Oxygen-Rich Atmosophere

Hundreds of species of bacteria, unicellular protists and deep-sea worms evolved to exist without oxygen. Most multicellular metazoa however, with rare exception, require oxygen.

"Oxygen flooded into the atmosphere as a pollutant, even a poison, until natural selection shaped living things to thrive on the stuff and, indeed, suffocate without it." ― Richard Dawkins

Oxygen is produced as a byproduct of photosynthesis. Microbes in the oceans produce half of Earth's atmospheric oxygen. Macro algae, like kelp, and land plants provide the remainder.

"...the life of the planet began the long, slow process of modulating and regulating the physical conditions of the planet. The oxygen in today's atmosphere is almost entirely the result of photosynthetic living, which had its start with the appearance of blue-green algae among the microorganisms." ― Lewis Thomas

View of the crescent moon through the top of the Earth's atmosphere
by NASA Earth Observatory

About 20 percent of the atmosphere consists of free molecular oxygen O₂ not bound to another element like atmospheric gases carbon dioxide and sulfur dioxide. Much of Earth's total oxygen is sequestered in oxides in rocks buried deep in Earth's interior.

How did Earth's initially oxygen-free atmosphere change to its oxygen-rich state?

Geological, isotopic, and chemical evidence suggest a Great Oxidation Event occurred about 2.3 billion years ago, although the actual causes remain contested and inconclusive.

A multidisciplinary team of researchers recently published findings that suggest the rise of atmospheric oxygen that occurred during the Great Oxidation Event, then again roughly 2 billion years later during deep-ocean oxygenation that coincided with the rise of animal life, resulted from the growth of continents in the presence of carbon life forms.

In Two-step rise of atmospheric oxygen linked to the growth of continents, the authors propose that the initiation of plate tectonics lead to a change in the composition of Earth's crust that, in turn, decreased the oxidative efficiency of Earth's surface, thus allowing molecular oxygen from photosynthesis to accumulate in the atmosphere:

In the first stage, the change in composition of Earth's crust from iron- and magnesium-rich mafic rocks to feldspar- and quartz-rich felsic rocks could have caused a decrease in the oxidative efficiency of the Earth's surface, allowing atmospheric O2 to rise. Over the next billion years, as carbon steadily accumulated on the continents, metamorphic and magmatic reactions within this growing continental carbon reservoir facilitated a gradual increase in the total long-term input of CO2 to the ocean–atmosphere system. Given that O2 is produced during organic carbon burial, the increased CO2 input may have triggered a second rise in O2. A two-step rise in atmospheric O2 may therefore be a natural consequence of plate tectonics, continent formation and the growth of a crustal carbon reservoir.
―Two-step rise of atmospheric oxygen linked to the growth of continents

REFERENCES

• Atmospheric Oxygen, GrokEarth, 16 June 2012.
• Late Night Thoughts on Listening to Mahler's Ninth Symphony, Lewis Thomas, Penguin Books, 1995.
• New answer to why Earth's atmosphere became oxygenated, Science Daily, 16 May 2016.
• Two-step rise of atmospheric oxygen linked to the growth of continents, Cin-Ty A. Lee, Laurence Y. Yeung, N. Ryan McKenzie, Yusuke Yokoyama, Kazumi Ozaki & Adrian Lenardic, Nature Geoscience, 16 May 2016.
• The first metazoa living in permanently anoxic conditions, Roberto Danovaro, Antonio Dell'Anno, Antonio Pusceddu, Cristina Gambi, Iben Heiner and Reinhardt Møbjerg Kristensen, BMC Biology,6 April 2010.
• The Greatest Show on Earth: The Evidence for Evolution, Richard Dawkins, Bantam Press, 2009.

Photosynthetic Phytoplankton

Most atmospheric oxygen comes from plant-life converting the Sun's energy into chemical energy via photosynthesis.

I cannot cause light; the most I can do is try to put myself in the path of its beam.
― Annie Dillard

Photosynthesis in aquatic plants like phytoplankton contributes roughtly 45% of atmospheric oxygen.

Phyto-
a botanical prefix derived from the Greek phyton meaning plant, flora, or vegetation.

Phytoplankton are single-celled, aquatic plants adrift and aplenty in the oceans, seas, and freshwater ecosystems. Phytoplankton are microscopic organisms that sustain aquatic animals like whales, shrimp, snails, and jellyfish.

The diatom Attheya Longicornis at 400x magnification

The main classes of phytoplankton are diatoms and dinoflagellates. Diatoms often form ribbon-shaped colonies, but with no means of propulsion, are subject to the flow of currents.

Dinoflagellates are swimmers with whip-like flagella used to propel themselves.

