Phytoplankton provide a lot of oxygen to the air. Some estimates say it's even more than half! However, phytoplankton are not the only source of oxygen.

Phytoplankton produce oxygen through photosynthesis. Photosynthesis is the process that plants use to produce their own food. During photosynthesis plants use the energy from the sun to convert carbon dioxide from the air into sugar molecules. When they do this they consume water and also carbon dioxide, and they produce oxygen.

Now that you know how phytoplankton produce oxygen, can you guess what other types of things might be producing oxygen? Anything that uses photosynthesis! This means that grass, trees, flowers, bushes, and all of the other plants are producing oxygen. That's where the rest of the oxygen comes from!

I'm impressed that you know what phytoplankton are! Most people do not. I study phytoplankton, so I'm glad to hear that you know what phytoplankton are and that you know one of the reasons why phytoplankton are so important to the earth.

The answer to your question is really easy, so I'm not going to come right out and tell you. You'll probably laugh when you figure it out. The main way oxygen is added to the earth's atmosphere is though photosynthesis. The main way it is taken out or removed is through respiration. During photosynthesis (photo = light, synthesis = to make), energy from sunlight is captured with special light-absorbing chemicals (called pigments) and this captured energy is used to make living tissue from the carbon molecules in carbon dioxide gas. Oxygen is released almost as a waste product. In respiration, the reverse occurs: the carbon molecules used to make new living tissue are taken from food. The energy for respiration comes from food and oxygen instead of sunlight, and carbon dioxide gas is released as a waste product. Which process do humans use to grow: photosynthesis or respiration?

Phytoplankton are one group of organisms that use photosynthesis to grow. Phytoplankton live in the water. There are other organisms that live on land that use photosynthesis to grow. Pigments, or light-absorbing chemicals, come in many colors but the main ones used in photosynthesis are called chlorophylls. Living tissue with a lot of chlorophyll looks green.

If you want to know what organisms are providing the other 50% of oxygen to the earth's atmosphere, walk outside and look for living organisms that are green!

Most of the rest of the Earth's oxygen is produced by plants growing on land, particularly in areas with high rainfall (be they tropical or temperate). A substantial portion of the Earth's oxygen is also produced by seaweed in offshore areas.