Maarg Manthan · Topic 5.6

Oxygen Cycle

Environment & Ecology › Nutrient Cycling (Biogeochemical Cycles) · Topic 5.6

The oxygen cycle is the movement of oxygen between the atmosphere, biosphere and lithosphere. Free oxygen makes up about 21 percent of the air. It is produced mainly by photosynthesis, when plants, algae and cyanobacteria split water and release oxygen, and by the breakdown of water vapour by ultraviolet light in the upper air. It is used up by respiration of all aerobic organisms, by combustion of fuels, by decomposition and by chemical weathering and oxidation of rocks and minerals. Most of Earth's oxygen is locked in rocks and minerals, and only a very small share is free in the atmosphere. Phytoplankton in the oceans produce roughly half of the free oxygen. The cycle is closely tied to the carbon cycle because photosynthesis and respiration are opposite processes.

5 min read · UPSC Prelims · Environment & Ecology · Nutrient Cycling (Biogeochemical Cycles)

At a Glance

Air share About 21 percent
Main source Photosynthesis
Main sinks Respiration, combustion
Largest store Rocks and minerals
Ocean source Phytoplankton
Exam link UPSC Prelims, Environment & Ecology: Nutrient Cycling (Biogeochemical Cycles)

Where Does It Fit?

This topic follows the nitrogen cycle. Oxygen links with every other cycle through respiration, combustion and oxidation, and it prepares for the sulphur and phosphorus cycles. Previous topic: Nitrogen Cycle and Human Impact on It. Next topic: Sulphur Cycle and Human Impact on It.

How Does Oxygen Move Through the Earth System?

Process	Role in the oxygen cycle
Photosynthesis	Releases oxygen from water. Main source of free oxygen. Land plants, algae and cyanobacteria all contribute
Respiration	Plants, animals and microbes use oxygen to break down food, producing carbon dioxide and water
Combustion	Burning of wood, fossil fuels and other matter consumes oxygen
Decomposition	Aerobic decay consumes oxygen
Weathering and oxidation	Oxygen reacts with minerals such as iron to form oxides, a slow sink over geological time
Photolysis of water	Ultraviolet light splits water vapour at high altitude, releasing small amounts of oxygen
Ozone formation	Oxygen molecules are split by UV in the stratosphere and recombine into ozone, which shields life from UV rays

Why Does the Oxygen Balance Matter and How Do Humans Affect It?

Steady level: production and consumption have been closely balanced for a long period, keeping oxygen near 21 percent
Link with carbon: photosynthesis consumes carbon dioxide and releases oxygen, while respiration and burning do the reverse
Great Oxidation Event: about 2.4 billion years ago, cyanobacteria raised oxygen levels and changed the planet
Aquatic oxygen: dissolved oxygen is far lower than in air and falls in warm, polluted water
Biochemical oxygen demand: the oxygen needed by microbes to decompose organic waste, a measure of water pollution
Eutrophication: algal blooms die and decay, using up dissolved oxygen and causing fish kills
Ocean deoxygenation: warming reduces oxygen dissolved in the sea and expands oxygen minimum zones
Air pollution and deforestation: cutting forests lowers oxygen production, but the atmosphere holds so much that global depletion is not a risk

What Else Should You Remember?

Atmospheric reservoir: holds about 1.2 million gigatonnes of oxygen
Residence time: oxygen atoms remain in the air for thousands of years
Amazon myth: the Amazon is often called the lungs of the Earth, but it uses most of the oxygen it produces, so its net contribution is small
Phytoplankton share: marine algae and cyanobacteria make about half of the oxygen
Prochlorococcus: a tiny ocean cyanobacterium, probably the most abundant photosynthetic organism
Dissolved oxygen: measured in milligrams per litre; fish need generally above 4 to 5 milligrams per litre
Dead zones: coastal areas with very low oxygen, found near river mouths with heavy nutrient load
Hypoxia: low oxygen condition in water
Anoxia: no oxygen at all
Oxygen minimum zones: ocean layers where oxygen is low, as in the Arabian Sea
Ozone layer: a belt of ozone in the stratosphere, protecting life from UV-B and UV-C
Ozone depletion: caused by chlorofluorocarbons and similar chemicals; the Montreal Protocol controls them
Ground-level ozone: a pollutant, formed from nitrogen oxides and volatile compounds
Oxygen for life: all aerobic organisms rely on it, and humans need about 21 percent in the air
Altitude: oxygen proportion is constant but pressure falls with height, making less available
Oxygen bar and plants: indoor plants add only tiny amounts
Oxidation: rusting of iron is an oxygen sink
Banded iron formations: rocks that record early oxygen produced by cyanobacteria
Fossil fuel burning: uses oxygen but only a tiny fraction of the total
Link to previous: see Nitrogen Cycle and Human Impact on It
Link to carbon: see Carbon Cycle: Steps, Long-Term and Short-Term Cycles, Carbon as a Climate Buffer
Link to ozone: see International Environmental Governance and Conventions macro
Caution: oxygen is a gaseous cycle but largely locked in rocks and water
Biological oxygen demand levels: clean rivers have BOD below about 3 milligrams per litre; check the latest Central Pollution Control Board norms
Exam hint: remember that photosynthesis and respiration are complementary

