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The Most Common Organism in the Oceans Harbors a Virus in its DNA

August 10, 2020

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Structure of a typical virus, in this case a phage. The appearance of these viruses has been likened to a miniature lunar lander. They are essential to the regulation of marine ecosystems
Graphic credit: by Chelsea Bonnain, Mya Breitbart and Kristen N. Buck - extracted from this Commons file, CC BY-SA 4.0; Wikipedia Commons

Marine viruses are defined by their habitat as viruses that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. A virus is a small infectious agent that replicates only inside the living cells of another organism. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea as well as other viruses.

Viruses are now recognized as ancient and as having origins that pre-date the divergence of life into the three domains. They are found wherever there is life and have probably existed since living cells first evolved. The origins of 1) _____ in the evolutionary history of life are unclear because they do not form fossils. Molecular techniques are used to compare the DNA or RNA of viruses and are a useful means of investigating how they arose. Some viruses may have evolved from plasmids - pieces of DNA that can move between cells - while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity.

Opinions differ on whether viruses are a form of life or organic structures that interact with living organisms. They are considered by some to be a life form, because they carry 2) _____ material, reproduce by creating multiple copies of themselves through self-assembly, and evolve through natural selection. However, they lack key characteristics such as a cellular structure generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as replicators and as “organisms at the edge of life.“

When not inside a cell or in the process of infecting a cell, viruses exist in the form of independent particles. A single viral particle is called a 3) _____. A viron consist of two or three parts: (i) the genetic material (genome) made from either DNA or RNA, long molecules that carry genetic information; (ii) a protein coat called the capsid which surrounds and protects the genetic material. The shapes of these virus particles range from simple helical and icosahedral forms for some virus species to more complex structures for others. Most virus species have virions that are too small to be seen with an optical microscope. The average virion is about one one-hundredth the linear size of the average bacterium.

A teaspoon of seawater typically contains about ten million marine viruses. Most of these viruses are 4) _____ which infect and destroy marine bacteria and control the growth of phytoplankton at the base of the marine food web. Bacteriophages are harmless to plants and animals, but are essential to the regulation of marine ecosystems. They supply key mechanisms for recycling ocean carbon and nutrients. In a process known as the viral shunt, organic molecules released from dead bacterial cells stimulate fresh bacterial and algal growth. In particular the breaking down of bacteria by viruses (lysis) has been shown to enhance nitrogen cycling and stimulate phytoplankton growth. Viral activity also affects the biological pump, the process which sequesters carbon in the deep ocean. By increasing the amount of respiration in the oceans, viruses are indirectly responsible for reducing the amount of carbon dioxide in the atmosphere by approximately 3 gigatonnes of carbon per year.

Marine microorganisms contribute about 70% of the total marine biomass. It is estimated marine viruses kill 20% of this biomass every day. Viruses are the main agents responsible for the rapid destruction of harmful algal blooms which often kill other marine life. The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms. Viruses are an important natural means of transferring genes between different species, which increases genetic diversity and drives evolution. It is thought viruses played a central role in early evolution before the diversification of bacteria, archaea and eukaryotes, at the time of the last universal common ancestor of life on Earth. Viruses are still one of the largest areas of unexplored genetic diversity on Earth.

The most common organism in the oceans, and possibly on the entire planet, is a family of single-celled marine bacteria called 5) _____ SAR11. These drifting organisms look like tiny jelly beans and have evolved to outcompete other bacteria for scarce resources in the oceans. We now know that this group of organisms thrives despite -- or perhaps because of -- the ability to host viruses in their 6) _____ . A study published in May in Nature Microbiology could lead to new understanding of viral survival strategies.

University of Washington oceanographers discovered that the bacteria that dominate seawater, known as Pelagibacter or SAR11, hosts a unique virus. The virus is of a type that spends most of its time dormant in the host's DNA but occasionally erupts to infect other cells, potentially carrying some of its host's genetic material along with it. This virus' two-pronged survival strategy differs from similar ones found in other organisms. The virus lurks in the host's DNA and gets copied as cells 7) _____, but for reasons still poorly understood, it also replicates and is released from other cells. The new study shows that as many as 3% of the SAR11 cells can have the virus multiply and split, or lyse, the cell -- a much higher percentage than for most viruses that inhabit a host's genome. This produces a large number of free viruses and could be key to its survival.

The authors had published a previous paper in 2019 looking at how marine phytoplankton, including SAR11, use 8) _____. That allowed the researchers to cultivate two new strains of the ocean-dwelling organism and analyze one strain, NP1, with the latest genetic techniques. The authors collected samples off the coast of Oregon during a July 2017 research cruise. They then 9) _____ the seawater several times and then used a sulfur-containing substance to grow the samples in the lab -- a difficult process, for organisms that prefer to exist in seawater. The authors found that a virus was complicating the task of sequencing the genome. Then they discovered a virus wasn't just in that single strain. The experiments showed that the virus' switch to replicating and bursting cells is more active when the cells are deprived of nutrients, lysing up to 30% of the host cells. The authors believe that bacterial 10) _____ that hitch a ride with the viruses could help other SAR11 maintain their competitive advantage in nutrient-poor conditions.

Sources:  University of Washington: Hannah Hickey, Robert M. Morris, Kelsy R. Cain, Kelli L. Hvorecny, Justin M. Kollman. Lysogenic host-virus interactions in SAR11 marine bacteria. Nature Microbiology, 2020; DOI: 10.1038/s41564-020-0725-x; "The most common organism in the oceans harbors a virus in its DNA;"  ScienceDaily.com; Wikipedia

ANSWERS: 1) viruses; 2) genetic; 3) virion; 4) bacteriophages; 5) SAR11; 6) DNA; 7) divide; 8) sulfur; 9) diluted; 10) genes

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