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.
No comments:
Post a Comment