Structural Study of the Cobetia marina Bacteriophage 1 (Carin-1) by Cryo-EM.

Most of studied bacteriophages (phages) are terrestrial viruses. However, marine phages are shown to be highly involved in all levels of oceanic regulation. They are, however, still largely overlooked by the scientific community. By inducing cell lysis on half of the bacterial population daily, their role and influence on the bacterial biomass and evolution, as well as their impact in the global biogeochemical cycles, is undeniable. Cobetia marina virus 1 (Carin-1) is a member of the Podoviridae family infecting the γ-protoabacteria C. marina. Here, we present the almost complete, nearly-atomic resolution structure of Carin-1 comprising capsid, portal, and tail machineries at 3.5 Å, 3.8 Å and 3.9 Å, respectively, determined by cryo-electron microscopy (cryo-EM). Our experimental results, combined with AlphaFold2 (AF), allowed us to obtain the nearly-atomic structure of Carin-1 by fitting and refining the AF atomic models in the high resolution cryo-EM map, skipping the bottleneck of de-novo manual building and speeding up the structure determination process. Our structural results highlighted the T7-like nature of Carin1, as well as several novel structural features like the presence of short spikes on the capsid, reminiscent those described for Rhodobacter capsulatus gene transfer agent (RcGTA). This is, to our knowledge, the first time such assembly is described for a bacteriophage, shedding light into the common evolution and shared mechanisms between gene transfer agents and phages. This first full structure determined for a marine podophage allowed to propose an infection mechanism different than the one proposed for the archetypal podophage T7. IMPORTANCE Oceans play a central role in the carbon cycle on Earth and on the climate regulation (half of the planet's CO2 is absorbed by phytoplankton photosynthesis in the oceans and just as much O2 is liberated). The understanding of the biochemical equilibriums of marine biology represents a major goal for our future. By lysing half of the bacterial population every day, marine bacteriophages are key actors of these equilibriums. Despite their importance, these marine phages have, so far, only been studied a little and, in particular, structural insights are currently lacking, even though they are fundamental for the understanding of the molecular mechanisms of their mode of infection. The structures described in our manuscript allow us to propose an infection mechanism that differs from the one proposed for the terrestrial T7 virus, and might also allow us to, in the future, better understand the way bacteriophages shape the global ecosystem.

Seasonal dynamics of marine protist communities in tidally mixed coastal waters.

Major seasonal community reorganizations and associated biomass variations are landmarks of plankton ecology. However, the processes of plankton community turnover rates have not been fully elucidated so far. Here, we analyse patterns of planktonic protist community succession in temperate latitudes, based on quantitative taxonomic data from both microscopy counts (cells >10 µm) and ribosomal DNA metabarcoding (size fraction >3 µm, 18S rRNA gene) from plankton samples collected bimonthly over 8 years (2009-2016) at the SOMLIT-Astan station (Roscoff, Western English Channel). Based on morphology, diatoms were clearly the dominating group all year round and over the study period. Metabarcoding uncovered a wider diversity spectrum and revealed the prevalence of Dinophyceae and diatoms but also of Cryptophyta, Chlorophyta, Cercozoa, Syndiniales and Ciliophora in terms of read counts and or richness. The use of morphological and molecular analyses in combination allowed improving the taxonomic resolution and to identify the sequence of the dominant species and OTUs (18S V4 rDNA-derived taxa) that drive annual plankton successions. We detected that some of these dominant OTUs were benthic as a result of the intense tidal mixing typical of the French coasts in the English Channel. Our analysis of the temporal structure of community changes point to a strong seasonality and resilience. The temporal structure of environmental variables (especially Photosynthetic Active Radiation, temperature and macronutrients) and temporal structures generated by species life cycles and or species interactions, are key drivers of the observed cyclic annual plankton turnover.

[Diversity and ecological importance of viruses in the marine environment]. / Diversité et importance écologique des virus dans le milieu marin.

The ocean is the largest reservoir of viruses on the planet with estimates of up to several billions per liter. These viruses represent a major driving force not only for the evolution and for structuring the microbial world, but also for the functioning and the balance of marine ecosystems. With the advances in high throughput sequencing techniques, we are beginning to uncover the diversity and the complexity of this marine virosphere. This review synthesizes milestones in the field of marine viral ecology, including the diversity of these fascinating microorganisms, their impact on microbial mortality and cycling of nutrients and energy in the ocean.

