Ancient Photosynthesizing Archaebacteria

One of the quests of origin of life researchers is the search for relatively simple proteins that might have converted sunlight into energy in early primitive cells. One archaebacteria at Yellowstone National Park has bacteriorhodopsin proteins in its membrane that pump hydrogen ions into the cell. Adenosine triphosphate ATP proteins are also embedded in their cellular membranes, which are powered by the hydrogen ion gradient between the inside and outside of the cell. This is an energy generation system that is much simpler than photosynthetic systems in algae and plants.

An archaebacterium called Halobacterium halobium has a protein, bacteriorhodopsin (Figure 6‑16), embedded in the membrane wall that pumps hydrogen ions through the membrane and out of the cell when a photon from the Sun impacts the retinal portion of bacteriorhodopsin. The hydrogen ions then pass back into the cell through the protein ATP synthase, giving ATP synthase the energy that it needs to convert adenosine diphosphate (ADP) to adenosine triphosphate (ATP). The ATP molecules then provide the energy that other proteins need to function. Modern cells also use the process of energy harvesting through ion gradients (ions behind a membrane) and powering ATP synthase as the ions pass through it.

The protein, bacteriorhodopsin in Halobacteria halobium changes shape when it absorbs a photon (Figure 6‑17). This is called photoisomerization. This causes it to release a hydrogen ion outside of the cell.

If the first photosynthesizers on earth were halobacteria, then the early earth might have been purple. The reason that the earth currently looks green is that plants harvest the sun's energy with chlorophyll. This is because chlorphyll does not use green light but reflects it. There are a few theories as to why chlorophyll do not absorb green light. One is that there is so much green light that it would damage chlorophyll. The other is that halobacteria were the first photosynthesizing organisms and they already absorbed green light. Thus, cyanobacteria evolved the capability to absorb the red and blue light not used by the halobacteria. If this was the case, then the world would have been purple prior to cyanobacteria because halobacteria reflected blue light + red light = purple light.

Archaebacteria are genetically closer than bacteria to eukaryotes such as plants and animals, but they are morphologically more similar to bacteria in that they do not have organelles or a nucleus for DNA. As with bacteria, green algae, and plants, they have a cell wall, but animals do not have a cell wall. Archaebacteria have a membrane that can adapt to higher temperatures than nonarchaebacteria. Most archaebacteria were wiped out when photosynthesis oxidized the atmosphere, but they are still common in locations where green algae cannot survive such as the deep ocean, anaerobic environments such as the human gut, and in the high temperature pools at Yellowstone National Park.

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Pool with archaebacteria at Yellowstone National Park. Credit: Jim Peako. National Park Service.