Marine viruses

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Marine viruses are tiny infectious particles that live in the ocean. Outside a cell, they exist as virions, which carry genetic material (DNA or RNA) protected by a protein coat. Most are too small to see even with a light microscope.

In the ocean, the vast majority of marine viruses infect bacteria. These bacteriophages are the most abundant biological entities in seawater—often far outnumbering their bacterial hosts. A single teaspoon of seawater can contain tens of millions of viruses.

Viruses play a major role in marine life and the global carbon and nutrient cycles. When they infect and burst open bacterial cells (a process called lysis), they release organic molecules that fuel new growth in bacteria and algae. This viral shunt helps recycle carbon and nutrients and can boost the biological pump, a process that moves carbon from the surface to the deep ocean. Because of these activities, marine viruses contribute to keeping oceans productive and influence atmospheric carbon dioxide levels.

Viruses are also key drivers of genetic diversity. They transfer genes between microbes, helping evolution and adaptation. Scientists still debate whether viruses are alive in the traditional sense, since they lack a cellular structure and can’t reproduce on their own, but they clearly interact with living organisms in complex, important ways.

Diversity and types abound in the sea. Tailed phages (Caudovirales) used to seem dominant, but non-tailed and giant viruses are now known to be widespread across many depths and regions. Giant viruses can be as large as about a micron, and some even carry very large genomes. Virophages are small viruses that hijack giant viruses to replicate and can sometimes protect the host organism by inhibiting their giant virus. Notable marine examples include CroV (which can kill Cafeteria roenbergensis, a tiny zooplankton), the Emiliania huxleyi virus (which infects a common marine alga) and various others like HaV and Mavirus.

Host range is broad. Bacteriophages infect bacteria (including cyanobacteria), while other viruses target archaea, algae, fungi, and even some marine animals. Phycodnaviridae are large DNA viruses that infect algae and can influence algal blooms, which in turn affect fisheries and the broader food web.

In the deep ocean and near vents, viruses continue to drive microbial life and nutrient cycling. They cause the death of many prokaryotes in sediments, releasing elements like nitrogen, carbon, and phosphorus back into the environment and sustaining nutrient turnover. Some marine viruses may even help microbes survive extreme conditions by moving genes around.

Viruses affect a wide range of marine life, from microscopic plankton to fish and marine mammals. They contribute to disease outbreaks in wild populations and in aquaculture, such as rhabdoviruses in fish and herpesviruses in some species. They also occur in sea stars and other echinoderms, sometimes linked to mass die-offs.

Detecting and studying these invisible players has evolved from electron microscopes to fluorescence methods and advanced genetic sequencing. Researchers map where viruses live, how many there are, which hosts they infect, and how they move genes around in the ocean. Viral activity varies with depth, distance from shore, and ocean conditions, influencing local and global ecosystems.

Overall, marine viruses are the ocean’s most abundant and diverse life forms. They shape microbial communities, regulate major biogeochemical cycles, and help drive evolution, all while remaining largely mysterious—rich with potential for new discoveries about how the planet’s oceans work.