Title: Diversité et importance écologique des virus dans le milieu marin. Abstract: L'océan est le réservoir le plus important de virus sur la planète avec des abondances allant jusqu'à plusieurs milliards par litre. Ces virus sont une force directrice majeure pour l'évolution et la structuration du monde microbien, mais aussi pour le fonctionnement des grands cycles biogéochimiques dans les écosystèmes marins. Grâce aux techniques de séquençage à haut débit, nous commençons à entrevoir la diversité et la complexité de cette virosphère marine. Cette synthèse décrit les découvertes importantes dans le domaine de l'écologie virale marine et aborde la diversité de ces micro-organismes fascinants, leur impact sur la mortalité microbienne et les cycles de nutriments et d'énergie dans l'océan.

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.

Diversity and dynamics of relevant nanoplanktonic diatoms in the Western English Channel.

In the ocean, Bacillariophyta are one of the most successful protistan groups. Due to their considerable biogeochemical implications, diatom diversity, development, and seasonality have been at the center of research, specifically large-sized species. In comparison, nanoplanktonic diatoms are mostly disregarded from routine monitoring and are often underrepresented in genetic reference databases. Here, we identified and investigated the temporal dynamics of relevant nanodiatoms occurring in the Western English Channel (SOMLIT-Astan station). Coupling in situ and laboratory approaches, we revealed that nano-species from the genera Minidiscus and Thalassiosira are key components of the phytoplankton community that thrive in these coastal waters, but they display different seasonal patterns. Some species formed recurrent blooms whilst others were persistent year round. These results raise questions about their regulation in the natural environment. Over a full seasonal cycle at the monitoring station, we succeeded in isolating viruses which infect these minute diatoms, suggesting that these mortality agents may contribute to their control. Overall, our study points out the importance of considering nanodiatom communities within time-series surveys to further understand their role and fate in marine systems.

Viral Attachment to Biotic and Abiotic Surfaces in Seawater.

Viruses influence microbial community structure and biogeochemical cycles in marine environments. Viral attachment to nonhost surfaces could influence host viral infection rates; however, the prevalence of such viral attachment is not investigated quantitatively. We used coastal seawater viral assemblages and, as models, marine vibriophage (SIO-2) and enterobacteriophages (T2 and T4) to investigate their attachment to probable nonhost marine bacteria. We also studied viral attachment to colloids and other abiotic surfaces in seawater. Centrifugation experiments with bacterium-virus mixtures showed substantial viral loss in the supernatant presumably due to the viral attachment to bacteria. This attachment (0.04 to 24 viruses µm-2 [bacterial surface area]) varied with bacterium-virus combinations. Surprisingly, filtering seawater on 0.2-µm Anodisc or polycarbonate filters retained ∼12 to 84% of viruses presumably attached to ≥0.2-µm-sized particles and/or the filter surface. Enzymatic digestion followed by epifluorescence and atomic force microscopy suggested that 7 to 25% of the total viruses were attached via ß-glycosidic linkages. Furthermore, a substantial proportion (7 to 48%) of viruses became attached to model abiotic surfaces (polycarbonate, polypropylene, and glass), and this has significance for laboratory protocols as well as studies of virus ecology in particle-rich marine environments. Substantial attachment of viruses to nonhost surfaces could influence virus-driven biogeochemical cycles and microbial community structure.IMPORTANCE Viruses play important roles in altering microbial community structure and biogeochemical cycles in marine environments. Viral attachment to nonhost surfaces can influence host viral infection rates; however, the prevalence of viral attachment to nonhost surfaces and the ratio of attached viruses to total viruses are little known. We used coastal seawater viral assemblages and used marine vibriophage (SIO-2) and enterobacteriophages (T2 and T4) as models to investigate their attachment to abiotic and biotic surfaces in seawater. Viral attachment was observed on several surfaces, such as nonhost bacteria, polymers, filters, cover glasses, and tube surfaces. This study cautions against commonly used protocols that require viral incubation and seawater fractionation. More importantly, these results could influence virus-driven biogeochemical cycles and microbial community structure in the ocean.

Picoeukaryotes of the Micromonas genus: sentinels of a warming ocean.

