A
dinosaur-killing asteroid may have wiped out much of life on Earth 65 million
years ago, but now scientists have discovered how smaller organisms might have
survived in the darkness following such a catastrophic impact.
Survival
may have depended upon jack-of-all-trades organisms called mixotrophs that can
consume organic matter in the absence of sunlight. That would have proved
crucial during the long months of dust and debris blotting
out the sun, when plenty of dead or dying organic matter filled the Earth's
oceans and lakes.
"Mixotrophs
are very good at stabilizing situations by using whatever resources are there,
and can often provide what resources there aren't," said Harriet Jones, a
biologist at the University of East Anglia in the UK. "They're very good
at coping in extreme environments, and enabling other organisms to
live."
Jones and
her colleagues tested the limits of mixotrophs by subjecting them to six months
of low light or complete darkness. The mixotrophs not only thrived, but also
surprised researchers by helping sunlight-dependent organisms also survive
pitch black conditions.
Simulating
catastrophe
Scientists
have long debated the overall impact of the K-T extinction that may have
heralded the end of the dinosaurs, but most researchers agree that such an
event would have thrown up enough dust and debris to darken Earth's skies for
about six months. A lack of sunlight would have killed off a majority of
plants, eliminating the food supply for animals higher up the food chain.
Many
scientists assumed that even smaller organisms would struggle just to stay
alive during months of almost complete darkness. Some previous studies even
looked at how some organisms such as mixotrophs can survive low light and low
food conditions. But no one had tried to test how well mixotrophs would survive
the catastrophic environment following something such as the K-T event, Jones
said.
"The
literature was always saying in that biological production would cease in a
post-catastrophic environment," Jones noted. "We felt that because of
what mixotrophy algae could do, that wasn't always the case."
Jones joined
forces with Charles Cockell, a microbiologist at the Open University based in
the UK who specializes in catastrophic
environments, as well as other researchers. They tested both freshwater and
ocean mixotrophs under conditions ranging from low light to complete darkness
for six months, and added food sources during short-term experiments to
simulate decaying organic matter.
However,
Jones and her colleagues also wanted to see how mixotrophs fared when living together
with phototrophs, or light-dependent organisms. They tested mixotrophs and
phototrophs separately and together under the different light conditions.
Live
together or die alone
Turns out
that the mixotrophs survived all the experiments, and some even grew under the low
light conditions. Their ability to consume other organisms or organic matter
helped them rebound quickly after low light returned, perhaps similar to the
clouds of dust and debris finally beginning to clear.
But the
real shock came from how well light-dependent organisms did when living with
the mixotrophs. No photosynthesis could take place under the complete darkness,
but the phototrophs mostly managed to survive based on nutrients cycled by the active
mixotrophs.
"We
were extremely surprised at how well phototrophs did during six months
darkness, when they can't eat at all," Jones said. Such findings may cause
researchers to rethink how well certain life forms survived the catastrophic
impacts that dot Earth's geological record.
Furthermore,
the mixotroph activity allowed the phototroph populations to rebound quickly
back to normal within a month. And in the end, both mixotrophs and phototrophs
tended to fare better when living together.
"So
long as mixotrophs are cycling nutrients, [phototroph] algae can take off
quickly and get the life cycle going," Jones explained.
Life
lessons for survival
Only one low light condition saw phototrophs fail to survive
while living with mixotrophs. The phototrophs may have used too much energy
trying to do photosynthesis in the weak light, or perhaps the hungry mixotrophs
simply fed on their fellow organisms.
"You
can only do so much in a flask, and obviously the mix of species would be much
greater in a natural environment," Jones pointed out.
Still, the
overall results suggest how mixotrophs provide a cushion against catastrophe
for certain ecosystems, and may even prevent huge population crashes. The
research is further detailed in the July/August issue of the journal Astrobiology.
Jones and
her colleagues plan to conduct more studies with greater mixes of species, in
an environment that would more closely resemble the natural world. They also
want to shorten experiments to three months rather than six.
That looks
all well and good for the smaller organisms. But humans, who would have a much
harder time feeding themselves if the skies went dark, may want to plan on how to
prevent such catastrophic asteroid impacts in the future.
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