16 THE SCIENTIST | the-scientist.com
ALEXANDRA WORDENAfter the expedition, postdoc David
Needham decided to mine frozen sam-
ples for the presence of viruses. He hit
pay dirt. What he found was the largest
known virus to inhabit the ocean, and
only the second discovered that infects
predatory protists (PNAS, 116:20574–83,
2019). “By looking for the usual [micro-
bial] candidates, we initially missed that
there was quite a beautiful virus to be
assembled,” says Worden.
At 875 kilobases, the so-called
Choano Virus’s genome is impressive in
size, and the proteins it codes for are
particularly exciting to the researchers.
Among hundreds of protein sequences,
Worden’s team identified three for
rhodopsins—light-processing receptors
found in certain cellular membranes
of many living organisms, including
humans, who require rhodo psin in the
retina for sight. The viral genome also
codes for the components of a path-
way that synthesizes a molecule calledβ-carotene along with the enzyme that
processes it into retinal, the pigment that
does the actual light sensing within the
rhodopsin protein. By contrast, humans
and indeed most eukaryotes make only
part of the rhodopsin machinery and
have to obtain β-carotene from external
sources to make retinal.
To find out what these viral rhodo-
psins were possibly doing inside the
choanoflagellate hosts, the research-
ers expressed the viral sequences in E.
coli and found that light triggered the
proteins to start pumping protons. The
likely scenario is that the virus is actu-
ally helping the cell harvest energy from
the sun—“something you would never
expect in a predatory cell,” says Worden.
“Here was a virus bringing an entirely
new function, something that didn’t
exist in the host organism.”
That some organisms use rhodo psins
to generate chemical energy from light
is not a new discovery. Decades ago, sci-
entists reported that halophilic archaea
make use of rhodopsins for just that. And
in 2000, Worden’s coauthor Ed DeLong,
then at MBARI and now at the Univer-
sity of Hawai‘i at Mānoa, and colleaguesdescribed marine bacteria that also use
rhodopsins to capture light energy. Since
then, marine biologists have uncovered
this form of metabolism in a wide range
of microbial species in the ocean, accord-
ing to Laura Gómez-Consarnau of the
University of Southern California. Last
year, she and her colleagues quantified
the abundance of rhodopsins in seawa-
ter samples and estimated that, in some
parts of the ocean, the energy harvested
from sunlight by rhodopsins is more
than that captured by chlorophyll (Sci
Adv, 5:eaaw8855, 2019).
The latest paper is the first to show
a virus apparently giving a predatory
unicellular organism that functional-
i ty. Gómez-Consarnau says she suspects
choanoflagellates turn to the rhodopsin
mechanism to get energy when organic
matter is scarce, or perhaps to help speed
up their metabolisms.
“This hypothesis challenges us to
try to understand the infectious process
beyond the pathogenic consequences
for the host,” Jose Luis López, who stud-
ies marine rhodopsins at the University
of Buenos Aires but was not involved in
the work, writes in an email to The Sci-
entist. Rather than infecting their hosts
and lysing the cells to spread more virus,
perhaps Choano Viruses and others pro-
vide some survival benefit. The concept
has gained traction in human virology,
says Joaquín Martínez Martínez, a senior
research scientist at the Bigelow Labora-
tory for Ocean Sciences in Maine who was
not involved in the study, but it’s rather
new on the marine side, where viruses
have received far less scientific attention.
“So far we mostly have circumstan-
tial evidence” of a symbiotic relation-
ship between marine viruses and hosts,
says Martínez Martínez, who studies
these interactions. Part of the trouble
is that many of these organisms are
not cultured, making lab experiments
on them impossible—at least for now,
until more subjects are cultivated in the
lab. “Papers like this are very important
to keep building on circumstantial evi-
dence with more actual data.”
—Kerry GrensNOTEBOOKBUG CATCHER: Researchers collect seawater
from the side of a ship as part of a survey of
marine unicellular eukaryotes and the organisms
associated with them.