Does Algae Produce Oxygen?


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Just like aquatic plants, algae also produce oxygen via photosynthesis. When algae undergo photosynthesis, oxygen is released into the atmosphere as a by-product of the process. This process typically occurs during the day when light exposure is at its greatest. 

As Earth is only 29% terrestrial, and with a decline in our forests, it is no surprise that our large oxygen demand must come from elsewhere. The oceans cover 71% of the Earth’s surface, providing over half of the world’s oxygen. 

While aquatic plants produce some oxygen, it is, in fact, single-celled organisms called algae that produce the majority of our atmospheric oxygen. So, how can such a small creature produce so much oxygen? 

Algae & Oxygen Production

Algae are like microscopic plants without a plant’s root system, leaves, stems, leaves, and complex reproductive structures, however, they can still grow into seaweed and leafy kelps. To separate them from terrestrial plants, we can take a look at their photosynthetic pigments and unique cells. 

It is estimated that there are more than 1 million species of algae that are all capable of producing oxygen. The most impressive algal species is Prochlorococcus. Despite Prochlorococcus being the smallest photosynthetic organism on Earth, it produces the most oxygen in the ocean. 

How Does Algae Produce Oxygen?

When nutrients are introduced into aquatic environments, some are absorbed into the sediment, but most of the nutrients are quickly dissolved into the water. It is these nutrient-rich waters that allow algae to reproduce. 

During photosynthesis, algae use energy from sunlight to drive the process. This allows the algae to convert carbon dioxide (CO2) and water into oxygen (O2) and sugar. The algae consume the sugars for food and release some oxygen into the atmosphere. 

It is important to note that photosynthesis is slightly different in algal species, as they have different pigmentations to terrestrial plants. Chlorophyll pigments absorb blue and red light, carotenoids absorb blue and green, and phycobiliproteins absorb blue or red, therefore some algae can absorb more light than their terrestrial relatives, depending on the wavelength of light that penetrates the water. 

When Do Algae Produce Oxygen?

Just like their plant relatives, algae produce oxygen during the day, when light intensity is at its greatest, as a by-product of photosynthesis. During the night, the algae consume oxygen in the water, but the amount they consume is far less than they produce during the day. 

However, there are a few factors that can reduce the amount of oxygen the algae produces without reducing their oxygen demand during the night. In simpler terms, this means that the algae consume more oxygen than they produce. 

For example, on days with high cloud cover or little wind movement, photosynthesis and oxygen production from the algae are greatly reduced. Oxygen depletion caused by weather can have dramatic effects on fish health, like weakening their immune systems, and in some cases fish death. 

Factors That Influence Oxygen Production In Algal Communities

  • Light intensity
  • Water movement (calm days reduces water mixing)
  • CO2 concentration
  • Temperature
  • An increase in nutrients (nitrogen & phosphorus) from agricultural run-off 

How Much Oxygen Does Algae Produce?

Oxygen levels in the ocean are difficult to measure as most of the ocean is still unexplored, and oxygen levels vary dramatically per season because of algal stimuli. However, scientists estimate photosynthesizing algae produce around 70% of all atmospheric oxygen.

What Happens To Oxygen Levels When Algae Die?

As algae only produce oxygen in the top layer of water, when they die, they sink to the seafloor or bottom sediment where they stop producing oxygen. Once they reach the seafloor or bottom sediment, they get decomposed by bacteria. During this process, the bacteria consume any remaining oxygen, which decreases the amount of dissolved oxygen (DO) in the lower regions of the water body. 

Oxygen Depletion & Excessive Algae Growth

With increases in global pollution, many bodies of water are suffering from algal blooms due to the water becoming more nutrient-rich. While algal blooms are naturally occurring, extreme cases are not, and most of the time they create dead zones or hypoxic zones. 

Hypoxic areas decrease the amount of life as oxygen is depleted dramatically. Mobile aquatic animals can usually survive hypoxic waters as they can move to other areas, however juvenile animals, eggs, and plants are restricted, so die or decay which contributes to algal blooms, often resulting in dead zones. 

When oxygen levels become too low, it can also affect the life stages of aquatic organisms, causing a reduction in life, and in severe cases a trophic cascade (where the whole ecosystem is affected). Lethal DO levels for fish are between 1 and 3 mg/L, which is why it is important to test DO levels in different water bodies.

If you have an aquarium, and it is suffering from low DO levels, it is critical to increase oxygen in the fish tank immediately to prevent fish loss. 

How To Measure Oxygen In Aquatic Systems

Dissolved oxygen can be measured by titration, colorimetry, or a sensor and meter. Modern methods either use an electrochemical or optical sensor (oxygen sensor or dissolved oxygen kits are recommended), which are considered the most accurate way to measure oxygen in aquatic systems. 

Instructions on how to use a DO probe/sensor can be found here


The oceans cover 71% of the Earth’s surface. It is estimated that photosynthesizing algae are responsible for approximately 70% of all atmospheric oxygen.

Algae produce oxygen via photosynthesis, releasing oxygen into the atmosphere as a by-product of the process. This process typically occurs during the day when light exposure is at its greatest. 

If you have any questions regarding oxygen, or you would like to learn more about other water quality measurements, characteristics, or applications for DO, please do not hesitate to contact our world-class team at Atlas Scientific.

Dissolved Oxygen Probes & Sensors

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