Photosynthetic picoeukaryotesx in the genus Micromonas show among the widest latitudinal distributions on Earth, experiencing large thermal gradients from poles to tropics. Micromonas comprises at least four different species often found in sympatry. While such ubiquity might suggest a wide thermal niche, the temperature response of the different strains is still unexplored, leaving many questions as for their ecological success over such diverse ecosystems. Using combined experiments and theory, we characterize the thermal response of eleven Micromonas strains belonging to four species. We demonstrate that the variety of specific responses to temperature in the Micromonas genus makes this environmental factor an ideal marker to describe its global distribution and diversity. We then propose a diversity model for the genus Micromonas, which proves to be representative of the whole phytoplankton diversity. This prominent primary producer is therefore a sentinel organism of phytoplankton diversity at the global scale. We use the diversity within Micromonas to anticipate the potential impact of global warming on oceanic phytoplankton. We develop a dynamic, adaptive model and run forecast simulations, exploring a range of adaptation time scales, to probe the likely responses to climate change. Results stress how biodiversity erosion depends on the ability of organisms to adapt rapidly to temperature increase.

Therapeutic Potential of a New Jumbo Phage That Infects Vibrio coralliilyticus, a Widespread Coral Pathogen.

Biological control using bacteriophages is a promising approach for mitigating the devastating effects of coral diseases. Several phages that infect Vibrio coralliilyticus, a widespread coral pathogen, have been isolated, suggesting that this bacterium is permissive to viral infection and is, therefore, a suitable candidate for treatment by phage therapy. In this study, we combined functional and genomic approaches to evaluate the therapeutic potential of BONAISHI, a novel V. coralliilyticus phage, which was isolated from the coral reef in Van Phong Bay (Vietnam). BONAISHI appears to be strictly lytic for several pathogenic strains of V. coralliilyticus and remains infectious over a broad range of environmental conditions. This candidate has an unusually large dsDNA genome (303 kb), with no genes that encode known toxins or implicated in lysogeny control. We identified several proteins involved in host lysis, which may offer an interesting alternative to the use of whole bacteriophages for controlling V. coralliilyticus. A preliminary therapy test showed that adding BONAISHI to an infected culture of Symbiodinium sp. cells reduced the impact of V. coralliilyticus on Symbiodinium sp. photosynthetic activity. This study showed that BONAISHI is able to mitigate V. coralliilyticus infections, making it a good candidate for phage therapy for coral disease.

A New Freshwater Cyanosiphovirus Harboring Integrase.

Pelagic cyanobacteria are key players in the functioning of aquatic ecosystems, and their viruses (cyanophages) potentially affect the abundance and composition of cyanobacterial communities. Yet, there are few well-described freshwater cyanophages relative to their marine counterparts, and in general, few cyanosiphoviruses (family Siphoviridae) have been characterized, limiting our understanding of the biology and the ecology of this prominent group of viruses. Here, we characterize S-LBS1, a freshwater siphovirus lytic to a phycoerythrin-rich Synechococcus isolate (Strain TCC793). S-LBS1 has a narrow host range, a burst size of ∼400 and a relatively long infecting step before cell lysis occurs. It has a dsDNA 34,641 bp genome with putative genes for structure, DNA packing, lysis, replication, host interactions, DNA repair and metabolism. S-LBS1 is similar in genome size, genome architecture, and gene content, to previously described marine siphoviruses also infecting PE-rich Synechococcus, e.g., S-CBS1 and S-CBS3. However, unlike other Synechococcus phages, S-LBS1 encodes an integrase, suggesting its ability to establish lysogenic relationships with its host. Sequence recruitment from viral metagenomic data showed that S-LBS1-like viruses are diversely present in a wide range of aquatic environments, emphasizing their potential importance in controlling and structuring Synechococcus populations. A comparative analysis with 16 available sequenced cyanosiphoviruses reveals the absence of core genes within the genomes, suggesting high degree of genetic variability in siphoviruses infecting cyanobacteria. It is likely that cyanosiphoviruses have evolved as distinct evolutionary lineages and that adaptive co-evolution occurred between these viruses and their hosts (i.e., Synechococcus, Prochlorococcus, Nodularia, and Acaryochloris), constituting an important driving force for such phage diversification.

First Viruses Infecting the Marine Diatom Guinardia delicatula.

