17 August 2019 | New Scientist | 9AN ORGANISM found off the coast
of Japan could explain one of the
great mysteries of evolution: the
origin of complex cells.
A successful attempt to grow
the organism in the lab is “very
influential and monumental”,
says Thijs Ettema at Wageningen
University in the Netherlands.
For much of Earth’s history,
life existed only as simple,
single-celled organisms like
bacteria. However, today
the planet is also home to
eukaryotes – organisms, from
trees to elephants to us, that are
all made of more complex cells.
While bacterial cells have little
internal structure, the eukaryotic
cells of animals, plants and fungi
are more intricate. A major
difference is that they contain
mitochondria – structures that
supply them with energy.
It is thought that the
mitochondria in your cells are
descended from a bacterium that
was engulfed by another cell and
then set up home there. The
question is: what organism did
the swallowing, effectively
becoming the first complex cell?
Clues started to emerge in 2010,
when a team sampled sediment
at the bottom of the Arctic Ocean
near a set of hydrothermal vents
called Loki’s Castle, named after a
Norse god. Five years later, Ettema
and his colleagues found DNA
from an unknown microorganism
lurking in this mud sample.
It was from an archaean, a
single-celled life form from a
group of organisms that split from
bacteria early in the history of life.
But it had genes thought to be
unique to the more advanced
eukaryotes – including some used
to deform the outer membranes
of cells, hinting at an ability to
swallow smaller objects. The
team named it Lokiarchaeota.
Several related archaea have
since been identified from their
DNA. In keeping with the Norse
mythology theme, this group
of organisms is now called the
Asgard archaea, after the realm
of the Norse gods.
Ettema and others believe the
Asgard archaea hint at the origin
of eukaryotes. An Asgard-like
archaean that lived billions of
years ago could have swallowed
a bacterium to become the firsteukaryote – setting evolution
on the path towards humanity.
However, all studies of this
group have been based on
found DNA rather than whole
cells. Nobody had seen a living
Asgard archaean – until now.
In 2006, before anyone had
heard of Asgard archaea, a
submersible collected sediment
2500 metres down in the Nankai
Trough off Japan. Later that year,
Hiroyuki Imachi of the Japan
Agency for Marine-Earth
Science and Technology and
his colleagues began trying to
grow microorganisms from the
sample in an incubator.
After hearing Ettema talk about
Asgard archaea at a conference
in 2014, Imachi worked with
colleagues to devise a special
mixture of nutrients and other
chemicals for the sample. This has
now finally enabled a species of
Asgard archaean to be grown and
studied in a lab for the first time
(bioRxiv, doi.org/gf5z2n).
The team has photographedand studied the microbe, which
is named Prometheoarchaeum
syntrophicum. “Several groups
worldwide are trying this,
including my lab,” says Ettema.
“They’ve beaten us to it.”
P. syntrophicum has two
surprising features. The first is
that it doesn’t live on its own.
The team found it can only grow
alongside another archaean,
Methanogenium. It grows even
better when Halodesulfovibrio
bacteria are also present.
P. syntrophicum appears to
break down amino acids into
simpler chemicals, including
hydrogen, which the othermicrobes then eat. Close
partnerships like these could
have been a stepping stone
towards an Asgard archaean
engulfing a cooperative
bacterium 2 billion years ago.
P. syntrophicum is also an
unusual shape for an archaean.
It has long protrusions, a bit like
the tentacles of an octopus, and
its microbial partners nestle
within these. Imachi suggests that
these explain how complex cells
originated. If an Asgard archaean
had bacteria in its arms, and the
arms fused into a single lump,
a bacterium would have ended
up inside the Asgard archaean.
This is possible, says Ettema,
but we don’t know yet if other
Asgard archaea have these arms
too. “It will be very exciting if
we can cultivate other Asgard
members to see if they have
similar types of protrusions,” he
says. “Only then will we be able to
infer the ancestral characteristics
of the Asgard cell from which
eukaryotes evolved.” ❚Evolution
Michael Marshall
TH
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AGESYour single-celled relative
Deep-sea microbe offers us a glimpse of our early origins
What is life?
Physicist Paul Davies says an answer is in sight
newscientistlive.com/what-is-lifeP. s y nt r ophicu m (above)
gives clues to how complex
life began. It was isolated
in the sea off Japan (below)“ The microbe is unusual.
It has long protrusions,
a bit like the tentacles
of an octopus”HIROYUKI^ IMAC
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