https://www.nytimes.com/2018/04/13/science/virosphere-evolut
ion.html

Trillions Upon Trillions of Viruses Fall From the Sky Each
Day

By JIM ROBBINSAPRIL 13, 2018

Viruses attached to a fragment of a bacterial cell wall.
"Viruses modulate the function and evolution of all living
things," scientists wrote last year. "But to what extent
remains a mystery." Credit Biophoto Associates/Science
Source

High in the Sierra Nevada mountains of Spain, an
international team of researchers set out four buckets to
gather a shower of viruses falling from the sky.

Scientists have surmised there is a stream of viruses
circling the planet, above the planet's weather systems but
below the level of airline travel. Very little is known
about this realm, and that's why the number of deposited
viruses stunned the team in Spain. Each day, they
calculated, some 800 million viruses cascade onto every
square meter of the planet.

Most of the globe-trotting viruses are swept into the air by
sea spray, and lesser numbers arrive in dust storms.

"Unimpeded by friction with the surface of the Earth, you
can travel great distances, and so intercontinental travel
is quite easy" for viruses, said Curtis Suttle, a marine
virologist at the University of British Columbia. "It
wouldn't be unusual to find things swept up in Africa being
deposited in North America."

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The study by Dr. Suttle and his colleagues, published
earlier this year in the International Society of Microbial
Ecology Journal, was the first to count the number of
viruses falling onto the planet. The research, though, is
not designed to study influenza or other illnesses, but to
get a better sense of the "virosphere," the world of viruses
on the planet.
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Generally it's assumed these viruses originate on the planet
and are swept upward, but some researchers theorize that
viruses actually may originate in the atmosphere. (There is
a small group of researchers who believe viruses may even
have come here from outer space, an idea known as
panspermia.)

Whatever the case, viruses are the most abundant entities on
the planet by far. While Dr. Suttle's team found hundreds of
millions of viruses in a square meter, they counted tens of
millions of bacteria in the same space.

Mostly thought of as infectious agents, viruses are much
more than that. It's hard to overstate the central role that
viruses play in the world: They're essential to everything
from our immune system to our gut microbiome, to the
ecosystems on land and sea, to climate regulation and the
evolution of all species. Viruses contain a vast diverse
array of unknown genes -- and spread them to other species.

Last year, three experts called for a new initiative to
better understand viral ecology, especially as the planet
changes. "Viruses modulate the function and evolution of all
living things," wrote Matthew B. Sullivan of Ohio State,
Joshua Weitz of Georgia Tech, and Steven W. Wilhelm of the
University of Tennessee. "But to what extent remains a
mystery."
Photo
Viruses reproduce by attaching to a bacterium and injecting
their own genes. Ancient viral DNA eventually became part of
the nervous system of modern humans, playing a roll in
consciousness, nerve communication and memory formation.
Credit Biozentrum, University of Basel/Science Source

Do viruses even fit the definition of something alive? While
they are top predators of the microbial world, they lack the
ability to reproduce and so must take over the cell of a
host -- called an infection -- and use its machinery to
replicate. The virus injects its own DNA into the host;
sometimes that new genes are useful to the host and become
part of its genome.

Researchers recently identified an ancient virus that
inserted its DNA into the genomes of four-limbed animals
that were human ancestors. That snippet of genetic code,
called ARC, is part of the nervous system of modern humans
and plays a role in human consciousness -- nerve
communication, memory formation and higher-order thinking.
Between 40 percent and 80 percent of the human genome may be
linked to ancient viral invasions.

Viruses and their prey are also big players in the world's
ecosystems. Much research now is aimed at factoring their
processes into our understanding of how the planet works.

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"If you could weigh all the living material in the oceans,
95 percent of it is stuff is you can't see, and they are
responsible for supplying half the oxygen on the planet,"
Dr. Suttle said.

In laboratory experiments, he has filtered viruses out of
seawater but left their prey, bacteria. When that happens,
plankton in the water stop growing. That's because when
preying viruses infect and take out one species of microbe
-- they are very specific predators -- they liberate
nutrients in them, such as nitrogen, that feed other species
of bacteria. In the same way, an elk killed by a wolf
becomes food for ravens, coyotes and other species. As
plankton grow, they take in carbon dioxide and create
oxygen.

One study estimated that viruses in the ocean cause a
trillion trillion infections every second, destroying some
20 percent of all bacterial cells in the sea daily.

Viruses help keep ecosystems in balance by changing the
composition of microbial communities. As toxic algae blooms
spread in the ocean, for example, they are brought to heel
by a virus that attacks the algae and causes it to explode
and die, ending the outbreak in as little as a day.

While some viruses and other organisms have evolved together
and have achieved a kind of balance, an invasive virus can
cause rapid, widespread changes and even lead to
extinction.

West Nile virus has changed the composition of bird
communities in much of the United States, killing crows and
favoring ravens, some researchers say. Multiple extinctions
of birds in Hawaii are predicted as the mosquito-borne
avipoxvirus spreads into mountain forests where it was once
too cold for mosquitoes to live.

When species disappear, the changes can ripple through an
ecosystem. A textbook example is a viral disease called
rinderpest.
Photo
An engraving showing a cattle inspection at a market in 19th
century London, when rinderpest, a viral disease, was
rampant in Europe and Africa, wiping out some herds
entirely. Credit Universal History Archive/UIG, via Getty
Images

The Italian army brought a few cattle into North Africa, and
in 1887 the virus took off across the continent, killing a
broad range of cloven-hoofed animals from Eritrea to South
Africa -- in some cases wiping out 95 percent of the herds.

"It infected antelope, it infected wildebeest and other
large grazers across the whole ecosystem," said Peter
Daszak, the president of Ecohealth Alliance, which is
working on a global project to catalog viruses likely to
pass from animals to humans.

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"The impact was not just on the animals. But because they
are primary grazers and they died off in huge numbers,
vegetation was impacted, and it allowed trees to grow where
they would have been grazed away," he said.

"The large acacia trees on the plains of Africa are all the
same age and were seedlings when rinderpest first came in
and the wildlife died," Dr. Daszak said. In other places,
far less grazing created a hospitable habitat for the tsetse
fly, which carries the parasites that cause sleeping
sickness.

"These kinds of ecological changes can last for centuries or
even millennia," Dr. Daszak said.

Combined with drought, large numbers of people died from
starvation as rinderpest spread. An explorer in 1891
estimated two-thirds of the Masai people, who depended on
cattle, were killed.

"Almost instantaneously, rinderpest swept away the wealth of
tropical Africa," wrote John Reader in his book "Africa: A
Biography of a Continent."

With intensive vaccinations, rinderpest was completely wiped
out, not only in Africa but globally in 2011.

The beneficial effects of viruses are much less known,
especially among plants. "There are huge questions in wild
systems about what viruses are doing there," said Marilyn
Roossinck, who studies viral ecology in plants at
Pennsylvania State University. "We have never found
deleterious effects from a virus in the wild."

A grass found in the high-temperature soils of Yellowstone's
geothermal areas, for example, needs a fungus to grow in the
extreme environment. In turn, the fungus needs a virus.

Tiny spots of virus on the plant that yields quinoa is also
important for the plant's survival. "Little spots of virus
confer drought tolerance but don't cause disease," she said.
"It changes the whole plant physiology."

"Viruses aren't our enemies," Dr. Suttle said. "Certain
nasty viruses can make you sick, but it's important to
recognize that viruses and other microbes out there are
absolutely integral for the ecosystem."
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