The marine diatom Guinardia delicatula is a cosmopolitan species that dominates seasonal blooms in the English Channel and the North Sea. Several eukaryotic parasites are known to induce the mortality of this species. Here, we report the isolation and characterization of the first viruses that infect G. delicatula. Viruses were isolated from the Western English Channel (SOMLIT-Astan station) during the late summer bloom decline of G. delicatula. A combination of laboratory approaches revealed that these lytic viruses (GdelRNAV) are small tailless particles of 35-38 nm in diameter that replicate in the host cytoplasm where both unordered particles and crystalline arrays are formed. GdelRNAV display a linear single-stranded RNA genome of ~9 kb, including two open reading frames encoding for replication and structural polyproteins. Phylogenetic relationships based on the RNA-dependent-RNA-polymerase gene marker showed that GdelRNAV are new members of the Bacillarnavirus, a monophyletic genus belonging to the order Picornavirales. GdelRNAV are specific to several strains of G. delicatula. They were rapidly and largely produced (<12 h, 9.34 × 104 virions per host cell). Our analysis points out the host's variable viral susceptibilities during the early exponential growth phase. Interestingly, we consistently failed to isolate viruses during spring and early summer while G. delicatula developed important blooms. While our study suggests that viruses do contribute to the decline of G. delicatula's late summer bloom, they may not be the primary mortality agents during the remaining blooms at SOMLIT-Astan. Future studies should focus on the relative contribution of the viral and eukaryotic pathogens to the control of Guinardia's blooms to understand the fate of these prominent organisms in marine systems.

Micromonas versus virus: New experimental insights challenge viral impact.

Viruses have recurrently been hypothesized as instrumental in driving microbial population diversity. Nonetheless, viral mediated co-existence of r/k-strategists, predicted in the Killing-the-Winner (KtW) hypothesis, remains controversial and demands empirical evidence. Therefore, we measured the life strategy parameters that characterize the relevant system Micromonas-Micromonas Virus (MicV). A large number of host and viral strains (37 and 17, respectively) were used in a total of 629 cross-infectivity tests. Algal and viral abundances were monitored by flow cytometry and used to calculate values of growth rate, resistance capacity, and viral production. Two main assumptions of the KtW model, namely (1) a resistance-associated cost on growth and (2) a negative correlation between resistance and viral production capacity, were mildly observed and lacked statistical significance. Micromonas strains infected by more MicV strains presented higher lysis and viral production rates as the number of infectious virus strains increased, suggesting a 'one-gate' regulation of infection in this system. MicV strains demonstrated a vast range of virion production capacity, which unexpectedly grew with increasing host-range. Overall, the significant trends observed in here demonstrate strong co-interactions at different levels between Micromonas and MicV populations, however, the role of viruses as major driving force in phytoplankton fitness wasn't explicitly observed.

Temperature is a key factor in Micromonas-virus interactions.

The genus Micromonas comprises phytoplankton that show among the widest latitudinal distributions on Earth, and members of this genus are recurrently infected by prasinoviruses in contrasted thermal ecosystems. In this study, we assessed how temperature influences the interplay between the main genetic clades of this prominent microalga and their viruses. The growth of three Micromonas strains (Mic-A, Mic-B, Mic-C) and the stability of their respective lytic viruses (MicV-A, MicV-B, MicV-C) were measured over a thermal range of 4-32.5 °C. Similar growth temperature optima (Topt) were predicted for all three hosts but Mic-B exhibited a broader thermal tolerance than Mic-A and Mic-C, suggesting distinct thermoacclimation strategies. Similarly, the MicV-C virus displayed a remarkable thermal stability compared with MicV-A and MicV-B. Despite these divergences, infection dynamics showed that temperatures below Topt lengthened lytic cycle kinetics and reduced viral yield and, notably, that infection at temperatures above Topt did not usually result in cell lysis. Two mechanisms operated depending on the temperature and the biological system. Hosts either prevented the production of viral progeny or maintained their ability to produce virions with no apparent cell lysis, pointing to a possible switch in the viral life strategy. Hence, temperature changes critically affect the outcome of Micromonas infection and have implications for ocean biogeochemistry and evolution.

Les virus au service de l'écologie marine.

In less than 50 years, marine viruses shifted from meaningless entities to major players of the oceanic ecosystems. These parasites numerically dominate marine microbial communities and mostly infect micro-organisms (bacteria, microalgae and other protists) that constitute the basis of trophic levels in the ocean. Viruses that replicate though a lysogenic cycle affect genetic expression of the host and promote horizontal gene transfer within the marine microbial communities. Viruses that replicate through a lytic cycle contribute to the control of host population and the release of a large amount of organic matter in the ocean. From the genetic manipulation of their hosts to the modification of the biogeochemical cycles, the marine viruses play a pivotal role for the structure and the functioning of their environment and cannot be excluded from ecological models anymore. This review presents the impact of viruses on the marine environment by focusing on three integration scales: the cell, the community and the ecosystem.

