How Plants Changed the Atmosphere

Build-up of oxygen in the atmosphere

2.1 - 1.5 Billion Years ago

Oxygen which has been present in only trace amounts for 2.5 billion years, begins to build-up in the atmosphere.

Some oxygen formed through photo-dissociation of water vapour:

2H20 => 2H2- + O2

However most of the oxygen was probably produced as a by-product of photosynthetic autotrophs using light energy to split water molecules and so build organic compounds.

Primitive unicellular forms resembling modern blue-green algae (cyanobacteria) released oxygen, which accumulated in the atmosphere and was deposited in iron oxide beds on the floor of the ocean.

Other primitive bacteria such as the purple bacteria, contain simplified photosystems that do not release oxygen. Recent research has suggested that that non-oxygen-producing bacteria species such as the purple and green bacteria are the most ancient photosynthetic bacteria.

Another group of non-oxygen-producing bacteria, known as heliobacteria, evolved later and appear to have been the precursors of the forms that produce oxygen as a byproduct.

The heliobacteria appear to be the most closely related to the common ancestor of the oxygen-producing photosynthetic cyanobacteria.

Relatives of cyanobacteria appear to have given rise to chloroplasts in algae and green plants - the chloroplasts are the small bodies in plant cells that carry out photosynthesis in modern algae and other plants. This occurred through a process of engulfment where these primitive cyanobacteria were captured, engulfed and enslaving by other cells to become the solar driven carbohydrate factories within the cells.

Chloroplasts contain their own RNA and are thought to have been derived from cells which were once independent.

A similar process is thought to have involved the engulfing and enslavement of other bacteria to form mitochondria - the energy powerhouses of cells using oxidative phosphorylation to use oxygen and carbohydrate to release energy, carbon dioxide and water.

These complex cells with organelles entrapped within them, and nuclei became the 'eukaryotes' - the next stage in were in evolution of unicellular organisms. In some ways these forms can be regarded as the first type of 'multi-cellular organism' - though they are not generally recognized as such. The organelles represent cells within cells.

It was the power of this organization, and more complex and adaptable structure which led to the explosion of types and species. The higher efficiency of aerobic respiration and the development of photosynthesis for generating new complex molecules to feed upon was also crucial for this expansion.

The oxygen that was produced was toxic to most forms present at the time and its build-up may have cause the first mass extinction on the planet opening up new habitats and opportunities for eukaryotes respiring oxygen.

Final Phases in Build-up of Oxygen in the Atmosphere

The final phase of development of the modern atmosphere was the removal of the last remnants carbon dioxide (from 1-5% down to 0.04% today) and the build-up of oxygen to modern day proportions (from about 10% ? to 21% today).

Recent research has suggested that an earlier spread of plants onto the vacant land surfaces may have cause this, perhaps supported by the development of a protective ozone layer.

This final phase of the changes in the atmosphere may have been responsible for the dramatic cooling of the earth and the intense glaciations that took place from 700 t0 580 millions of years ago that heralded the Cambrian explosion of life.

For this to occur plants would have had to develop 300 millions years earlier than previously thought.

An article published in the August issue of Science suggest that land plants may have developed at 700 million years ago, much sooner than the 480 million year date, and that land fungi may have developed 1.3 billion years ago.

This article suggests that land plants and fungi may have caused the Snowball earth - the cooling of the Earth's temperature that preceded the Cambrian Explosion of Life.

The suggestion is that the abundant plants would have removed the remaining remnants of carbon dioxide and increased the amount of oxygen to the level needs to support aerobic organisms.

The proposed process is a reversal of the global warming we see today, where the release of more carbon dioxide by humans is triggering a greenhouse effect and increasing earth's temperature.

Fossil evidence suggest that bacteria formed microbial mats on land as early as three billion years ago.

Fossilized remnants and other biochemical evidence from South Africa suggest that photosynthetic bacteria (primarily blue-green cyanobacteria may have colonized the wet surface of clay-rich soil during rainy seasons, but were blanketed by aerosol deposits laid down during subsequent dry seasons. Such mats may have formed in surface pools, water edges, and other wet spots on land.

Oxygen - a major Extinction and Stimulus for Increased Complexity of Life

The first prokaryotes would probably have used the raw materials that surrounded them perhaps feasting on the 'Primeval Soup'?

As their number grew, and as their source of food may have declined with the increase in organisms and the rise in oxygen levels in the atmosphere, the prokaryotes adapted to their new environment by developing photosynthesis and other means of using the sun's energy to build structure (negative entropy) upon which to feed.

Simple blue-green algae still form an efficient part of the biosphere today. These bacteria may therefore represent the first and most successful adaptation of life to its environment.

The transition to oxygen-rich atmosphere occurred about 2.0 billion years ago and eukaryotes, cells that contain a nucleus, appeared between two and one billion years ago.

There is a potential problem here as most eukaryotes are aerobic and yet there is evidence of them occurring about 2 billion years ago when oxygen level had just began to rise in the atmosphere.

The probable answer is that the earliest eukaryotes were probably anaerobic types such as the modern Gardia. These types live without oxygen and without mitochondria, which are presumably were originally independent prokaryotes that were acquired by eukaryotes by endosymbiosis (engulfing without digestion).

Eukaryotes keep their DNA structures within the nucleus and have between 10 to 1000 times more than is found in prokaryotes

Evolution has led to the increasing specialization of cells in their function and structure, and to the development of larger, more complex cells, such as organs and the large living creatures that incorporate them.

Although life has been in existence on Earth for at least 3.5 billion years, during most of that time it consisted mainly of microorganisms. For 500 million years there were only prokaryotes and for another 1000 million years there were only unicellular types (prokaryotes and eukaryotes). This represents about half the time life has been on earth.

Quite recently in geological time, about 600 million years ago, there was a sudden increase in the distribution, number and variety of organisms - due probably to the build-up of significant amounts of oxygen to a threshold level, and the availability of new habitable environments (wetlands and land surfaces).

This was the beginning of the Cambrian period and is noted for the appearance of organisms with hard parts - shells, carapaces and skeletons - that were well preserved in sedimentary rocks to form the fossils that we find today.

TALK ABOUT THE GREEN HOUSE EFFECT, AND CLIMATE CHANGE

PLANTS DID IT MILLIONS OF YEARS AGO - THEY CHANGED THE ATMOSPSHERE AND PROBABLY THE CLIMATE, AS WELL IN WAYS THAT HERALDED THE ADVENT OF HIGHER FORMS OF AEROBIC LIFE.

WE AND MOST OF OUR FELLOW AEROBES COULD NOT SURVIVE IN THE PRIMITIVE ANAEROBIC ATMOSPHERE THAT PRECEDED THE PLANTS (INCLUDING PHOTOSYNTHETIC BACTERIA).

PLANTS CREATED AND SUSTAINED THE OXYGEN RICH ATMOSPHERE ON PLANET WATER. (EVEN IF IT WAS MERELY A BY-PRODUCT OF RESPIRATION)

THEY DID IT BY SPLITTING THE WATER MOLECULES, WHICH ARE REFORMED WHEN AEROBES LIKE OURSELVES RESPIRE.

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OzH2O - Water the Crux of Life on Planet Water

Origin of Life on Planet Water - Index