Czech scientists have discovered a new giant virus in Římov reservoir. It infects freshwater algae and has been named Budvirus

Although we have freshwater ecosystems such as lakes, ponds, reservoirs and rivers all around us, their microscopic representatives, especially viruses and bacteria, are still a little-explored area. A drop of water can contain a million bacteria and ten times more viruses, but only a handful of them have been described. Recent methods, such as environmental DNA analysis, are making great strides in the study of the aquatic microworld. This is also one of the methods used by the Czech scientific team. "The way it works is that we extract all the genetic material from a water sample, analyse it using DNA sequencing methods, and from this, we can trace which species of organisms were present in the water. This provides us with clues to find new viruses or bacteria," explains Rohit Ghai, head of the Microbial Ecology and Evolution Laboratory at the Biology Centre CAS. Using these molecular clues, the scientists then search for the existing virus in other water samples. They also try to bring it to the laboratory, preserve it in laboratory conditions and subject it to detailed examination.

Flasks of Rhodomonas culture. The big red one in the middle is a healthy culture (not infected by virus);  the green/yellowish ones on the sides are Rhodomonas cultures infected with giant viruses. Photo: Helena Henriques Vieira, BC CAS

Hydrobiologists first detected the Budvirus this spring, at a time when microscopic algae grow rapidly in the water. They knew that thanks to planktonic predators (such as protozoa, rotifers or cladocera) and nutrient depletion, these algae would disappear again in a short time. But they have now confirmed that a newly discovered giant virus is a major contributor to this algal decline and that its activity is especially important during spring plankton growth. "Budvirus is the first virus ever described that infects cryptophytes of the genus Rhodomonas, which are some of the most common phytoplankton algae. It can be assumed to be representative of a globally distributed group of viruses in freshwater ecosystems and to occur worldwide," says team member Helena Henriques Vieira. Budvirus capsid has an icosahedral shape measuring 200 nanometres, roughly ten times larger than the usual size of viruses. Its genome encodes over 400 proteins, half of them with an unknown function. There are many algae viruses of similar size called „Giant Viruses“, and Budvirus belongs to them. Using environmental DNA the researchers discovered that genetically almost identical giant viruses could be found in many lakes in Europe or other continents. In this respect, studying one location intensively has a worldwide impact on the study of microbes in freshwaters.

Freshwater ecosystems are very dynamic environments, full of complex interactions between organisms - from microscopic bacteria, viruses, algae, and protozoa to larger organisms such as fish. Their mutual interactions, whether predation, parasitism, competition or symbiosis, have a major impact on the balance of the ecosystem and its ability to respond to external changes such as temperature fluctuations, changes in water chemistry or the introduction of new species. With the new discovery, the team of Czech scientists is contributing to the understanding of the ecological and evolutionary interactions between viruses and their hosts in freshwater environments. "It is important to have a thorough understanding of these ecosystem players and their interactions. When unforeseen changes occur in the water, we will know what happened," adds Rohit Ghai.

Publication

Henriques Vieira H., Bulzu P., Kasalický V., Haber M., Znachor P., Piwosz K., Ghai R. (2024) Isolation of a widespread giant virus implicated in cryptophyte bloom collapse. The ISME Journal 18: wrae029.

DOI: 10.1093/ismejo/wrae029