The marine bacteria Cobetia marina DSMZ 4741 synthesizes an unexpected K-antigen-like exopolysaccharide.

We have studied the exopolysaccharide produced by Cobetia marina DSMZ 4741, a marine bacterium isolated from coastal seawater. This strain is able to produce a polysaccharide in presence of carbon sources as glucose, mannitol and alginate. The maximum production occurs in aerobic condition, during the end of the exponential phase. The polymer is a non-viscous, acidic heteropolysaccharide of 270kDa constituted of a repeating unit of: This kind of chemical structure is generally related to K-antigen polysaccharide of pathogenic Escherichia coli strains. This is the first time this type of EPS is described from a marine bacterium. Moreover the polysaccharide exhibits a pyruvate substitution on its 3-deoxy-d-manno-oct-2-ulosonic acid (KDO) residue never encountered before. The discovery of such an unexpected EPS with high biotechnological potential is a new incentive for a better exploration of bioactive marine resources.

'Ménage à trois': a selfish genetic element uses a virus to propagate within Thermotogales.

Prokaryotic viruses play a major role in the microbial ecology and evolution. However, the virosphere associated with deep-sea hydrothermal ecosystems remains largely unexplored. Numerous instances of lateral gene transfer have contributed to the complex and incongruent evolutionary history of Thermotogales, an order well represented in deep-sea hydrothermal vents. The presence of clustered regularly interspaced short palindromic repeats (CRISPR) loci has been reported in all Thermotogales genomes, suggesting that these bacteria have been exposed to viral infections that could have mediated gene exchange. In this study, we isolated and characterized the first virus infecting bacteria from the order Thermotogales, Marinitoga piezophila virus 1 (MPV1). The host, Marinitoga piezophila is a thermophilic, anaerobic and piezophilic bacterium isolated from a deep-sea hydrothermal chimney. MPV1 is a temperate Siphoviridae-like virus with a 43.7 kb genome. Surprisingly, we found that MPV1 virions carry not only the viral DNA but preferentially package a plasmid of 13.3 kb (pMP1) also carried by M. piezophila. This 'ménage à trois' highlights potential relevance of selfish genetic elements in facilitating lateral gene transfer in the deep-sea biosphere.

Exopolysaccharide biosynthesis and biodegradation by a marine hydrothermal Alteromonas sp. strain.

Alteromonas macleodii subsp. fijiensis biovar deepsane is a deep-sea ecotype exopolysaccharide-producing bacteria isolated from the polychaete annelid Alvinella pompejana. The high molecular weight biopolymer HYD657 produced by this strain, is the first marine exopolysaccharide (EPS) to be commercialized for cosmetic use. Depolymerization methods are necessary to elucidate the complete structure of this EPS and to generate potentially bioactive oligosaccharides. Enzymatic methods are useful for elucidating polysaccharide structure because they specifically cleave glycosidic bonds and do not require harsh chemical conditions. The HYD657 EPS is structurally complex and no commercially available enzymes are able to effectively degrade it. Here, we present the first results on the endogenous enzymatic depolymerization of a marine EPS of biotechnological interest by the producing strain. Enzymatic activity was detected in the bacterial lysate and was able to decrease the apparent molecular size of the EPS, releasing mainly oligosaccharides. The reduced form of the native polysaccharide showed a slightly modified osidic composition, particularly in terms of molar ratio. Several exoglycosidase activities were measured in the bacterial lysate using paranitrophenyl-osides.

High occurrence of viruses in the mucus layer of scleractinian corals.

Viruses attract increasing interest from environmental microbiologists seeking to understand their function and role in coral health. However, little is known about their main ecological traits within the coral holobiont. In this study, a quantitative and qualitative characterization of viral and bacterial communities was conducted on the mucus of seven different coral species of the Van Phong Bay (Vietnam). On average, the concentrations of viruses and bacteria were, respectively, 17- and twofold higher in the mucus than in the surrounding water. The examination of bacterial community composition also showed remarkable differences between mucus and water samples. The percentage of active respiring cells was nearly threefold higher in mucus (m = 24.8%) than in water (m = 8.6%). Interestingly, a positive and highly significant correlation was observed between the proportion of active cells and viral abundance in the mucus, suggesting that the metabolism of the bacterial associates is probably a strong determinant of the distribution of viruses within the coral holobiont. Overall, coral mucus, given its unique physicochemical characteristics and sticking properties, can be regarded as a highly selective biotope for abundant, diversified and specialized symbiotic microbial and viral organisms.

