“Our data show that many cells of eukaryotic organisms - protists - contain DNA from the most
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, stramenopils, chlorophytes, serkozoic, 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 fed on 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, 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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