Prokaryotes And The Evolution Of Metabolism

In this lecture we are concerned with the Archaean and prokaryote evolution.
Prokaryotes are composed of cells that contain:


Some might suppose, on seeing such structurally simple organisms, that the Archaean was a period of relatively little evolutionary change. This is not the case because the prokaryotes are biochemically very diverse and have played a central role in the evolution of most of the important biochemical or metabolic pathways used by life on the planet today.

Evolution of the Prokaryotes

Twenty years ago, the most primitive prokaryotes were thought to be cyanobacteria (also called the blue-green algae). Why?

More recently:
1. Fossils of non-photosynthetic organisms found that date to 3.8 bya.
2. Photosynthesizing organisms while certainly passive in appearance are actually doing something that is biochemically very complex. How could something as complex as photosynthesis evolve quickly when the intermediate steps of photosynthesis give no advantage that could be favored by natural selection ?

What could have happened?
1. The primitive ocean, because of outgassing and concentration of organic compounds was a rich organic soup. The first organisms to arise would be the ones that used this rich store of organic chemicals. They would have been heterotrophs.
anabolism catabolism

ATP is used to store energy and when it is broken down into ADP through the loss of phosphate ions. ATP will form by naturally occuring chemical processes, and it may have accumulated in the Earth's early oceans. The earliest forms of life would only need to take in ATP from the environment and break it down into ADP to obtain energy.

However, as life forms multiplied, this free ATP would have been used up. At this point in time, prokaryotes that were able to create ATP through simple chemical pathways appeared and were favored by natural selection (in other words they need to also be able to do anabolism). We have such prokaryotes still living today - they are called the Archebacteria

Archebacteria

These are the only type of prokaryotes that could have survived in the world 3.8 billion years ago. Archaea are similar to other prokaryotes in most aspects of cell structure and metabolism (they have DNA in a circular loop, a cell wall (although its chemical composition is unique), ribosomes, flagella may be present, lack membrane-bound organelles).

Individual archaeans range from 0.1 to over 15 µm in diameter, and some form aggregates or filaments up to 200 µm in length. They occur in various shapes, such as spherical, rod-shaped, spiral, lobed, or rectangular.


However, their genetic transcription and translation do not show the typical bacterial features, but are extremely similar to those of eukaryotes.

Several other characteristics also set the Archaea apart.

The most striking chemical differences between Archaea and other living things lie in their cell membrane.

Remember: The basic unit from which cell membranes are built is the phospholipid .

 


Like bacteria and eukaryotes, archaea possess glycerol based phospholipids. However, three features of the archaeal lipids are unusual:

Like Bacteria, Archaea have a cell wall. In bacteria, however, the cell wall is made up of a material called peptidoglycan, while it is made of a variety of other materials.

Habitats

Archaea are usually harmless to other organisms and none are known to cause disease. Many archaeans are extremophiles (that is they live in extreme environments). Some live at very high temperatures, often above 100°C. Others are found in very cold habitats or in highly saline, acidic, or alkaline water. However, other archaeans are mesophiles, and have been found in environments like marshland, sewage, and soil. Many methanogenic archaea are found in the digestive tracts of animals such as ruminants, termites, and humans.

Important to our story of evolution are one of there - the Methanogens

  • anaerobes - intolerant to O2.
  • Organisms able to generate energy from chemical reactions involving inorganic molecules that they take in from the environment.

    • Most archebacteria are extreme heterotrophs and would have depleted the organic soup and caused an evolutionary crisis. We can call it the living world's first energy crisis. The precursors of the organic compounds would still be naturally occurring. Natural selection would favor those organisms which could produce their own complex organic molecules from the naturally occurring precursors. This led to the evolution of the Eubacteria (means true bacteria : as opposed to archebacteria) - Cell structure a little more complex (cell walls contain murine, mesosomes-complex membrane folds that are the site of many biochemical pathways) but more importantly, these bacteria had more complex metabolic pathways that allowed fermentation to produce energy.

    Eubacteria

    1. Fermenting bacteria



    The bonanza of free glucose in the biotic soup could not last. If the earliest heterotrophs continued to consume glucose molecules at a rate greater that they could be produced by slow geochemical reactions, then eventually life would have died out.

    In this early famine, organisms with the ability to produce their own glucose would have been favored by natural selection. Life on earth today depends primarily on photosynthesis as the process of glucose production - a solution invented more than 2 billion years ago.

    The earliest photosynthetic organisms lived in an anaerobic atmosphere.

    2. Photosynthetic Bacteria
    Trapping light energy and converting it to chemical energy is the process of photosynthesis. Most organisms that do it use chlorophyll. Chlorophyll is a porphyrin molecule.


    Porphyrins are ring-shaped molecules that combine with metal ions and can be formed from chemicals present on the early earth. Porphyrins include hemoglobin (which contains iron), chlorophyll (which contains magnesium), and vitamin B12 (which contains cobalt). These molecules are able to act as catalysts and cause electrons to be exchanges. Chlorophyll can capture light energy and store it by raising the energy within the porphyrin ring. This energy then causes electrons to move through the photosynthesis reaction resulting in formation of glucose.

    Two types:

    Cyanobacteria accomplished this by evolving enzymes that lowered the energy threshold and a more complex photosystem: Splitting water in cyanobacterial photosynthesis generates oxygen as a waste product. Cyanobacteria created huge mats (called stromatolites) and produced a great deal of oxygen. These mats can be seen commonly in Archaean fossil rocks and, in a few places where they are protected from animal predators, they still form today (for example in saline lagoons in Australia:


    About 2300 million years ago (2.3 bya) oxygen gas began accumulating in the atmosphere. This created an enormous problem for existing life forms:

    This leads to a third type of eubacteria:

    3. Facultative anaerobic bacteria

    These anaerobic bacteria were the first to have evolved the ability to produce special molecules that bind oxygen and neutralize it. These molecules, primarily porphyrins, are still used today by microorganisms and their descendants to bind oxygen.


    Finally, the ability to use the reactive properties of oxygen to generate ATP would have appeared:

    4. Aerobic Bacteria
    In respiration, pyruvate is converted into Acetyl CoA. Acetyl CoA enters a series of reactions (known as the krebs cycle) where it is broken down to give 30 molecules of ATP, carbon dioxide, and water.




    Summary (in the form of a cladogram):