Viruses are found everywhere on Earth, from the atmosphere to the depths of the ocean. Surprisingly, given the abundance and richness of nutrients in viruses, no organisms are known to consume them. In the journal Frontiers in Microbiology, researchers publish the first convincing evidence that two groups of ecologically important marine protists are virus eaters, catching their “prey” through phagocytosis – absorption.
“Our data shows that many cells of eukaryotic organisms – protists – contain DNA from a wide variety of non-infectious viruses, but not bacteria, which is compelling evidence that they feed on viruses and not bacteria. This came as a big surprise as these results contradict current scientists’ understanding of the role of viruses and protists in marine food webs, ”said Dr. Ramunas Stepanauskas, director of the Single Cell Genomics Center at Bigelow Ocean Science Laboratory in East Boothbay, Maine, USA.
Stepanauskas and his colleagues took seawater samples from two locations: in the northwestern Atlantic Ocean in the Gulf of Maine, USA in July 2009, and in 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, including alveolates, stramenopils, chlorophytes, serkozoan, picozoic, and choanozoic. 19% of SAGs – single amplified genomes – from the Gulf of Maine and 48% from the Mediterranean were associated with bacterial DNA, suggesting that these protozoa ate bacteria. The most common viral sequences were found in 51% of Gulf of Maine SAGs and 35% of Mediterranean SAGs. Most of these were from viruses known to infect bacteria, supposedly parasites of the bacterial prey of protozoa.
But Choanozoic and picozoic protozoa, which were found only in the Gulf 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 SAGs of the choanozoic and picozoic protozoa studied were associated with the viral sequences of bacteriophages and CRESS-DNA viruses, 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 could potentially be a good addition to a carbon-rich diet, which could 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.