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The methods that scientists use to tally up DNA
viruses often miss RNA viruses, so it's been hard for
scientists to estimate how many RNA viruses are one
Earth.
The scientists first scooped up water from off a pier on
Oahu (about 115 litres all told). They then passed the
water through fine filters to exclude bacteria and
larger organisms. Through a series of additional steps,
they concentrated the viruses down further and then
extracted all the DNA and RNA from their samples.
The scientists then measured how much of each kind
of molecule was present in the sea water. Based on
the mass of RNA in an average virus, they could then
estimate how many RNA viruses were in the sample.
Their conclusion was remarkable: RNA viruses made
up between 38 and 63% of the viruses in the seawater.
In other words, about half of the viruses in the ocean
are RNA viruses.
This can be surprising when you consider the number
of hosts that RNA and DNA viruses can infect. In the
ocean, bacteria (hosts to DNA-based bacteriophages)
far outnumber eukaryotes (hosts to RNA viruses).
But it's important to bear in mind that eukaryotes are
better as virus incubators. A single eukaryote cell can
spew out far more viruses than a single bacterium.
Bacteria can't churn out viruses the way you can, my
dear eukaryotic reader.
This new study is just a first cut at estimating the
number of RNA viruses on Earth. It's possible that for
some reason the waters off the coast of Hawaii are
weirdly good places to find them. The scientists
themselves acknowledge that they may be
overestimating the number of RNA viruses through
some flaw in their methods. But when I contacted an
expert on global virus surveys, Curtis Suttle of the
University of British Columbia, for his opinion, he gave
a thumbs-up. "I think it is an interesting and
provocative paper," he told me. "We will have to see if
it holds up."
Some people say viruses are truly alive, and some say
they aren't. But if they aren't, then we need some
name that can encompass the things that are "really"
alive, along with the things that carry genes, can
evolve, and carry out all the other life-ish tasks that
viruses do.