Structure of Virioplankton in the Charente Estuary (Frane) : Transmission Electron Microscopy versus Pulsed Field Gel Electrophoresis

International audienc

Detection of Ostreid Herpesvirus 1 (OsHV-1) DNA in seawater by PCR: influence of water parameters in bioassays

Since 1991, herpesvirus infections have been reported among larvae and juveniles of various bivalves. Most of the studies focused on detection of viral infections of economically important species. However, the persistence of bivalve herpesviruses; in the marine environment is poorly documented. The present study concerns the role of seawater parameters in Ostreid Herpesvirus I (OsHV-1) detection by polymerase chain reaction (PCR). Viral DNA extracted from purified particles or virions present in infected oyster larvae were detected by PCR after storage in different media at different temperatures. The lowest detection threshold was found using distilled water or Tris EDTA buffer. In seawater, the threshold was higher. The use of sterile media permitted detection of viral DNA stored over a longer period. Storage temperature also had a significant influence on detection, with lower temperatures promoting DNA detection over a longer period. In summary, water parameters such as temperature influenced detection of OsHV-1 DNA by PCR. However, the PCR technique may also be successfully applied to samples in natural seawater. Indeed, the PCR technique permitted detection of naked viral DNA at 100 ng l(-1) in seawater in bioassays

Impact of the oyster Crassostrea gigas on a microbial community in Atlantic coastal ponds near La Rochelle

To assess the in situ impact of oysters Crassostrea gigas on planktonic protist and bacteria communities and the potential contribution of protozoa to their food resource intake, the abundance and the diversity of protists and bacteria were followed in 2 Atlantic coastal ponds, with and without oysters. The protist biomass in such ponds was high, with a maximum in spring of 982 mug C l(-1) and a minimum in winter of 179 pg C l(-1). Whatever the season, the presence of oysters (20 m(-2) corresponding to an average of 23 mg dry weight m(-2)) induced a significant decrease in >5 pm protist abundance. On the contrary, planktonic organisms <5 pm, such as Chlorophyta flagellates and bacteria, developed similarly in both ponds. It can be assumed that such depletion in micro-sized protists was especially related to the grazing activity of C. gigas, which efficiently retains >5 pm particles. In spring, oyster grazing triggered dramatic changes in the protist community by lowering the taxonomic diversity. In autumn and winter, the presence of oysters deeply influenced the taxonomic structure of the protist communities: > Fun protists could only develop in the control pond, whereas they were removed by filtration in the oyster pond; on the contrary, >5 Fun protists that were not retained were favoured in the oyster pond. The results showed that hetero/mixotrophic protists represent an important potential resource in coastal ponds: flagellates >5 pm were the main protist resource for C. gigas; ciliates represented the second resource, with a substantial contribution in autumn; diatoms and dinoflagellates, though efficiently removed, represented a weak carbon resource. Our study supports the hypothesis that oysters may access the strong bacterioplanktonic production through hetero/mixotrophic protists, which would thus allow the transfer of carbon from the microbial loop towards C. gigas

Potential effect of freshwater virus on the structure and activity of bacterial communities in the Marennes-Oléron bay (France).

12 páginas, 6 figuras, 2 tablas.Batch culture experiments using viral enrichment were conducted to test the response of a coastal bacterial community to autochthonous (i.e., co-existing) or allochthonous riverine viruses. The effects of viral infections on bacterial dynamics and activity were assessed by epifluorescence microscopy and thymidine incorporation, respectively, whereas the effect of viral infection on bacterial community composition was examined by polymerase chain reaction-single strand conformation polymorphism 16S ribosomal RNA fingerprinting. The percentages of high nucleic acid-containing cells, evaluated by flow cytometry, were significantly correlated (r 2 =0.91, n=12, p<0.0001) to bacterial production, making this value a good predictor of active cell dynamics along the study. While confinement and temperature were the two principal experimental factors affecting bacterial community composition and dynamics, respectively, additions of freshwater viruses had significant effects on coastal bacterial communities. Thus, foreign viruses significantly reduced net bacterial population increase as compared to the enrichment treated with inactivated virus. Moreover, freshwater viruses recurrently and specifically affected bacterial community composition, as compared to addition of autochthonous viruses. In most cases, the combined treatment viruses and freshwater dissolved organic matter helped to maintain or even enhance species richness in coastal bacterial communities in agreement to the ‘killing the winner’ hypothesis. Thus, riverine virus input could potentially influence bacterial community composition of the coastal bay albeit with modest modification of bulk bacterial growth.This work was supported by grants from the French ‘Programme National en Environnement Côtier’ (PNEC, ART 5), the ‘Contrat de Plan Etat Région’ (CPER 2000–2005) and the ‘Conseil Général de Charente-Maritime’ (PhD grant to J.-C. Auguet). EO Casamayor is a ‘Ramon y Cajal’ Fellow supported by project REN 2003-08333 from the Spanish Ministry of Education and Science (MEC) and the European Community. At the final writing period for this work, J.-C. Auguet benefited from a SEUI-MEC Spanish postdoctoral fellowship.Peer reviewe

Variability and Host Density Independence in Inductions-based Estimates of Environmental Lysogeny

Temperate bacterial viruses (phages) may enter a symbiosis with their host cell, forming a unit called a lysogen. Infection and viral replication are disassociated in lysogens until an induction event such as DNA damage occurs, triggering viral-mediated lysis. The lysogen–lytic viral reproduction switch is central to viral ecology, with diverse ecosystem impacts. It has been argued that lysogeny is favoured in phages at low host densities. This paradigm is based on the fraction of chemically inducible cells (FCIC) lysogeny proxy determined using DNA-damaging mitomycin C inductions. Contrary to the established paradigm, a survey of 39 inductions publications found FCIC to be highly variable and pervasively insensitive to bacterial host density at global, within-environment and within-study levels. Attempts to determine the source(s) of variability highlighted the inherent complications in using the FCIC proxy in mixed communities, including dissociation between rates of lysogeny and FCIC values. Ultimately, FCIC studies do not provide robust measures of lysogeny or consistent evidence of either positive or negative host density dependence to the lytic–lysogenic switch. Other metrics are therefore needed to understand the drivers of the lytic–lysogenic decision in viral communities and to test models of the host density-dependent viral lytic–lysogenic switch