Why Does It Matter for the Exam?

Sources: photosynthesis
Sinks: respiration, burning
Share: 21 percent
Current affairs link: ocean deoxygenation and dead zones
Common errors: saying forests produce most net oxygen or confusing ozone with oxygen

As an analytical point, global oxygen is not in danger, but local and aquatic oxygen is. Pollution, nutrient overload and warming cut dissolved oxygen, which makes oxygen stress a freshwater and ocean problem rather than an atmospheric one.

Why Is This Topic Important?

Source Photosynthesis
Sink Respiration, fire
Stress Water oxygen

What Came Next?

The next topic covers the sulphur cycle and human impact on it. Next topic: Sulphur Cycle and Human Impact on It.

Key People and Terms

Photolysis Splitting by light
Hypoxia Low oxygen
BOD Biochemical oxygen demand
Ozone O3 in stratosphere
Oxidation Reaction with oxygen
Dead zone Oxygen-poor water

Exam Corner

Points to Remember

Air has 21 percent oxygen.
Main source: photosynthesis.
Phytoplankton give about half.
High BOD means polluted water.
Ozone shields from UV.

Do Not Confuse With

Oxygen and ozone O2 for breathing versus O3 UV shield
Hypoxia and anoxia Low versus zero oxygen
Photosynthesis and respiration Release versus use of oxygen

Memory Hook

Plants and plankton make it, life and fire use it, rocks keep most.

Mains Angle

Questions ask for sources, sinks and links. These points help.

Cycle Sources, sinks
Link Carbon
Stress Aquatic

A Question You May Face

An original practice question, not a past paper question.

“Describe the oxygen cycle and explain how nutrient pollution and warming are reducing oxygen in aquatic ecosystems.”

How to Answer

Cycle Source and sink.
Link Carbon.
Stress Water.
Remedy Cut pollution.

GS Relevance

Prelims: sources and sinks of oxygen, link with the carbon cycle, ozone. GS Paper 3: ecology and pollution.

Frequently Asked Questions

What is the oxygen cycle?

The oxygen cycle describes how oxygen moves between the atmosphere, living things, water and rocks. Photosynthesis releases oxygen into the air and water, respiration, combustion, decay and oxidation of minerals use it, and a large amount of the oxygen on Earth is locked in rocks and compounds such as water and carbon dioxide.

What are the main sources of atmospheric oxygen?

The main source of atmospheric oxygen is photosynthesis by land plants, algae and cyanobacteria. Marine phytoplankton produce about half of the total. A very small amount is also formed when ultraviolet light splits water vapour in the upper atmosphere, but this is minor compared with photosynthesis.

How is the oxygen cycle linked to the carbon cycle?

Photosynthesis takes in carbon dioxide and releases oxygen, while respiration, decomposition and burning use oxygen and release carbon dioxide. The two cycles are therefore linked and balance each other. Burial of organic carbon in sediments over geological time allowed oxygen to build up in the atmosphere.

Is the Amazon the lungs of the Earth?

The idea is a popular simplification. The Amazon rainforest produces a lot of oxygen by photosynthesis but also uses almost the same amount in respiration and decay, so its net contribution to atmospheric oxygen is small. Its real global value lies in storing carbon, regulating climate and supporting biodiversity.

What is biochemical oxygen demand?

Biochemical oxygen demand, or BOD, is the amount of dissolved oxygen needed by microorganisms to break down organic matter in a water sample over five days at 20 degrees Celsius. High BOD means heavy organic pollution and low oxygen for fish, so it is used as a measure of water quality.

PYQ Practice — Statement Analysis

1 Photosynthesis is the main source of free oxygen in the atmosphere.

True

Along with plankton.

2 Oxygen is the most abundant gas in the atmosphere.

False

Nitrogen is, at about 78 percent.

3 High BOD indicates polluted water.