show that the honey bee gut virome consists of a complex and diverse phage community that likely plays an important role in regulating strain-level diversity in the bee gut and that holds promise as an experimental model to study bacteria-phage dynamics in natural microbial communities.