The latest study, which was published in the journal Science on Thursday (June 9), focuses on viruses that include RNA, a molecular cousin of DNA. In human illness, RNA viruses abound; coronaviruses and influenza viruses, for example, are both RNA-based. Scientists are just now learning about the variety of RNA viruses that may be found in the water, as well as the spectrum of animals that they can infect.
"We are certainly sure that most RNA viruses in the ocean are infecting microbial eukaryotes, such as fungi and protists, and to a lesser extent, invertebrates," said co-first author Guillermo Dominguez-Huerta, who was a postdoctoral scholar in viral ecology at Ohio State University (OSU) at the time of the study. Eukaryotes are creatures that have complex cells with a nucleus that stores their genetic material.
Fungi and protists, which include algae and amoebas, are viral hosts that take carbon dioxide from the atmosphere and so control how much carbon is stored in the ocean. According to Steven Wilhelm, the principle investigator of the Aquatic Microbial Ecology Research Group at the University of Tennessee Knoxville, who was not involved in the current study, RNA viruses likely alter how carbon travels through the ocean at large by infecting these species.
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In an email to Live Science, Wilhelm said, "Given the quantity of RNA virus particles, knowing they can accomplish this continues to develop the tale of how significant viruses are in the world with respect to how energy and carbon move."
(Wilhelm has worked on projects unrelated to the current study with several of the study authors, including Matthew Sullivan and Alexander Culley.)
Dominguez-Huerta and his colleagues discovered more than 5,500 previously undiscovered RNA viruses in the world's waters earlier this year.
The scientists evaluated 35,000 water samples gathered from 121 locations in the five seas by the Tara Oceans Consortium, an ongoing global study exploring the impact of climate change on oceans, for the study, which was published April 7 in the journal Science(opens in new tab). Plankton — microscopic creatures that move in the current and can serve as hosts for RNA viruses — was abundant in these water samples. The researchers sorted through all of the RNA in the planktons' cells to discover a specific bit of genetic code called the RdRp gene, which they used to find the viruses.
"That's the sole... coding sequence that's consistent across all RNA viruses," said Dominguez-Huerta, who now works as a scientific consultant for Virosphaera; nonetheless, the RdRp gene isn't found in cells or other viruses.
In order to categorize all of the RNA viruses detected in the plankton, the researchers advised tripling the number of RNA virus phyla — the wide taxonomic category just below "kingdom" — from five to ten.
Following that, the researchers wanted to learn more about how these viruses spread throughout the world and whose hosts they attack.
The viral populations might be divided into four broad zones, according to the researchers: Arctic, Antarctic, Temperate and Tropical Epipelagic (near to the ocean surface), and Temperate and Tropical Mesopelagic (approximately 656 to 3,280 feet (200 to 1,000 meters) below the surface. Interestingly, despite having a broader range of hosts to infect in warmer seas, the diversity of viruses tended to be highest in the polar zones.
"Viruses don't care about how cold the water is when it comes to variety," said co-first author Ahmed Zayed, a research scientist in OSU's Department of Microbiology. This discovery suggests that multiple viruses fight for the same hosts at the poles, according to Zayed of Live Science.
The researchers employed a variety of methods to find these viral hosts, including comparing the genomes of RNA viruses with known hosts to those of the new viruses, and seeking for unusual snippets of viral RNA in host cell genomes, where fragments of RNA can sometimes get left behind. Many RNA viruses in the water infect fungus and protists, while some infect invertebrates and a small percentage infect bacteria, according to this study.
According to Dominguez-Huerta, the study also determined that 95 of the viruses had "taken" genes from their host cells. These genes assist the host in directing metabolic activities within the cell. This finding shows that the viruses tampered with their hosts' metabolisms in some way, most likely to increase the generation of new viral particles, according to the scientists.
In the past, Dominguez-Huerta remarked, certain smaller-scale investigations had hinted at this gene-swiping potential.
After determining which hosts the ocean viruses are most likely to infect, the researchers discovered that about 1,200 of them may be involved in carbon export, which is the process by which carbon is extracted from the atmosphere, incorporated into marine organisms, and then "exported" to the deep sea as those organisms sink to the seafloor after death.
According to the Monterey Bay Aquarium Research Institute, the deeper these carbon stocks sink, the longer they'll be held in the water before being cycled back into the atmosphere (opens in new tab). As a result, carbon export is a significant issue that scientists consider when developing climate change models. Infection of marine creatures by RNA viruses, according to the current study, may be another previously unknown driver driving carbon flow in the seas, as the viruses affect the cellular activity of the hosts they infect.
Because viruses frequently burst out of their hosts after quickly multiplying inside them, RNA viruses may potentially promote carbon flow by tearing their hosts apart and pouring stored carbon into the ocean, Wilhelm added.
