Viral Planet

Viral Planet

Dr. Arwyn Edwards

The White Cliffs of Dover

• The cliffs are composed of chalk, which is formed primarily from coccoliths, structures made of calcium carbonate (CaCO₃).

• Coccoliths: Ornamental structures located on the surfaces of marine algae cells, specifically coccolithophores.

• The most prominent coccolithophore studied is Emiliania huxleyi.

  • Found in almost all non-polar waters.

  • Can dominate local plankton populations, comprising up to 75% of plankton abundance in a region.

• Photosynthesis Contribution: Plankton contribute approximately 50% to global photosynthesis, which supports life on Earth.

Coccolithophore Blooms and Their Impacts

• A typical bloom of coccolithophores is detectable through remote imaging, with specific focus on light reflections when cells perish and release coccoliths, marking a shift from a bloom to a "doom."

Viral Dynamics in Coccolithophore Blooms

• Coccolithophores are significantly affected by algal viruses, such as Coccolithovirus Ehux-86.

• With numerous host cells available, the likelihood of viral infections increases.

  • This leads to the adage “viruses kill the winner.”

  • The phenomenon results in a viral shunt in the carbon cycle, where lysed cells leak organic carbon and nutrients.

Numerical Data Relating to Coccolithophores and Viruses

• Chalk and Coccolithophore Data:

  • A typical piece of chalk weighs approximately 10 grams.

  • Mass of coccoliths per coccolithophore cell is around 3 picograms.

  • The estimated number of coccolithophore cells in one piece of chalk is approximately $3.3 imes 10^{12}$ cells.

  • Roughly 30% of cells in a bloom are visibly infected by viruses.

  • This translates to about $1 imes 10^{12}$ cells being virus-infected in a piece of chalk.

  • Each infected cell can produce around 800 new virus particles, culminating in approximately $8 imes 10^{14}$ virus particles generated per piece of chalk, or 800 trillion virus particles per 10 grams.

• The White Cliffs of Dover:

  • Best estimated density of 2700 kg/m³, with dimensions of 25km long, 110m high, and 500m thick, equating to a total weight in trillions of tons.

  • This results in an estimate of $8 imes 10^{31}$ virus particles linked to the evolution of the cliffs, which equals eight times the number of stars in the observable universe.

• On a global scale, there are currently around $1 imes 10^{31}$ virus particles on Earth, which if placed end to end could stretch approximately 100 million light years.

• Current statistics show there are about $1 imes 10^{23}$ virus infections per second occurring in Earth's oceans, responsible for the removal of about 20-40% of marine bacterial cells daily.

• The total mass of carbon represented by these virus particles in oceans is estimated to equal that of 75 million blue whales.

The East African Rift Valley Lakes

• Hosts significant populations of Lesser Flamingos (Phoeniconaias minor), which thrive on alkali-loving cyanobacteria, specifically Arthrospira fusiformis.

• Fluctuations in the populations of Arthrospira are influenced by viral infections, which can result in dramatic crashes in Flamingo populations—a phenomenon known as a trophic cascade.

The Viral Shunt in the Anthropocene

• Recent studies highlight the interaction between environmental factors and viral dynamics:

  • Sender et al. (2021) discusses the total number and mass of SARS-CoV-2 virions.

  • Carlson et al. (2022) explore increased viral transmission risks due to climate change.

  • Miner et al. (2021) highlight biogeochemical risks stemming from Arctic permafrost degradation.

• Global peak biomass of SARS-CoV-2 estimated between 0.1-10 kg.

• A notable reduction in CO₂ emissions was recorded in April 2020, amounting to a decline of 17 megatons per day, suggesting a potential 2–200 billion fold carbon impact relative to the mass of the virus.

Definition and Characteristics of Viruses

• Viruses: Submicroscopic in size and considered obligate intracellular parasites.

• A virus particle is referred to as a virion, composed of a genome housed within a protective protein layer.

  • Viruses can be either “naked” (only protein) or “enveloped” (with an outer lipid membrane).

• Viral genomes can consist of either DNA or RNA (but never both).

  • DNA can be single-stranded (ss) or double-stranded (ds).

  • RNA can also be ss or ds. An example includes Ebola, which is ssRNA.

  • RNA viruses may be further classified as positive or negative sense:

  • Positive sense: Directly translatable (akin to mRNA).

  • Negative sense: Requires transcription into mRNA before translation.

  • Retroviruses like Human Immunodeficiency Virus (HIV) make a DNA copy of their RNA intermediate prior to replication.

The Debate: Are Viruses Alive?

• The status of viruses being termed "alive" is complex and nuanced:

  • Considered acellular life forms.

  • Extracellular viruses are inert and can only degrade through environmental factors (enzymes, UV, radiation).

  • Viruses must infect living host cells in order to replicate.

  • Tropism: Viruses demonstrate specificity, infecting particular species, cells, tissues, or organs.

• Within host cells, viruses can either:

  1. Create new virus particles by commandeering the host cell's machinery for genome replication, protein synthesis, and particle assembly, ultimately leading to cell lysis or budding.

  2. Remain dormant within the host genome (known as lysogeny, latency, or persistence).

• The concept of the virocell posits that viruses are considered "alive" only when inside a host cell.