Whip-like tail of a dinoflagellate

All phytoplankton convert the Sun's energy into sustenance via photosynthesis within the euphotic zone. The term euphotic comes from the Greek roots for well lit: εὖ "well" + φῶς "light".

Euphotic Zone
Sunlit water extending from the water's surface to a depth where the light intensity falls to 1% of that at the surface. 

Phytoplankton thrive where sunlight can penetrate and fuel their activity. About 90% of all marine life exists in the euphotic zone.

The oxygen in today's atmosphere is almost entirely the result of photosynthetic living, which had its start with the appearance of blue-green algae among the microorganisms.
— Lewis Thomas

REFERENCES

• Atmospheric Oxygen, GrokEarth, 18 January 2014.
• Phytoplankton, Wikipedia.
• What are Phytoplankton?, NOAA.

Changing Atmosphere

Earth amassed its atmosphere from the gases of cooling rocks.

“The earth was initially very hot and without an atmosphere. In the course of time it cooled and acquired an atmosphere from the emission of gases from the rocks.”
― Stephen Hawking, A Brief History of Time

Today life persists in a thin band of Earth's atmosphere called the troposphere which has a mean height of 11 miles (~ 17 km) in the middle latitudes straddling the equator.

Layers of the atmosphere

source: NASA

While it's stands to reason that the atmosphere would continue to change over time, in recent decades the rate of change in the composition and characteristics of the atmosphere has become cause for concern, particularly when compared to the adaptive capacity of living organisms.

A range of public figures from naturalist David Attenborough, to science educator Bill Nye and entrepreneur Elon Musk have expressed concern over the impact of human activity on the composition and characteristics of the atmosphere.

On human population growth and industrialization:

"I would be absolutely astounded if population growth and industrialization and all the stuff we are pumping into the atmosphere hadn't changed the climatic balance. Of course it has. There is no valid argument for denial."
― David Attenborough

On burning carbon-based fuels creating gases that trap the sun's radiant energy:

"Burning carbon-based substances like oil, gas, and especially coal, produces billions of tons of extra carbon dioxide each year. Methane gas from cows and pigs and other animals on our large farms ends up in the atmosphere as well, trapping more of the sun's energy as heat." ― Bill Nye

On the rapidly accumulating carbon dioxide in the atmosphere:

"We're running the most dangerous experiment in history right now, which is to see how much carbon dioxide the atmosphere... can handle before there is an environmental catastrophe." ― Elon Musk

Earth's standard surface gravity of 9.8 m/s² holds down the atmosphere, while atmospheric escape is the loss of atmospheric gases to outer space.

“The oldest, easiest to swallow idea was that the earth was man's personal property, a combination of garden, zoo, bank vault, and energy source, placed at our disposal to be consumed, ornamented, or pulled apart as we wished.”
― Lewis Thomas, The Lives of a Cell: Notes of a Biology Watcher

REFERENCES

• Atmosphere, Wikipedia.
• A Brief History of Time, Stephen Hawking, 1988.
• The Lives of a Cell: Notes of a Biology Watcher, Lewis Thomas, 1978.

Atmospheric Oxygen

Blue-green algae bloom

Photosynthesis on land (~ 55%) and photosynthesis in the ocean (~ 45%) account for nearly 100% of Earth's atmospheric oxygen.

A fraction comes from the breakup of water (photolysis) from ultraviolet radiation.

"The oxygen in today's atmosphere is almost entirely the result of photosynthetic living, which had its start with the appearance of blue-green algae among the microorganisms."
― Lewis Thomas

The annual gain of atmospheric oxygen is shown below:

Photosynthesis (land) Photosynthesis (ocean) Photolysis of N2O Photolysis of H2O	16,500 13,500 1.3 0.03
Total	~ 30,000
units 1010 kg O2 per year

Plants, algae, and cyanobacteria perform photosynthesis which maintains atmospheric oxygen and supplies all of the organic compounds and most of the energy necessary to sustain life in Earth's biosphere.

"One hundred and fifty years ago, the monster began, this country had become a place of industry. Factories grew on the landscape like weeds. Trees fell, fields were up-ended, rivers blackened. The sky choked on smoke and ash, and the people did, too, spending their days coughing and itching, their eyes turned forever toward the ground. Villages grew into town, towns into cities. And people began to live on the earth rather than within it."
― Patrick Ness, A Monster Calls

An LED screen in Tiananmen Square showing the rising sun
in a tourism advertisement for China’s Shandong province
while Beijing is shrouded in smog. 16 January 2014.

REFERENCES

• Oxygen Cycle, Wikipedia.
• China starts televising the sunrise on giant TV screens because Beijing is so clouded in smog, by James Nye, Daily Mail, 17 January 2014.
• Late Night Thoughts on Listening to Mahler's Ninth Symphony, Lewis Thomas, 1995.
• Photosynthesis, Wikipedia.