Where does oxygen in the air come from?

Article by Isabel Key and Mina Frost.

It’s common knowledge that trees release oxygen, but it’s not just trees [1]. In fact, more than half of oxygen in the atmosphere comes from marine “phytoplankton” [1]! ‘Plankton’ are microscopic plant-like organisms; some are algae, others are bacteria [3] that are too weak to swim against a current, and instead simply drift in the water [6]. Most of them are invisible to the naked human eye [1].

Phytoplankton use sunlight, nutrients, and CO2 to make sugars that they use to grow, and produce oxygen in the process [4]. This is exactly what plants on land do too – it’s called photosynthesis [4].

Phytoplankton existed long before plants on land did [1]. They were the first organisms to do photosynthesis, and the evolution of photosynthesis radically changed the composition of both the air and sea [1,3]. For millions of years ago, there was very little oxygen and a huge amount of CO2 in the atmosphere compared to now, a legacy of 4 billion year old volcanoes [1]. When phytoplankton evolved and started taking in lots of CO2 and releasing lots of oxygen (O2), this made the air ‘breathable’ and more similar to how it is today [1]. There was a sudden leap in atmospheric oxygen about 700 million years ago [10]. This change caused the “Cambrian Explosion”, a period in which many diverse animal species evolved in the seas [9], and may have allowed animals and plants to move onto land [1].

It’s interesting how such tiny living things, if there are enough of them, can completely alter the course of life on the planet. Without phytoplankton, the atmosphere would not be able to support life like we see today [1]. Moreover, since plants evolved from algae, we wouldn’t even have any plants [5]!

Phytoplankton are still hugely important in regulating climate today [3], and there is great concern that their decline will cause acceleration of global warming [7, 8].


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