[meteorite-list] How Organisms Survived Asteroid Impacts

[Sterling K. Webb]

http://www.space.com/scienceastronomy/090910-am-impact-mixotrophs.html

Life in the Dark: How Organisms Survived Asteroid Impacts 
By Jeremy Hsu -- Astrobiology Magazine
10 September 2009

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.

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.

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.

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