By creating a new genetic family tree of the world's most primitive bacteria and comparing it to the geochemistry of ancient iron and sulfur deposits, Carrine Blank of Washington University has found evidence that instead of Cyanobacteria being very ancient, they may have appeared much later, perhaps as much as a billion years later, than previously assumed. Blank will present the results of her research at the annual meeting of the Geological Society of America in Denver on Tuesday, Oct. 29.
"What paleontologists and geologists have had to do is reconstruct evolutionary events because biologists haven't had a very good evolutionary tree of bacteria," says Blank. To get a better family tree, Blank took advantage of growing genome archives and studied 38 genes in the whole gene sequences of 53 species of extant bacteria, including Cyanobacteria. By mapping out the rates of change in the slowest-changing genes, Blank was able to generate a bacterial evolutionary history that shows cyanobacteria branching off last.
If correct, Blank's tree essentially flip-flops the traditional order in which bacteria appeared on the scene.
Traditionally, it has been thought that Cyanobacteria came on stage very early in Earth's history, perhaps at least 3.5 billion years ago. They produced the first abundant oxygen molecules. All that oxygen bound to the abundant free iron in the oceans and rained to the seafloor – creating the economically important banded iron formations. The advent of atmospheric oxygen also caused sulfide minerals on land to break down into sulfates and wash into the oceans – where sulfur-loving bacteria gobbled them up. The earliest geological evidence for sulfur bacteria is changes in sulfur isotopes – indicating organisms are preferentially using isotopes of the element – that began about 2.4 billion years ago. This was followed by a sudden rise in oxygen in the atmosphere at about 2.2 or 2.3 billion years ago.
"The (traditional) model was that the cyanobacteria were present all the time," says Blank. Reasonable as all this sounds, it doesn't match the genetic evolutionary tree, she says.
In Blank's version of the story, the sulfur-loving bacteria came on the scene at about 2.4 billion years ago, and the Cyanobacteria came along at least 100 million years later, she says. Because banded iron formations were formed much earlier than these dates, Cyanobacteria are not likely to have led to their creation, she explains.
Blank's model could explain the puzzling lack of actual cyanobacteria fossils in the earliest days of the banded iron formations. It could also resolve an apparent contradiction regarding the biochemistry of Cyanobacteria, says Blank. The contradiction is that cyanobacteria have a surprisingly advanced biochemistry that was the product of a long evolutionary history. In other words, cyanobacteria must have evolved from more primitive photosynthetic bacteria.
If Blank is correct, her revised evolutionary history of the bacteria raises a difficult question: If cyanobacteria came later, where did the Earth's earliest oxidants come from which produced banded iron formations? There are many competing theories on this matter, Blank says. Among them are hypotheses that call on inorganic reactions in the oceans and the air to release limited amounts of oxidants. There is even the possibility that there was also an early and so-far undiscovered iron oxidizing microbe that may have produced banded iron formations as a result of their metabolism, Blank says.
Blank's cyanobacteria research was conducted as part of her recent doctoral thesis at the University of California at Berkeley. Her bacterial phylogeny research is currently under review for publication in the journal Molecular Phylogenetics and Evolution. Blank is an Assistant Professor of Molecular Geobiology in the Department of Earth and Planetary Sciences at Washington University in St. Louis.
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The abstract for this presentation is available at: http://gsa.confex.com/gsa/2002AM/finalprogram/abstract_46069.htm
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