Iron organic speciation determination in rainwater using cathodic stripping voltammetry.

A sensitive method using Competitive Ligand Exchange-Adsorptive Cathodic Stripping Voltammetry (CLE-ACSV) has been developed to determine for the first time iron (Fe) organic speciation in rainwater over the typical natural range of pH. We have adapted techniques previously developed in other natural waters to rainwater samples, using the competing ligand 1-nitroso-2-naphthol (NN). The blank was equal to 0.17±0.05 nM (n=14) and the detection limit (DL) for labile Fe was 0.15 nM which is 10-70 times lower than that of previously published methods. The conditional stability constant for NN under rainwater conditions was calibrated over the pH range 5.52-6.20 through competition with ethylenediaminetetraacetic acid (EDTA). The calculated value of the logarithm of ß'(Fe(3+)(NN)(3)) increased linearly with increasing pH according to log ß'(Fe(3+)(NN)(3)) (salinity=2.9, T=20 °C). The validation of the method was carried out using desferrioxamine mesylate B (DFOB) as a natural model ligand for Fe. Adequate detection windows were defined to detect this class of ligands in rainwater with 40 µM of NN from pH 5.52 to 6.20. The concentration of Fe-complexing natural ligands was determined for the first time in three unfiltered and one filtered rainwater samples. Organic Fe-complexing ligand concentrations varied from 104.2±4.1 nM equivalent of Fe(III) to 336.2±19.0 nM equivalent of Fe(III) and the logarithm of the conditional stability constants, with respect to Fe(3+), varied from 21.1±0.2 to 22.8±0.3. This method will provide important data for improving our understanding of the role of wet deposition in the biogeochemical cycling of iron.

Capsomer dynamics and stabilization in the T = 12 marine bacteriophage SIO-2 and its procapsid studied by CryoEM.

We report the subnanometer cryo-electron microscopy (cryoEM) reconstruction of a marine siphovirus, the Vibrio phage SIO-2. This phage is lytic for related Vibrio species with significant ecological importance, including the broadly antagonistic bacterium Vibrio sp. SWAT3. The three-dimensional structure of the 800 Å SIO-2, icosahedrally averaged head of the tailed particle revealed a T = 12 quasi-symmetry not previously described in a bacteriophage. Two morphologically distinct types of auxiliary proteins were also identified; one species bound to the surface of hexamers, and the other bound to pentamers. The secondary structure, evident in the electron density, shows that the major capsid protein has the HK97-like fold. The three-dimensional structure of the procapsid form, also presented here, has no "decoration" proteins and reveals a capsomer organization due to the constraints of the T = 12 symmetry.

Dynamics and diversity of newly produced virioplankton in the North Sea.

Viral diversity has been studied in a variety of marine habitats and spatial and seasonal changes have been documented. Most of the bacteriophages are considered host specific and are thought to affect fast growing prokaryotic phylotypes more than slow growing ones. We hypothesized that viral infection and consequently, lysis occurs in pulses with only a few prokaryotic phylotypes lysed at any given time. Thus, we propose that the newly produced viruses represent only a fraction of the viral diversity present at any given time. Virioplankton diversity was assessed by pulsed-field gel electrophoresis in the surface waters of three distinct areas of the North Sea during the spring and summer. Bulk virioplankton diversity was fairly stable in these waters. Viral diversity produced by the indigenous bacterioplankton, however, exhibited day-to-day variability with only a few bands produced at any given time. These bands frequently matched bands of the in situ virioplankton; however, bands not present in the band pattern of the in situ virioplankton community were also found. These new bands probably indicate infection and subsequent release of viruses from bacterioplankton phylotypes previously not infected by these specific viruses. Overall, our results demonstrate that viral infection and lysis are rather dynamic processes. The main targets of viral infection are changing apparently on time scales of hours to days indicating that viral infection might effectively regulate and maintain bacterioplankton diversity.