Scientists discover giant new virus that infects freshwater algae

Scientists from the Biology Center of the Czech Academy of Sciences discovered forty new freshwater viruses infecting aquatic microorganisms this year. The first, which they isolated and described in detail, was named Budvirus in homage to the capital of South Bohemia, České Budějovice. It belongs to the “giant viruses” and infects unicellular algae called cryptophytes.

Researchers have confirmed that this virus plays an important role in the ecosystem, as it controls the proliferation of algae, thus helping to maintain the balance of the aquatic environment. All viruses were discovered in the Římov reservoir near České Budějovice, which has been regularly monitored by South Bohemian hydrobiologists for five decades and is one of the most studied freshwater reservoirs in Europe. The work is published in The ISME magazine.

Although we are surrounded by freshwater ecosystems such as lakes, ponds, reservoirs and rivers, their microscopic representatives, especially viruses and bacteria, still remain an underexplored area. A drop of water can contain a million bacteria and ten times as many viruses, but only a handful of these have been described. Recent methods, such as environmental DNA analysis, are making great progress 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, analyze it using DNA sequencing methods, and from there 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 CAS Biology Center. Using these molecular clues, scientists then look for the virus existing in other water samples. They also try to bring it to the laboratory, keep it in laboratory conditions and subject it to detailed examination.

The newly discovered Budvirus probably inhabits many lakes in Europe or other continents

Hydrobiologists first detected Budvirus this spring, at a time when microscopic algae grow rapidly in water. They knew that thanks to planktonic predators (like protozoa, rotifers or cladocerans) 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 algae decline and that its activity is particularly important during the spring growth of plankton.

“Budvirus is the first virus ever described that infects cryptophytes of the genus Rhodomonas, which are among the most common phytoplankton algae. It can be assumed to be representative of a group of viruses distributed globally in ecosystems freshwater and is present all over the world,” explains team member Helena Henriques Vieira.

The Budvirus capsid has an icosahedral shape measuring 200 nanometers, about ten times larger than the usual size of viruses. Its genome codes for more than 400 proteins, half of which have an unknown function. There are many algae viruses of similar size called “giant viruses”, and Budvirus is one of them.

Using environmental DNA, researchers discovered that genetically almost identical giant viruses could be found in many lakes in Europe or other continents. In this regard, the intensive study of a site has a global impact on the study of freshwater microbes.

Exploring ecosystem players and their interactions helps understand unexpected changes

Freshwater ecosystems are highly dynamic environments, filled with 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 this new discovery, the team of Czech scientists contributes to the understanding of 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 unanticipated changes occur in the water, we will know what happened,” adds Ghai.