What are viruses? Marine Viruses I & II. OCN 626 Marine Microplankton Ecology. Characteristics, Abundance, and Diversity

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1 OCN 626 Marine Microplankton Ecology Marine Viruses I & II Characteristics, Abundance, and Diversity What Are Viruses? What are they made of? How do they replicate? Are they alive? What are viruses? Infectious agents that can replicate only inside a host cell Relatively simple: A set of instructions (DNA or RNA) A protective coat No motility No metabolism Bacteriophage Epsilon 15

2 Baltimore Classification Group I: Double-stranded DNA viruses Group II: single-stranded DNA viruses Group III: double-stranded RNA viruses Group IV: positive-sense single-stranded RNA viruses Group V: negative-sense single-stranded RNA viruses Group VI: positive-sense single-stranded RNA viruses that replicate through a DNA intermediate (retroviruses) Group VII: Double-stranded DNA viruses with ssrna intermediates A continuum of extrachromosomal Genetic Rebels Tranposons - no direct mechanism of cell entry or exit (must hitch a ride) Plasmids - direct transfer between hosts, no extracellular stage Viruses - cell entry and exit mechanisms with extracellular stage LEVINE, A. J. (1992). Viruses. Scientific American Library, New York.

3 Many shapes and Sizes Siphovirus (bacteria) Influenza (Humans, birds, pig, seals) Tobamovirus (Tobacco plant) Calicivirus (vertebrates) Viruses Infect All Marine Organisms Turtle Herpesvirus Infection Marine mammals Turtles Fish Crustaceans Bivalves Protozoa Macroalgae (kelp) Microalgae (phytoplankton) Bacteria Bacteriophage Infection

4 Phases of the infection cycle In this example burst size = 6 Modes of Bacteriophage Replication Abundance and Distribution of Viruses Fuhrman J. A in Microbial Ecology of the Oceans, Kirchman, D. A. (Ed) Wiley and Sons N.Y. Prokaryotes ( 10 9 per l) Viruses ( per l) Steward unpublished, Steward et al. (1996) MEPS, Steward et al Deep-Sea Res.

5 TEMPORAL VARIATIONS Long term Short term Bratbak et al. (1996) FEMS Microbiol. Ecol. 19: Jiang and Paul (1994) Mar. Ecol. Progr. Ser Viruses vs. Bacteria Literature Summary Steward unpublished compilation Fuhrman (2000) in Microbial Ecology of the Oceans, Kirchman (ed) Wiley & Sons, New York

6 Visibly infected bacteria Visibly infected bacteria Generally <1 up to about 4% of bacteria in the ocean are at a stage of infection where viruses are visible inside Seems like a small fraction, but must consider the viral replication cycle! With additional assumptions can calculate bacterial mortality

7 Bacteriophage Replication If only 20% of infected cells are visible, must multiply by 5 to get TOTAL infected cells. Actual factor is in the range of 4 to 10 Proctor et al. (1993) Microb. Ecol. 25: [average factor reported as 5.4] Weinbauer et al. (2002) Aquat. Microb. Ecol. 27: [average factor reported as 7.1] Viral Mortality of Bacteria Environment Mean FVIC (%) FIC (%) Mortality (%) Ref. MARINE L.I. Sound E. Caribbean Sea W. Caribbean Sea Sargasso Sea Gulf Stream Arctic N Adriatic Sea Solar Salterns FRESHWATER Lake Pluβsee Danube River Lake Constance MEAN Proctor and Fuhrman (1990) 5. Weinbauer and Höfle (1998) 2. Steward et al. (1996) 6. Mathias et al. (1995) 3. Weinbauer and Peduzzi (1995) 7. Hennes and Simon (1995) 4. Guixa-Boixareu et al. (1996) Table Adapted from Binder (1999)

8 VIRAL INFECTION CAN TERMINATE BLOOMS (putative E. Huxleyi virus) Nagasaki et al J. Plank. Res. 16: Bratbak et al MEPS 93: July Host Range Varies Widely but can be very specific Host Strains Viral Isolates

9 VIRAL INFLUENCE ON CARBON CYCLING ALGAL LYSIS EFFICIENTLY TRANSFERS CARBON TO BACTERIA VIRAL INFLUENCE ON CARBON CYCLING BACTERIAL LYSATES STIMULATE ACTIVITY OF NON-INFECTED CELLS

10 CO2 DMS Viral Lysis vs. Grazing Trophic Sink Very Selective C, N, P, Trace Metals Viral Loop Microbial Loop Trophic Link Less Selective Other Consequences of Infection Transduction Lysogenic Conversion Infection Immunity New Enzymes Toxin Production Estimate: 43% of marine isolates harbor prophages

11 LYSOGENIC CONVERSION IN VIBRIO CHOLERAE PHAGE MOBILIZATION OF TOXIN GENE (TST) IN STAPHYLOCOCCUS AUREUS Importance of Lysogeny Lysogenic bacteria are common Contribution to viral production seems generally low but induction could trigger mass lysis Natural induction (e.g. temperature, UV) may be occasionally important Significance of lysogenic conversion in the marine environment is not known Importance of Transduction Some marine bacteria have been isolated which spontaneously release generalized transducing particles at high frequency (Chiura, H.X Aquat. Microb Ecol. 13: 75-83) Can transfer genes among bacteria: generate new combinations of genes repair of mutated genes Quantitative importance in marine environment remains unknown

12 Summary Consequences of Viral Infections Increase respiration & remineralization in the food web Terminate some phytoplankton blooms Drive species successions Enhance microbial diversity Mediate transduction Change bacterial phenotype through lysogenic conversion