4.2 Marine viruses and their ecological roles

Marine viruses are the ocean's most abundant life forms, outnumbering bacteria 5-25 times. They infect a wide range of organisms, from bacteria to eukaryotes, and their distribution varies across marine environments. Coastal and nutrient-rich waters have higher viral concentrations than the open ocean.

These tiny powerhouses significantly impact marine ecosystems. They kill 20-40% of marine bacteria daily, altering microbial communities and nutrient cycles. Viruses also drive evolution through gene transfer and selective pressure, shaping the genetic landscape of marine microorganisms.

Marine Viruses: Abundance, Diversity, and Distribution

Abundance and distribution of marine viruses

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• Marine viruses are the most abundant biological entities in the oceans
  • Estimated to be $10^{30}$ viruses in the global oceans which is an incredibly large number
  • Exceed bacterial abundance by 5-25 times meaning there are far more viruses than bacteria in the ocean
• High diversity of marine viruses infecting a wide range of marine organisms
  • Bacteria, archaea, and eukaryotic microorganisms are all susceptible to viral infection
  • Exhibit diverse morphologies such as tailed (bacteriophages), polyhedral (adenoviruses), and filamentous viruses (Ebola)
• Viral distribution varies across different marine environments
  • Higher viral abundance in coastal and nutrient-rich waters (estuaries, upwelling zones) compared to oligotrophic open ocean
  • Vertical distribution: viral abundance typically decreases with depth in the water column as there are fewer hosts deeper down

Ecological Roles and Impacts of Marine Viruses

Impact on microbial populations

• Viral lysis is a major cause of microbial mortality in the oceans
  • Estimated that 20-40% of marine bacteria are killed daily by viruses which is a significant portion of the bacterial population
  • Contributes to the release of dissolved organic matter (DOM) and nutrients back into the water for other organisms to use
• Viral lysis can alter microbial community composition
  • Selective infection and lysis of specific microbial groups (e.g. cyanobacteria) can shift the balance of species
  • Maintains microbial diversity by preventing dominance of a single species (kill-the-winner hypothesis) allowing other species to thrive
• Impact on biogeochemical cycles
  1. Viral lysis releases cellular contents, including carbon, nitrogen, and phosphorus
  2. Enhances nutrient recycling and supports microbial growth (viral shunt)
  3. Influences carbon flux and storage in the ocean by shuttling carbon between different pools (dissolved vs. particulate)

Role in gene transfer and evolution

• Marine viruses facilitate horizontal gene transfer (HGT) among microorganisms
  • Transduction: viruses can package host DNA and transfer it to another host upon infection, spreading genes between organisms
  • Contributes to the spread of genetic material, including antibiotic resistance (multidrug resistance) and metabolic genes (photosynthesis)
• Influence on microbial evolution
  • Viral infection exerts selective pressure on microbial populations, favoring resistant or tolerant strains
  • Drives the evolution of defense mechanisms, such as CRISPR-Cas systems and restriction-modification systems
  • Coevolution between viruses and their hosts leads to genetic diversification and adaptation, like an evolutionary arms race

Methods for Studying Marine Viruses

Methods for studying marine viruses

• Microscopy techniques provide visual information about viruses
  • Transmission electron microscopy (TEM): visualizes viral morphology and ultrastructure at high resolution
  • Epifluorescence microscopy: uses fluorescent dyes (SYBR Green) to enumerate viruses based on their DNA content
• Flow cytometry enables rapid and quantitative analysis of viruses
  • Rapid enumeration of viruses based on their size and fluorescence after staining with dyes
  • Allows for high-throughput analysis of viral abundance and distribution in many samples
• Molecular techniques offer insights into viral genetics and host-virus interactions
  • Metagenomics: sequencing of viral genomes directly from environmental samples to assess diversity and discover new viruses
  • PCR-based methods: detection and quantification of specific viral groups (T4-like phages) or genes (photosystem genes)
  • Transcriptomics: studying viral gene expression and host-virus interactions during infection cycles