ORIGIN OF METABOLISM What was the earliest life and where did it get its energy?

ORIGIN OF METABOLISM What was the earliest life and where did it get its energy?

Geological stratigraphy, together with radioactive dating, show the sequence of events in the history of the Earth. Note the entry for cyanobacteria and stromatolites only one billion years after the formation of the Earth. Last impact heating ~3500

The oldest actual fossils of microbial life date back some 3.6 to 3.5 billion years, which is a few hundred thousand years after the first chemical signs of life occurred. These fossils consist of delicate chains of microbes that look exactly like blue-green bacteria...or cyanobacteria, which are still in existence today. (Niele, p. 8) Were there actually living cells present 3.6 billion years ago? Did they form stromatolites? Were they cyanobacteria? If they were not cyanobacteria, what were they?

Stromatolites are layered rocks formed under thin microbial mats. Cyanobacterial mats: primarily Microcoleus chthonoplastes Stromatolites in Hamelin Pool, Shark Bay, Western Australia

Stromatolites can be recognized by their layered appearance. More stromatolites in Hamelin Pool, Shark Bay, Western Australia

Some ancient stromatolites are huge. Stromatolite from the Canadian Northwest Territories

Bill Schopf, a professor at UCLA, found microfossils in ancient stromatolites (Apex chert in Western Australia) and suggested that they had been cyanobacteria (oxygen-producing, photosynthetic bacteria. The fossils can be compared to micrographs of real cyanobacteria.

Martin Brasier, from Oxford University, asserts that Schopf s microfossils are inorganic carbonaceous precipitates. These inclusions, photographed from the Apex chert samples in the London Museum of Natural History, show more variation than the ones published by Schopf.

By comparison, here are recognized microfossils, but these are from the Bitter Springs chert, only about 850 million years old.

Were there actually living cells present 3.6 billion years ago? Evidence pro: Banded formations like stromatolites Carbonaceous microfossils(?) Evidence against: Variation in the microfossils Deposition of carbon (graphite) abiotically in a hot, reducing environment If ancient stromatolites were biogenic, were they formed by cyanobacteria? Problems: High temperatures A billion years before accumulation of oxygen Cyanobacteria are complex; advanced If they were not cyanobacteria, what were they?

Ancient stromatolites were formed a billion years before the accumulation of oxygen

How do stromatolites grow? (1) Bacterial slime catches fine silt at high tide (2) Photosynthesis uses up CO 2 and makes it easy for carbonate to crystallize on the mats... (Cowen, History of Life, 3rd ed.) 2 HCO 3 - Ca 2+ H 2 CO 3 CO 3 2- CaCO 3 (ppt) H 2 O CO 2 photosynthesis

Hypothesis: stromatolites 3.5 billion years ago were produced by organisms that: tolerated periodic episodes of high temperature existed in an anaerobic environment produced slime took up CO 2 Clue: stromatolites are more complex than they seem. There may be eight different zones in a stromatolite, each with its own microenvironment and each only a few millimeters thick. The zones vary from super-rich in oxygen...to zones without oxygen or light, saturated with H 2 S (hydrogen sulfide). (Cowen, writing about present-day stromatolites in History of Life, 3rd ed.)

Archaea tolerate (sometimes require) high temperatures Hot springs at Mount Lassen Some Archaea reduce CO 2, using H 2 as reductant

Other Archaea tolerate high salt and can have a weird type of photosynthesis Halobacterium in evaporation ponds at Blenheim, New Zealand (This is a cyclic photophosphorylation, like that of R. viridis, but using a different photoreceptor.)

Archaea under the microscope can look similar to the structures seen in fossil stromatolites. Halobacterium

Green and purple bacteria fix CO 2 in anaerobic conditions. Purple sulfur bacteria use light plus electrons from H 2 S to reduce CO 2 in the absence of O 2. light --> electric charge gradient --> H + gradient --> ATP --> electron transport --> oxidation/reduction Green sulfur bacteria use light plus electrons from H 2 S or H 2 to reduce CO 2 in the absence of O 2. Chromatium in Lake Ciso, in northeastern Spain Green sulfur bacteria (black region) below Chromatium in seashore sand.

Thus, it is possible that stromatolites were formed --CO 2 was taken up and CaCO 3 was precipitated-- by organisms that were simpler than cyanobacteria and non-o 2 -forming. Green, purple sulfur bacteria H 2 S + CO 2 --> Carbohydrate, H 2 O, S If the energy came in the form of reduced compounds, non-photosynthetic organisms could participate. Methanogens (archaea) H 2 + CO 2 --> CH 4, H 2 O, Carbohydrates Thiobacillus (bacteria) H 2 S + NO 3 - +CO 2 --> Carbohydrate, N 2, SO 4 2-

Conclusions Were there actually living cells present 3.6 billion years ago? Did they form stromatolites? Were the cells cyanobacteria? If they were not cyanobacteria, what were they?