ORIGIN OF METABOLISM Where did early life get its energy? How did cell structures become complex?

ORIGIN OF METABOLISM Where did early life get its energy? How did cell structures become complex?

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

Fossil stromatolites can be recognized by their layered appearance.

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.

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

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

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

Some Archaea reduce CO 2, using H 2 as reductant Hot springs at Mount Lassen

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 photosynthetic organisms that were simpler than cyanobacteria and non-o 2 -forming. Green, purple sulfur bacteria H 2 S + HCO 3-1 --> Carbohydrate, H 2 O, S, CO 3-2 Green sulfur bacteria, photosynthetic archaea H 2 + HCO 3-1 --> Carbohydrate, H 2 O, CO 3-2 If the energy came in the form of reduced compounds, non-photosynthetic organisms could participate. Methanogens (archaea) H 2 + HCO 3-1 --> CH 4, H 2 O, Carbohydrates, CO 3-2 Thiobacillus (bacteria) H 2 S + NO 3 - + HCO 3-1 --> Carbohydrate, N 2, SO 4 2-, CO3-2

Origin of Eukarya! Named the "greatest single evolutionary discontinuity"! Most important in terms of evolutionary innovation, leading to wide range of new adaptations! What was (were) the ancestor(s) of the first eukaryotes! Was it a single event, or many?! If there was a single key effect, what was it?! What is basic (might help understand origin)?! What is derived (after origin, even if facilitated by basic changes)?! Double-membrane-bounded organelles have been focus of attention: nucleus, mitochondrion, plastids

Plasma membrane hypothesis! Nucleus from infolding of plasma membrane! A similar mechanism has been proposed for the origin of mitochondria and plastids Mimivirus hypothesis--claverie Giant marine DNA virus formed the nucleus

Endosymbiotic hypothesis - - Margulis! Nucleus from symbiosis of archaean in bacterium (or vice versa)! Mitochondria from symbiosis of alpha-proteobacterium (includes E. coli, non-sulfur purple photosynthetic bacteria, Kreb cycle?) in nucleated host! Plastid from symbiosis of photosynthetic bacterium in nucleated host

! Endosymbiosis stabilized by loss of genetic material from symbiote (organelle) to nucleus, and the import of certain nuclear enzymes into symbiote needed for function

Evidence: Structural similarities between plastids (and mitochondria) and bacteria:! Circular DNA! Bacterial type ribosomes! Plasma membrane of (some) bacteria and the inner membrane of mitochondria have similar electron transport systems and ATP synthases! Other enzymes Present day endosymbioses show ease of symbiosis! Chlorella in Hydra and dinoflagellates in corals! Rhizobium in legume root nodule! Wolbachia in insects! Others more arcane:

Pelomyxa palustris! Single cell with nucleus but no Golgi, E.R., mitochondria, plastids, or spindle; instead, has 3 kinds of obligate endosymbiotic archaeans (2 methanogens)! Amoeboid, microaerophilic (pond mud), no mitosis (nuclear fission) Mixotricha paradoxa! Single cell in termite gut (symbiont: digests wood and excretes products)! No mitochondria; two kinds of spirochaetes and one rod bacterium on surface; internal bacteria symbiont (=> "beast with five genomes )

Summary: most eukaryotes DNA, prot synthes i s Energy metabolism Nucleus Archaean Bacterial Mitochondrion Bacterial Bacterial Plastid Bacterial Bacterial A+B1 => cell with nucleus, + B2 ( -proteobacterium) => cell with mitochondrion, + B3 (photosynthetic bacterium) => cell with plastid

Question: one endosymbiotic event or many? Prokaryote divergence Endosymbiosis Eukaryotic evolution Cyanos Red Green Brown Cyanos Red Green Brown Two models: how can we distinguish between them?! Are plastid genes of different algae more similar to each other than to cyanobacteria?

Prokaryote divergence Endosymbiosis Eukaryotic evolution Cyanos Red Green Brown Cyanos Red Green Brown In which model would you expect the genes of red, green, and brown algae to be most similar to each other? (a) The left model (b) The right model (c) No difference

Prokaryote divergence Endosymbiosis Eukaryotic evolution Cyanos Red Green Brown Cyanos Red Green Brown In which model would the genes of red, green, and brown algae be equally different from cyanobacteria? (a) The left model (b) The right model (c) No difference

One primary endosymbiosis Initial divergence Secondary endosymbioses Further divergence One model for endosymbiosis in algal evolution Tertiary endosymbioses P. J. Keeling et al., Science 306, 2191b (2004)

Summary Although there are claims relating ancient stromatolytes to modern, cyanobacterial stromatolytes, the first cells lived in an anoxic environment and most likely did not get energy from oxygenic photosynthesis. The first cells were prokaryotic. Endosymbiosis involving bacteria and archaea produced eukaryotic cells. Although endosymbiosis can occur in the present, algae and plants seem to have evolved through a single primordial endosymbiosis of a cyanobacteria, followed in some cases by secondary endosymbiotic events.