Virus Life Cycle - Lytic Cycle

• Phases of the generalized lytic cycle of a virus:

  1. Attachment: Virus attaches to the host cell.

  2. Penetration: The viral genome enters the host cell.

  3. Replication and Translation: The viral genome is replicated and translated to produce viral proteins.

  4. Assembly: New virions are assembled.

  5. Release: New virions are released from the host cell, either by lysis or budding.

Decision-Making in Viruses

• Some specific viruses have been known to exhibit decision-making capabilities regarding their infection strategy, such as choosing between lysis or lysogeny. This is well-documented in bacteriophages.

The Origins of Viruses

• The origins of viruses involve multiple theories:

  1. Progressive Evolution: Suggests viruses evolved from mobile genetic elements like transposons that acquired functions enabling transmission between cells.

  2. Regressive Evolution: Proposes viruses descended from once-independent cells that became obligate intracellular parasites, relying heavily on host machinery for replication.

  3. Virus-First Evolution: Argues that viruses originated before cellular life and had properties allowing independent replication.

• Current consensus highlights the likelihood that RNA-based life forms preceded DNA-based organisms.

Historical Context: The Discovery of Viruses

• Originally termed "filterable virus" or "filterable poison" based on the ability to induce disease via filtered fluids.

  • Chamberland filters were instrumental in studies leading to this discovery.

• The first virus identified was the plant virus Tobacco Mosaic Virus (TMV) in 1892, which demonstrated infectious properties when filtrate was introduced.

• Yellow Fever Virus was the first human virus discovered in 1898, through experimentation involving human volunteers.

• The first virus observable under a light microscope was the vaccinia virus (cowpox virus).

Bacteriophages (Phages)

• Bacteriophages, or “phages”, are viruses specifically targeting bacteria and are considered the most prevalent life forms on Earth.

• They were first observed by Fredrick Twort in 1915 and later studied by Felix d'Hérelle in 1917, who discovered their therapeutic potential.

• Use in bacteriophage therapy became prominent in the Soviet Union, thanks to advancements from the Eliava Institute.

  • Strains of phages infecting bacterial pathogens are isolated and purified for treatment.

• Future advantages in phage therapy may include the design of AI-optimized phages to combat antibiotic-resistant bacteria.

  • Pros: Specific targeting, rapid action, and potential efficacy against antibiotic-resistant strains.

  • Cons: May require specific testing, evolving host resistance, and possibility of generating an immune response.

Classifications of Animal Viruses

• Animal viruses are categorized into seven groups based on genome organization and replication strategies, known as the Baltimore classes:

  • Group I: dsDNA viruses (e.g., Poxviruses)

  • Group II: ssDNA viruses (e.g., Canine parvoviruses)

  • Group III: dsRNA viruses (e.g., Rotaviruses)

  • Group IV: +ssRNA viruses (e.g., Coronaviruses)

  • Group V: –ssRNA viruses (e.g., Measles)

  • Group VI: ssRNA-RT viruses (e.g., HIV)

  • Group VII: dsDNA-RT viruses (e.g., Hepatitis B)

Outcomes of Viral Infections

• Infection consequences vary depending on:

  • Type of virus, interaction with host cells, and immune responses.

• Latent Infections: Average infection duration in households without children is about 3.5 weeks; households with children is 18 weeks; and those with six children is approximately 45 weeks.

• Acute Infections: Rabies virus, a zoonotic virus, has virtually a 100% fatality rate across mammalian hosts.

• Chronic Infections: Certain oncoviruses, like HTLV, EBV, HPV, and HBV, have links to cancer development.

Studying Viruses

1. Visibility: Viruses are too small for light microscopy; they can be seen through fluorescence microscopy with antibody staining or SYBR nucleic acid staining. Transmission electron microscopy is typically used for detailed visualization (e.g., Ebola).

2. Detection and Quantification: Utilizes molecular techniques, such as Nucleic Acid Amplification Tests (NAATs) like qPCR and LAMP, along with immunological assays like the Enzyme-linked Immunosorbent Assay (ELISA).

3. Propagation Requirements: Viruses must infect compatible living cells; common hosts include animal models (notably ferrets for influenza) and cell culture.

4. Virus Modification: Techniques in genetic engineering are utilized for creating vaccines, gene therapies, and for research purposes.

5. Viral Sequencing: No universal marker gene (unlike 16S rRNA genes for bacteria). Techniques involve real-time genomic epidemiology (e.g., NextStrain) and viral metagenomics, which enrich viruses before shotgun sequencing.

6. Safety Protocols: Requires stringent safety measures; facility-level containment is necessary for handling certain viruses (e.g., Filovirus research at Porton Down). Safety measures are vital across all virus types.

Summary

1. Viruses: The most abundant and diverse life forms on the planet.

2. Characteristics: Submicroscopic, acellular obligate intracellular parasites made of a genome encapsulated in protein.

3. Not alive: They are inanimate outside of host cells.

4. Ecosystem Roles: Viruses play critical but often underappreciated roles in ecological functions.

5. Evolutionary Perspective: Highly diverse and polyphyletic origins.

6. Replication: They depend on living host cells for replication.

7. Bacteriophages: Potentially valuable in overcoming antibiotic resistance.

8. Classification: Seven groups of animal viruses classify based on genome organization and replication strategies.

9. Diseases: Viruses cause a variety of diseases across different host organisms.

10. Research Techniques: Evolved specialized methods for studying viruses throughout the past century.