Viruses and Their Influence on Marine Life

I know what you’re thinking: not another virus article! But I want to show you the positive side, the one we all need so badly right now. I want to take you on a journey through the ocean, and show you what good viruses can do for the health of marine environments, as well as how they’ve shaped life as we see it today.

Even before Corona hit our society, viruses didn’t have the best reputation. The first things that come to mind when thinking about viruses are almost always negative: deadly diseases, plagues or nasty infections. For a long time, viruses were only seen as parasites, taking what they need from others and harming them in doing so. But viruses also shaped our planet and its inhabitants as we see it today. 

Exploring The World Of A Virus 

Before we can get a bit more sympathetic towards viruses, we need to better understand their viewpoint. A virus consists of virions, single particles mainly made up of two parts: a protein shell called a capsid, which encloses a genome of nucleic acid. The capsid comes in various shapes and sizes. For example, a herpes virus particle is 200 nanometres in size. A thousand times smaller than a single human blood cell. 

Since viruses can’t replicate on their own, they completely depend on their hosts to survive. In 2014 John Dennehy made a fascinating point: How long would it take a virus particle to drift one meter? He calculated it would take 79 years, though that stretched to infinity considering that virions drift randomly and not in one direction. A virus on its own is helpless, dependent on the movement and metabolism of others. 

The size of a virion. Illustration of the T4-Bacteriophage that infects the bacterium Escherichia coli. (Image Credit: Lara Beckmann, CC BY-NC-SA 4.0)

For this reason, it is a common view to exclude viruses from the world of living organisms. Still, there is much debate about this: what can we consider living and what not? A narrow and blurred boundary divides living organisms from viruses. Yet this somewhat subjective view of viruses as non-living has highly influenced science. It has meant that the evolutionary impact of viruses and their ecological interactions are still largely unexplored.

Contribution to Life in the Ocean

Although helpless on their own, viruses are the most successful type of organism on our planet. The biologist Curtis Suttle wrote that if we placed all the viruses present in our oceans end-to-end, they would span over 10 million light-years! It’s hard to visualize a light-year, but the comparison expresses the unbelievable vast number of viruses existing in the marine ecosystem alone. Yet their classification and diversity remain to be fully discovered, and almost every study dealing with viral diversity in organisms finds new strains and divergent lineages.

A large fraction can be found as Virioplankton, marine pelagic viruses that mainly live within the bodies of other planktonic groups including Prokaryotes, Archaea and Eukaryotes. Their true impact on these groups are largely unknown, but some studies suggest that viruses contribute to maintaining the equilibrium in planktonic species. The open ocean is an environment seemingly limited in resources, raising the question of why so many plankton organisms manage to co-exist in this habitat. Viruses can infect plankton and thus kill abundant species – making way for other, less frequent species. This way, viruses make sure that the massive diversity of planktonic species might be maintained and nurtured. 

An unprocessed plankton sample. The most abundant plankton are bacteria, but Copepods number out any other type in the zooplankton. (Image Credit: Lara Beckmann, CC BY-NC-SA 4.0)

Many planktonic organisms are ecologically important as primary producers and thus play a vital role in the complex marine carbon cycle – in short, the biologically driven intake of carbon into the ocean – a vital process for our atmosphere. Some studies even suggest a contribution of viruses to this process, called the viral shunt. Infected cells get degraded by viruses, called lysis. The organic material inside the cells including carbon is released in the water column, which contributes to transporting energy between the water layers. 

A Complicated Relationship

Viruses not only kill but live in intricate relationships with others. They even form symbiotic-like interactions. An example can be found in a specific kind of virus, called bacteriophages, which only infect bacterial cells. Jeremy Barr and colleagues found that epithelial cells are covered by these bacteriophages and that they can act as a first barrier against bacteria. Sort of an anti-bac layer that protects the underlying cells against bacterial infections. 

In corals, viruses are both friend and foe. Some studies showed that bacteriophages can be used to treat coral diseases caused by bacteria (so-called phage therapy), suggesting that these viruses might be good candidates for disease control. This method is also interesting in human medicine and might even be a potential substitute in a post-antibiotics era. Furthermore, viruses might also play a regulatory role in the entire microbial community, for example controlling and maintaining bacterial populations. Threatened by challenging environmental conditions, climate change and pollution, viruses might be a lead to a huge boost in the health and survival of coral in the future.

