Text 213, 162 rader
Skriven 2004-09-28 13:35:00 av Michael Ragland (1:278/230)
Ärende: Re: Extinction of anaerob
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joe@removethispart.gs.washington.edu (Joe Felsenstein) wrote in
news:cj9cfv$6ve$1@darwin.ediacara.org: 
In article <civ2db$2s2$1@darwin.ediacara.org>, Brett Aubrey
<brett.aubrey@shaw.ca> wrote: 
My notes suggest that the earliest photosynthesis was around ~3.1 BYA,
which concurrently began to create an oxidized atmosphere. This in turn
started the extinction of anaerobes and the rise of aerobic organisms. 
If these notes are relatively accurate, do we have an ballpark idea of
the length of time it took for the extinction of anaerobic organisms?
(e.g. tens or hundreds of millions of years?) 

Regards, Brett Aubrey. 


It seems to be taking forever. That extinction is still not complete. As
I sit here my gut is loaded with an anaerobic bacterium 
(Bacteroides), for example. So is yours. 

Joe Felsenstein


OK, so Brett didn't know how to properly phrase the question. But do you
know if there is any good information on just how quickly environments
changed from low to high redox potential, and how quickly populations
changed from predominantly obligate anaerobes to a mix of aerobic,
facultative, and anaerobic forms? 

Yours, 
Bill Morse 




According to some sources by 2 billion years ago, oxygen levels were
high enough that anaerobic prokaryotes were declining.  Oxygen-releasing
photosynthesis caused the atmosphere to become oxidizing rather than
reducing. Accumulation of O2 caused extinction of [many] anaerobic
organisms and rise of aerobic organisms. This is a general ballpark
figure. As far as exactly pinning down good information on just how
quickly environments changed from low to high redox potential, and how
quickly populations changed from predominantly obligate anaerobes to a
mix of aerobic, facultative, and anaerobic forms I don't think there may
be an answer to that. I suggest either you or Mr. Aubrey consult a
microbiologist.

Another site stated, "The anaerobes undergo a form of respiration called
fermentation. Among anaerobes, some can live in the presence or absence
of oxygen. These are called facultative anaerobes. Some are indifferent
to the presence of oxygen, but others have two respiratory pathways, one
that uses oxygen and one that does not. The other group of anaerobes,
the obligate anaerobes, are actually poisoned in the presence of oxygen

Finally, I read at http://connection.lww.com/Products/
burton/documents/ByChapter/Ch04.doc

Life in the Absence of Oxygen

Someday, you may overhear someone erroneously state that A Life is
impossible there (perhaps referring to one of the planets) because there
isn't any oxygen. But, you'll know differently! You'll be able to point
out that life is indeed possible in the absence of oxygen. Furthermore,
you'll be able to explain that organisms capable of life in the absence
of oxygen are called anaerobes.
      But, who discovered anaerobes? The credit for
discovering anaerobes can be given to three scientists: a 17th century
scientist, an 18th century scientist, and a 19th century scientist. Each
of them made scientific observations that contributed to our present
knowledge and understanding of anaerobes.
      In 1680, Anton van Leeuwenhoek performed an experiment
using pepper and sealed glass tubes. In a letter to the Royal Society of
London, he wrote that "animalcules developed although the contained air
must have been in minimal quantity." [Leeuwenhoek used the term
"animalcules" to refer to the tiny organisms that he observed, using the
simple, single lens microscopes, which he made.] 
      Lazzaro Spallanzani, an Italian scientist, performed
similar experiments in the latter half of the 18th century. He drew the
air from microbe-containing glass tubes, fully expecting the microbes to
die—but some did not. He wrote in a letter to a friend, "The nature of
some of these animalcules is astonishing! They are able to exercise in a
vacuum the functions they use in free air. ... How wonderful this is!
For we have always believed there is no living being that can live
without the advantages air offers it."
      It was Louis Pasteur who actually introduced the terms
"aerobe" and "anaerobe." In an 1861 paper, he wrote "these infusorial
animals are able to live and multiply indefinitely in the complete
absence of air or free oxygen. ... These infusoria can not only live in
the absence of air, but air actually kills them. ... I believe this is
... the first example of an animal living in the absence of free
oxygen." [The term "infusoria" was used by early microbiologists to
refer to microorganisms. Infusoria was later used to specifically refer
to ciliated protozoa, but the term is now obsolete.]
      We know now that anaerobes are quite common and that
they live in specific ecological niches. They can be found in soil, in
fresh- and saltwater sediments (mud), and in the bodies of animals and
humans. The indigenous microflora of humans contains many species of
anaerobes, some of which are opportunistic pathogens. Anaerobes cause a
wide variety of human diseases, including botulism, tetanus, gas
gangrene, pulmonary infections, brain abscesses, and oral diseases. It
was Louis Pasteur who, in 1877, discovered the first pathogenic
anaerobe—the bacterium that today is known as Clostridium septicum. 

The Oxygen Holocaust

In the beginning, all the world was anaerobic—there was no oxygen.
Scientists tell us that the first organisms were anaerobic
microorganisms that evolved some 3 to 4 billion years ago. Life on earth
then remained anaerobic for hundreds of millions of years.
      Then, about 2 billion years ago, the first worldwide
pollution crisis occurred. "The oxygen holocaust" (as described by
Margulis and Sagan) came about as the result of the evolution of the
purple and green photosynthetic microbes. These organisms were able to
make use of the hydrogen in water, by photosynthesis, leaving a waste
product called oxygen. Yes, the oxygen that we humans consider so
precious was originally a gaseous poison dumped into the atmosphere.
      As oxygen gains electrons (becomes reduced), highly
reactive, short-lived chemicals (called free radicals) are produced.
These free radicals wreak havoc with the organic compounds that are the
very basis of life. They destroy membranes and enzymes and are lethal to
cells.
      As stated by Lovelock, "the first appearance of oxygen
in the air heralded an almost fatal catastrophe for early life." Many
anaerobic microbes were immediately destroyed. The microbes able to
survive were those that responded to the crisis by developing ways to
detoxify and eventually exploit the dangerous pollutant. These were the
organisms that developed the ability to produce enzymes—like catalase,
peroxidase, and superoxide dismutase—that break down and neutralize
the various toxic reduction products of oxygen.
       Those organisms lacking such enzymes either died or
were forced to retreat to ecological niches devoid of oxygen, such as
soil and mud and deep within the bodies of animals. Those anaerobes that
constitute part of our own indigenous microflora, for example, lead a
rather pampered existence. We provide them with warmth and nutrients and
a safe haven from their worst enemy—oxygen.

So it seems Mr. Morse despite the oxidation of the atmosphere and the
destruction of many anarobes there were still ecological niches where
they could survive without oxygen. As far as facultative anaerobes this
appears to be an adaptation to an oxidizing atmosphere. Some are
indifferent to oxygen while others have different respiration pathways
which can allow them to both survive in the presence or absence of
oxygen.

I found a good website on the origins of prokaryotes and metabolic
diversity at http://www.mansfield.ohio-state.edu/~sabedon/campbl27.htm
It also includes Google search engine to search for many terms.

Michael Ragland

"It's uncertain whether intelligence has any long term survival value.
Bacteria do quite well without it."
 Stephen Hawking
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