Saturday, December 8, 2012

57. The Phototrophic and the Aerobic Energy Regimes in Our Evolutionary History

After the thermophilic energy regime (described in Part 56), the next to emerge was the phototrophic energy regime. It was dominated by solar energy as the source for the energy gradient.

This energy regime came about because some of the hyperthermophiles reached the surface of the sea, where they encountered sunlight. Chemical and biological adaptation and evolution followed, enabling them to develop a new metabolism which did not depend on the energy provided by the hydrothermal vents, but instead used solar energy through photosynthesis. These solar-energy-dissipating organisms established the phototrophic regime.

Two major survival tools emerged: Fixing of carbon dioxide; and the stripping of hydrogen from water (which liberated oxygen). The newly evolved microorganisms doing this were cyanobacteria or blue-greens (Marais 2000). They produced carbohydrates from carbon dioxide and water, and gradually built up the molecular-oxygen (O2) content of the Earth's atmosphere as a by-product. The dependence on DNA, proteins, and ATP continued as before.

The colour of the blue-greens comes from the chlorophylls in them. These pigments act as ‘molecular solar panels’, harvesting solar energy and converting it into chemical energy.

As taught in elementary chemistry classes, loss of electrons is oxidation, and gain of electrons is reduction (LEOGER). The blue-greens strip electrons from water molecules, thus releasing hydrogen for use, along with carbon dioxide, in the production of carbohydrates. Photosynthesis amounts to sunlight-driven conversion of carbon dioxide and water into carbohydrates and oxygen. Since airborne carbon dioxide is the only source of carbon that the blue-greens use, we can say that they create organic matter from inorganic matter.

Phototrophy literally means use of light as an energy source. In the phototrophic regime, solar light was the dominant energy source for the energy-dissipating pathway for the sustenance and further evolution of life. The blue-green bacteria were the chief drivers of the biochemical cycle during this regime. The oxygen-producing (‘oxygenic’) photosynthesis mechanism evolved by them enabled an increase in the organic productivity by two to three orders of magnitude, compared to what was done by the hyperthermophiles in the thermophilic regime.

The Earth's atmosphere in the early thermoic era was mostly carbon dioxide, and practically no molecular oxygen. Geochemical processes buried much of the carbon dioxide as silicate-carbonates, and biochemical processes converted this gas to bioorganic matter. Similarly, the molecular oxygen liberated by the photosynthesis processes was not available initially as atmospheric gas. Instead, much of it (~97%) was captured by rocks, volcanic gases, and upwelling oceanic iron particles. This was a slow but irreversible process. Only after it was completed (~2.2 billion years ago) did the oxygen gas start permeating the atmosphere surrounding the earth (Catling, Zahnle and McKay 2001).

Within a few hundred thousand years the atmospheric oxygen levels rose from less than 1% to ~15% of present-day levels. The air became more breathable.

Thus, for the phototrophic energy regime:

Energy source: Solar light.

Energy sink: Chemical energy.

Energy-dissipating pathways: Photochemical reactions; photosynthetic life forms; other solar-energy-dissipating superstructures in the ecosphere.

Chief drivers: The cyanobacteria (blue-greens).

The release of molecular oxygen as a waste product of the photosynthetic process by the blue-greens fell into a positive-feedback loop: Abundant availability of solar light made the population of the blue-greens to grow, producing more and more oxygen. But oxygen itself was poison to these organisms. That was a crisis situation indeed.

Therefore, EVOLUTIONARY ADAPTATION LED TO THE DEVELOPMENT OF A NEW KIND OF CELL, NAMELY THE EUKARYOTIC CELL. Such a cell had organelles, which have the feature that they are enclosed in membranes. The evolution of the eukaryotic cell resolved the crisis (see below).

In due course, more complex multi-cellular life forms emerged, and dominated this energy regime, the aerobic regime, in which respiration provided the main fuel-burning mechanism. Before the emergence of the eukaryotic cell, all life on Earth had existed as bacteria and archaea only (for over a billion years).

The atmospheric oxygen was conducive to the aerobes, but poison for the anaerobic blue-greens. But the blue-greens did not simply fade away in such a situation, as they were instrumental not only in the production of molecular oxygen but also food for the respiring aerobes. Therefore the build-up of oxygen in the atmosphere was a threat to both types of organisms: a direct threat to the anaerobes, and an indirect threat to the aerobes. THE EVOLUTION OF A SYMBIOTIC ‘PACT’ BETWEEN OXYGENIC PHOTOSYNTHESIS AND AEROBIC RESPIRATION WAS AT THE HEART OF THE OXO-ENERGY REVOLUTION, RESULTING IN THE EMERGENCE OF THE AEROBIC ENERGY REGIME. How?

This strategic alliance between ‘light eaters’ and ‘oxygen breathers’ not only saved the light-harvesting technology of the blue-greens, it also increased by an order of magnitude the photosynthetic metabolism. The eukaryotic cell design embodied sunlight-harvesting photosynthesis, and protection against oxygen toxicity. Its highly efficient metabolic combustion via aerobic respiration triggered the appearance of multicellular life forms, which, in turn, led to the emergence of still more complex life forms and ecosystems. Humans appeared on the scene in due course, and this was a development with unprecedented consequences.

The eukaryotic organisms have continued to coexist with the prokaryotic organisms (namely the bacteria and the archaea) in several schemes. In fact, the prokaryotes ‘maintain the foundation of all functioning ecosystems on this planet’ (Knoll 2003). An example is the nitrogen that bacteria make available for biological processes.

For the aerobic regime:

Energy source: Photosynthetic carbohydrates together with free oxygen.

Energy sink: Carbon dioxide plus water.

Energy-dissipating pathway: Aerobic respiration.

Chief driver: The eukaryotic cell.

Aerobic respiration produced 18 times more ATP (the cell fuel) from carbohydrates than the anaerobic processes prevalent till then. The aerobic regime saw a tremendous growth in biomass, which ended up ultimately as fossilised minerals.