My Genes Are Your Genes

The relationship between viruses and their hosts go even deeper: beyond the cells. Viruses make up a large part of other organisms’ genetic information. They steal and swap genes in a process called gene transfer. During replication of the viruses DNA (or RNA) parts of the hosts’ genome can get inserted into the virus genome. This gene transfer isn’t a one-way route: The host organisms gather genetic information from viruses in their genomes too. Some viruses, known as endogenous retroviruses (ERVs), can be inserted into the sex cells and become an integral part of the organism’s genome. 

Simplified illustration of the integration of viral genetic information into a host-cell genome. Jellyfish as a potential host. (Image Credit: Lara Beckmann, CC BY-NC-SA 4.0)  

Historically this has mainly been  studied in mammals. The human genome for example is estimated to include around 8% complete viral elements! Some associated genes are even suggested to have played a pivotal role in the evolution of the placenta in mammals. For marine organisms, this issue remains a big question mark and there are not many studies concerning this. However, Sidney Pierce and his team found a specific ERV in the marine slug Elysia, which possibly plays a large role in the slugs remarkable life cycle. Yet, the interplay between viruses and the genomes of marine organisms must be deeper and more fundamental than known and studied today. Especially since they share a long evolutionary history and it can be assumed that much of the genetic information of marine organisms has come about as a result of exchanges between viruses and their hosts. 

Some species of the sea slug Elysia live in close association with algae and are using the algae chloroplasts to gain energy. Scientists found retrovirus elements incorporated in the slugs genome – its function still unknown. (Image Credit: Klaus Stiefel, CC BY-NC 2.0)

We humans have our strict cabinets of thinking, and viruses still don’t fit into many of our biological concepts. Yet in the past few years, researchers have dived deeper into virus ecology and evolution. So far it shows us that we share our planet with a huge, unbelievable diversity of viruses and that we don’t know or understand most of their effects on our ecosystems. Still, studying viruses remains a challenge, and many of the mentioned examples are vague theories, often lacking confident results due to the difficulty of studying viruses in their natural realm.

Their true contribution to evolution and their ecological influence is still fairly unexplained, but their ubiquitous presence implies a great impact on the history of life on our planet.

Lara Beckmann is a master’s student at the University of Stockholm in the program ‘Biodiversity & Systematics’ and is currently working on her thesis project at the University Museum of Bergen focusing on the diversity of marine polyps and jellyfish of the group Hydrozoa. Follow Lara on Twitter here.

Further Reading & Literature

Dennehy, J.J. (2014). What Ecologists can tell Virologists. Annu. Rev. Microbiol. 2014. 68:117–3510. DOI: 1146/annurev-micro-091313-103436

Mölling, K. and Broecker, F. (2019). Viruses and Evolution – Viruses First? A Personal Perspective. Front. Microbiol. 10:523. DOI: 10.3389/fmicb.2019.00523

Suttle, C.A. (2007). Marine viruses – major players in the global ecosystem. Nature Reviews Microbiology 5:801. DOI: doi.org/10.1038/nrmicro1750

Suttle, C.A. (2005). Viruses in the sea. Nature 437, 356–361. DOI: 10.1038/nature04160

Barr, J. et al. (2013). Bacteriophage adhering to mucus provide a non-host derived immunity. PNAS 110 (26) 10771-10776. DOI: 10.1073/pnas.1305923110

Pierce SK, Mahadevan P, Massey SE, Middlebrooks ML. (2016). A Preliminary Molecular and Phylogenetic Analysis of the Genome of a Novel Endogenous Retrovirus in the Sea Slug Elysia chlorotica. Biol Bull. 231(3):236-244. DOI: 10.1086/691071

Peixoto, R.S. (2021). Coral Probiotics: Premise, Promise, Prospects. Annual Review of Animal Bioscience. 9:265–88. DOI: 10.1146/annurev-animal-090120-115444

Feature Image credit: Lara Beckmann, CC BY-NC-SA 4.0

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