Many Prokaryotic Cells Evolved Autotrophic Pathways

Even though the primitive heterotrophs probably evolved more and more elaborate biochemical pathways that enabled them to use a greater variety of the organic compounds free in their environment, and even though some of them probably evolved diverse methods of feeding on living and dead organic material, life would eventually have ceased if all nutrition had remained heterotrophic. The reason is not only that nutrients must have been used up much faster than they were being synthesized but also that the organisms themselves must have been altering the environment in ways that decreased the rate of abiotic synthesis of organic compounds. For example, their metabolism, which would have involved fermentation in the absence of molecular oxygen, would have released carbon dioxide into the atmosphere. Abiotic synthesis of complex organic compounds from carbon dioxide is much less likely than from methane or hydrogen cyanide as found in the early atmosphere.

 

That life did not become extinct as the supply of free organic compounds dwindled is attributable to the evolution of organisms capable of synthesizing their own food from inorganic nutrients. The first such autotrophic pathways were almost certainly chemoautotrophic, utilizing the energy in inorganic molecules such as molecular hydrogen (H₂), ammonia, nitrite, or sulfur. The energy released in chemical reactions involving these compounds was used to synthesize many of the organic compounds no longer available from the "soup," as well as many new compounds. Chemoautotrophic organisms are still found today, particularly in bogs and at volcanic vents on the ocean floor. Below are listed some of the Archaeae: note that some of them are chemoautotrophs.

 

Archaea	Pigments	Anaerobic/Aerobic?	Electron Donor 02 Produced?
methanogens = anaerobic  chemoautotrophs		anaerobic
halophiles = photosynthesizers	no chlorophyll; use a carotenoid (purple colored)	anaerobic	no electron donor per se: carotenoids used to establish proton gradient
thermoacidophiles = aerobic chemoautotrophs		aerobic (always?)

 

Sometime around 3.6 billion years ago, an enormously important biochemical event occurred: certain organisms acquired the ability to capture the sun's energy directly and use it to synthesize ATP. The first photosynthetic pathway was probably similar to that of cyclic photophosphorylation, during which light energy is used indirectly to synthesize ATP. The first photosynthetic autotrophs did not use chlorophyll as their light‑absorbing pigments, nor did they split water and produce oxygen. Instead they evolved a number of different light‑absorbing pigments and carried on a process similar to cyclic photophosphorylation. The present‑day anaerobic photosynthetic bacteria may be the direct descendants of those organisms.

 

Later, the much more complex pathways of linear photophosphorylation and carbon dioxide fixation evolved, probably appearing first in ancestors of cyanobacteria as early as 3 to 3.5 billion years ago. These organisms, like plants but unlike most photosynthetic bacteria, use water as the electron source in linear photophosphorylation and therefore release molecular oxygen as a by-product. From this time onward, the continuation of life on earth depended on the activity of the photosynthetic autotrophs. In the chart below are listed various types of photosynthetic bacteria, their pigments, and whether or not they carry on water-based photosynthesis (i.e., linear photophosphorylation). Note which of the bacteria] groups could have produced oxygen and contributed to the accumulation of oxygen in the atmosphere.

 

Photosynthetic Bacteria	Pigments	Anaerobic or aerobic	Electron donor (Oxygen produced?)
purple & green bacteria = photosynthesizers (PS I only in some)	no chlorophyll a or b; but other bacteriochloropylls;  Photosystem I only	anaerobic	H2, H2S, or organic compounds (no)
cyanobacteria  = photosynthesizers	chlorophyll a but not b + phycocyanins (blue) & phycoerythrins (red)	aerobic	water (yes); Photosystems I and II
prochlorophyta  = photosynthesizers	chlorophyll a and b + carotenoids	aerobic	water (yes); Photosystems I and II

 

The evolution of linear photophosphorylation using water as the electron donor probably administered the coup de grace to significant abiotic synthesis of complex organic compounds. An important by‑product of such photosynthesis is molecular oxygen (0₂). At first the molecular oxygen released by photosynthesis did not accumulate in the water and atmosphere; instead it combined with iron dissolved in the oceans to form iron oxides, which precipitated and settled on the ocean floor. Oxygen began to accumulate at low levels in certain habitats by about 2.4 billion years ago and is thought to have reached its present level of 21 percent in the atmosphere about 540 million years ago. Once molecular oxygen became a major component of the atmosphere, both heterotrophic and autotrophic organisms could evolve the biochemical pathways of aerobic respiration, by which far more energy can be extracted from nutrient molecules than is obtainable by glycolysis and fermentation alone.

 

As molecular oxygen accumulated, the so-called oxygen revolution took place and the atmosphere became an oxidizing atmosphere. Some of the molecular oxygen was converted into ozone (0₃), forming a layer high in the atmosphere. Once the ozone layer became thick enough (at least 500 million years ago), it effectively screened out most of the lethal high‑energy ultraviolet radiation from the sun. Now land was safe for living organisms and they could leave the ultraviolet‑absorbing oceans and move to land. Note that living organisms, once they arose, changed their environment in a way that destroyed the conditions that had made possible the abiotic origin of life.