Like most plants, many algae produce oxygen during the daylight as a by-product of photosynthesis. At night these algae consume oxygen, but usually much less than was produced during the daylight.

What percentage of oxygen does algae produce?

It is estimated to be more abundant than any other photosynthesizer on the planet, and to be responsible for producing 20 percent of the oxygen in the atmosphere.

Does green algae make oxygen?

[Global] Like all green plants, algae produce oxygen during the daylight hours as a by-product of photosynthesis. This is usually a major source of oxygen in fish ponds.