October 24, 2008 | David F. Coppedge

The Life and Death of Oxygen

The oxygen in our atmosphere has the energy equivalent of 20 thousand billion billion hydrogen bombs.  To maintain the oxygen level in our atmosphere, that amount of energy would have to be spent in manufacturing molecular oxygen every 4 million years (a thousandth the assumed age of the earth).
    Now that we have your attention, let’s think about the role of oxygen and life.  The statistics above were estimated by Paul G. Falkowski and Yukio Isozaki in Science this week.1  Unlike nitrogen, which is inert, oxygen is lively – it oxidizes, or burns things – not only in fire, but in cells, where the element must be handled gingerly by molecular machines to avoid damage.  That’s also why you take antioxidants in your food.  Keeping oxygen away from the primordial soup at the origin of life is understandably a serious problem (10/20/2008).
    Evolutionary biologists do not believe earth’s oxygen is primordial (i.e., that it formed when the earth formed).  They believe it was generated by living organisms when they evolved to use oxygen for electron capture in metabolism.  This conveniently keeps oxygen out of the picture at the origin of life (though some atmospheric oxygen forms spontaneously by the dissociation of water).  Oxygen could also be sequestered from the air in continental rocks: silicates, carbonates and sulfates.
    Oxygen reached levels of 10 to 30% only in the last 550 million years, evolutionists say.  Its 4-million-year lifetime is 0.4% the estimated 1 billion year lifetime of the atmosphere’s most abundant gas, nitrogen.  How did oxygen, with its relatively short lifetime, become the second most abundant gas in the atmosphere?  “The story is not as simple as it might first appear,” said Falkowski and Isozaki.  One has to calculate when and how it was first generated, and how it persists in its high concentration.
    Some oxygen is continuously formed by the breakup of water molecules by ultraviolet light in the atmosphere (at least till ozone forms and shields the upper atmosphere from excess UV).  If biology is the source, how does life produce it from water and minerals? 

The overwhelming source of O2 on Earth is photobiological oxidation of water; neither the evolution nor the mechanism of this process are completely understood.  Apparently it arose once in a single clade of bacteria and was then appropriated via a single event, in which one cell engulfed another (endosymbiosis) to form a new symbiotic organism.  The latter became the progenitor of all photosynthetic eukaryotes, including algae and higher plants.
    The core of the oxidation machinery is photosystem II, a large protein complex containing four manganese atoms that are photocatalytically oxidized to create electron holes upstream.

They stressed that this “arose” once most likely because of the improbability that a “large protein complex” of “oxidation machinery” could arise by chance.  Nevertheless, assuming plants and bacteria produce it, the equation is balanced by the animals that consume it:

On time scales of years to millennia, these reactions are closely coupled to the reverse process of respiration, such that net production of O2 is virtually nil.  That is, without burial of organic matter in rocks, there would be very little free O2 in the atmosphere.  Hence, the evolution of oxygenic photosynthesis was a necessary but not a sufficient condition to oxidize Earth’s atmosphere.

So the second problem is getting molecular oxygen up to the level of 10-30% that has been maintained for 500 million years.  If a small amount is subducted into the mantle by plate tectonics, or captured in stable continental rocks, an atmospheric excess could be built up to a stable concentration without runaway production.  “The balance between burial of organic matter and its oxidation,” they said, “appears to have been tightly controlled over the past 500 million years.”  This balance requires an ongoing process of long-term storage within the earth.  The picture becomes complicated by the fact that volcanoes can re-release oxygen back into the atmosphere.  “The presence of O2 in the atmosphere requires an imbalance between oxygenic photosynthesis and aerobic respiration on time scales of millions of years,” they said; “hence, to generate an oxidized atmosphere, more organic matter must be buried than respired.”
    How well do scientists know how oxygen concentration has varied over geologic time?  “Perhaps surprisingly, not very well.”  Comparison of isotopes in carbonates and sulfates provide clues.  They believe the initial oxygen concentration produced by the first photosynthetic bacteria was quite low.  It rose when eukaryotes appeared, and then, according to the evolutionary timeline, became much more abundant in the Neoproterozoic – corresponding to the period just before the Cambrian Explosion.  The eukaryotic oxygen increase would have had to coincide with enhanced subduction in the lithosphere.
    Was the Cambrian Explosion a cause or effect of the rise of oxygen?  They suggested the latter: “The burial of large amounts of organic carbon over the past 750 million years is mirrored in a substantial rise in atmospheric O2, which may have triggered the Cambrian explosion of animal life.”
    Another balance of geology and biology would have had to occur in the Carboniferous.  The doubling of oxygen production by trees and ferns had to be balanced by “further increases in burial efficiency” they said.  How the continental plates coordinated their behavior with the evolution of plants, they did not say.  Throughout the remainder of earth history, this balance was maintained within comparatively narrow limits – 10 to 23%.  “The relatively narrow range of variability suggests tight controls on the rate of burial and oxidation of organic matter on Earth’s surface.”  They did not say who or what is controlling these rates, other than to say that “the burial of organic carbon is roughly balanced by oxidation and weathering.”
    How valid is this story?  They think the broad picture is understood, but “the details remain sketchy” – particularly, how photosynthesis splits water, how oxygen concentration is controlled in the atmosphere.
    Could Woodward W. Fischer in Nature help the story?2  How good is the evidence to support the rise of the first photosynthetic bacteria?  “Go back to Archaean time, the interval of Earth’s history between about 4 billion and 2.5 billion years ago,” he began, “and we’re in largely unknown biological territory.
    While Fischer was concerned primarily with debunking claims of eukaryotes too early for comfort (i.e., before the rise of atmospheric oxygen), his report contained reason to doubt the validity of the timeline.  The new evidence may remove an embarrassing puzzle of how photosynthesis could arise 300 million years before the rise of atmospheric oxygen, but “does it close the gap between the morphological and molecular-fossil records of the evolution of eukaryotes?” he asked himself.  He answered himself, “Not yet.”  Other scientists are not conceding the debunking of 2.7-billion-year-old photosynthesis.  A news item about this on Nature News agrees the debate is far from over.
    For problems with oxygen at the birth of the solar system, see bullet one of the 09/24/2008 entry.

1.  Paul G. Falkowski and Yukio Isozaki, “The Story of O2,” Science, 24 October 2008: Vol. 322. no. 5901, pp. 540-542, DOI: 10.1126/science.1162641.
2.  Woodward W. Fischer, “Biogeochemistry: Life before the rise of oxygen,” Nature 455, 1051-1052 (23 October 2008) | doi:10.1038/4551051a.

OK; how convinced are you that the evolutionary storytellers are compelled by the evidence to embrace their billions of years saga of a history they cannot observe?  It’s a magical history, in which complex oxidation machines “arise” by some unspecified natural magic.  (Note that if something “arose once,” it is not following a natural law).
    Lacking evidence, they can build models that include the natural magic built-in.  By tweaking parameters here and there, and trying to debunk contrary evidence, they can get it to work – sort of.  It continues to amaze them how finely balanced it is.
    So much for this space fantasy.  The atmosphere on Darwin’s imaginary world is too rarefied to breathe.  Let’s head back to the real world.

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Categories: Origin of Life

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