Found marine protozoa that can feed on viruses

“Our data show that many cells of eukaryotic organisms - protists - contain DNA of the most

various non-infectious viruses, but not bacteria,which is convincing evidence that they feed on viruses, not bacteria. This was a big surprise because these results contradict current scientific understanding of the role of viruses and protists in marine food webs,” said Dr Ramunas Stepanauskas, director of the Single Cell Genomics Center at the Bigelow Laboratory for Ocean Sciences in East Boothbay, Maine, USA.

Stepanauskas and his colleagues took samples of the marinewaters at two sites: the Northwest Atlantic Ocean in the Gulf of Maine, USA in July 2009, and the Mediterranean Sea off the coast of Catalonia, Spain, in January and July 2016. They used modern unicellular genomics tools to sequence total DNA from 1,698 protists in water.

Researchers have discovered a number of protozoa, includingalveolates, stramenopiles, chlorophytes, sercozoans, picozoans and choanozoans. 19% of SAGs—single amplified genomes—from the Gulf of Maine and 48% from the Mediterranean were associated with bacterial DNA, indicating that these protozoans fed on bacteria. The most common viral sequences were found in 51% of Gulf of Maine and 35% of Mediterranean SAGs. Most were from viruses known to infect bacteria, thought to be parasites of protozoan bacterial prey.

But choanozoic and picozoic protozoa, whichfound only in the Bay of Maine sample, were different. Choanozoa, also known as choanoflagellates, are of great evolutionary interest as the closest living relatives of animals and fungi. Tiny (up to 3 μm) picozoines were first discovered twenty years ago and were originally called picobiliphytes. Until now, their food sources have been a mystery, as their food system is too small for bacteria but ample enough for viruses, most of which are less than 150 nm.

Each of the Choanozoic and Picozoic SAGs studiedprotozoa was associated with viral sequences of bacteriophages and viruses CRESS-DNA, but mostly without any bacterial DNA. However, the same sequences have been found in a wide variety of species.

The authors conclude that the Choanozoic and Picozoic species are likely to regularly consume viruses.

“Viruses are rich in phosphorus and nitrogen and potentiallycan be a good addition to a carbon-rich diet, which can include cellular prey or carbon-rich marine colloids, ”concludes Dr. Julia Brown, researcher at Bigelow's Ocean Science Laboratory and co-author of the study.

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