Introduction
Cyanobacteria
or the blue green algae belong to a group of bacteria which derive their energy
by the process of photosynthesis. The characteristic color of the organisms
gives them their name. They were the undisputed champions of early life or
microbial heroes of Earth history since they were the first to produce oxygen
by photosynthesis. Some 2.4 billion years ago, it is believed that the first
true global mass extinction was caused by the Cyanobacteria or blue green
algae.
The early atmosphere
During
that period, hydrogen compounds such as ammonia and methane gas were abundant
in the atmosphere as well as iron and nickel but oxygen was present only in
bound form as in H2O. Due to the absence of free oxygen, anaerobic
life form prospered. However, a newly evolved variety; the cyanobacteria had
started to survive too and were about to alter the course of history.
The added advantage: Photosynthesis
The unique
ability of the cyanobacteria of photosynthesis gave them an added advantage in
this primitive world. Consuming H2O instead of H2S or
other compounds as a source of electrons and hydrogen for fixing CO2,
they started to produce organic matter releasing oxygen as a byproduct. Now,
this oxygen was about to be as good as poison to the other anaerobic organisms
leading to the first mass extinction.
The spread of oxygen in ocean and atmosphere
The huge amount
of O2 released by the ever
thriving population of the newly evolved cyanobacteria during the process of
photosynthesis started to accumulate in the oceans all over the world. The huge
amount of iron in the ocean started to react with the oxygen forming the red
colored iron oxide which began to deposit at the ocean bed. With the passage of
time, the banded iron formations (BIFs)
came into existence which are noting but distinctive layers of
sedimentary rocks formed by the iron oxide deposits. As time went by, the
oceans reserve of iron dwindled resulting in the saturation of the world ocean
with oxygen. Next, the oxygen began to release from the ocean into the
atmosphere some 2.4 billion years ago.
Ozone layer and GOE
With the
increasing amount of O2 in
the atmosphere, eventually the ozone layer was formed by the continuous
conversion of the diatomic oxygen
into its tri-atomic form 03 i.e. ozone .The ozone layer in the
troposphere started to proliferate the process of photosynthesis by acting as a shield against the Sun's ultraviolet
radiation thereby protecting Earth's carbon-based life forms including the cyanobacteria.
The shielding effect also increased the longevity of the
oxygen molecules
in the troposphere. All of these resulted in even more oxygen concentration in
the atmosphere all over the world, a phenomenon known as The Great Oxygenation
Event (GOE), or the Oxygen Catastrophe or Oxygen Crisis. Vast majority
of the anaerobes were exterminated resulting in possibly the first and the
biggest ever mass extinction. It is really hard to imagine that the life
supporting oxygen had such a horrific past.
What took it so long?
The
cyanobacteria were present long before The Great Oxygenation Event (GOE),
yet it took millions of years for it to happen. This is because initially the
ancient oceans were shared by both cyanobacteria and the methanogens, with the
later being the dominant player. The methanogens consumed hydrogen and carbon
dioxide releasing methane thereby making the early atmosphere methane rich.
Whatever oxygen that was released by the cyanobacteria was being neutralized by
the methane forming carbon dioxide and water. However, some 2.7 million years
ago, the nickel concentration of the BIFs suddenly started to dwindle and by
the time of GOE, the drop was almost 50%. As nickel is required by the
metabolic enzymes of the methanogens for the production of methane, they began
to starve and their population began to drop.
Oxygen as poison
The oxygen acted
as a poison to the anaerobe by the process of oxidation or slow burning of the
important life sustaining molecules, such as proteins and chromosomes inside
the bacteria. This resulted in the execution of almost all of the bacteria in
the Great Oxygen Poisoning of about 2.4 billion years ago, with the exception
of few. Some of the anaerobes managed to survive by escaping to places out of
reach of oxygen such as the mud at the swamp bottoms and cracks of hydrothermal
vents under the sea and adapted to life accordingly. Even today, the
descendants of these early anaerobes can be found in these remote areas
depicting the hardship their ancestors had to witness.
However, since
the cyanobacteria themselves lived in the anaerobic condition earlier, oxygen
should have been toxic to them too. One theory assumes that during the
"Snowball Earth" period, the UV radiations, unchecked by ozone as
there was no ozone layer then, entered the earth's atmosphere to react with the
glacial ice resulting in the formation of hydrogen peroxide and hydrogen. The
hydrogen peroxide, thus formed, got frozen and remained trapped. Once the ice
melts, the hydrogen peroxide broke down into molecular oxygen and helped the evolution
of oxygen-mediating enzymes. Similarly, few of the anaerobes also survived.
The impact of GOE
The GOE was
responsible for the thriving of the aerobic live form of earth whose energy
harnessing capabilities were far superior to the anaerobic ones. Thus, it was a
big boon for the development of complex life form on earth as without it the
current high level of oxygen would not have been possible. With the increase in
oxygen level, the stratospheric ozone layer between 12 and 30 miles above the
surface also increased, shielding the ocean from the harmful UV rays. The
bottom dwelling photosynthetic organisms got a chance to come up and increase
their output of oxygen, further building up the stratospheric ozone.
On the contrary, GOE was also responsible for
the biggest mass extinction on the planet including the climate-warming
methanogens. Moreover, the high amount of oxygen in the atmosphere resulted in
the conversion of methane (CH4 ) into CO2.As we know, CH4
is a far more effective green house gas than CO2, therefore the
temperatures of the earth plunged triggering the Huronian glaciation, possibly
the longest snowball that Earth ever witnessed. The temperature drop was as low
as -50 degrees Celsius. Had the temperatures dropped by another 30 degrees or
had the earth been a little more distant orbit, the planet would have been
converted into an icy wasteland as CO2 freezes into dry ice at -78 degrees.
Eventually through evolution of the survivors of the GOE and volcanic
activities, the CH4 and CO2 restored to normal level
resulting in the icy temperatures to
return to the hospitable range.
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