When we think about the marine environment images related to large cetaceans, paradisiacal beaches or even coral reefs usually come to mind. But did you know that we also find organisms that play a very important role not only for marine life but for the entire planet?

We are referring to phytoplankton, which is often forgotten.

But… what exactly is phytoplankton?

Phytoplankton is a diverse group of aquatic plant organisms that are at the mercy of ocean currents. Specifically, they are found in the photic zone of the ocean, i.e. the surface layer 200 metres deep where solar radiation penetrates. However, its highest concentration is in the first 50 metres.

Approximately, the plant plankton is composed of more than 20,000 species. They range from cyanobacteria to unicellular algae being the most important groups diatoms, dinoflagellates and coccolithophores, although many other groups are represented, most of which are microscopic.

How important is phytoplankton on the planet?

Firstly, it is important to know they are the main primary producers in the marine ecosystem. They are the starting point for the circulation of energy and nutrients through the food chains.
Ever since we were children, we have had the idea forests and rainforests are the main producers of oxygen on our planet, in other words, terrestrial vegetation is the planet’s lungs. Much of society even refers to the Amazon rainforest as the great lung of the planet, when in fact it represents only a small portion of oxygen production. Approximately, it produces between 6 and 9 % of the oxygen that would be found on the planet each year. This is not to downplay the importance of the Amazonia, its role and that of many other forests and rainforests is important for the planet. However, just as these are widely valued, the importance that the ocean and the life in it take part in our lives must be recognised and acknowledged.

This is where phytoplankton, the true planet’s lungs, flourish. All the organisms composing it are photosynthetic autotrophs, using CO2, water and solar energy to produce organic compounds, releasing oxygen during the process. Species that make up phytoplankton produce between 50 and 85 % of all the oxygen produced annually on the planet through the reaction called photosynthesis. In particular, the most striking organisms belong to the genus Prochlorococcus, being the most abundant photosynthesisers on the planet. The amount of oxygen they produce is equivalent to that generated by all terrestrial vegetation. Actually, as some of the most knowledgeable phytoplankton scientists point out, all the oxygen generated in the marine environment is consumed by the other living creatures in the ocean. Therefore, only a very small portion of the oxygen produced (0.1 %) reaches the atmosphere.

Secondly, phytoplankton is not only important because it is the great lung of the planet, furthermore, it captures a large part of CO2 through photosynthesis too. Specifically, 40 % of the gas released since 1750 by burning fossil fuels has been absorbed by the marine environment thanks to phytoplankton, with the ocean acting as a major sink. In other words, it is mitigating climate change by reducing CO2 levels in the air. Currently, we find around 420 ppmv of CO2 in the air environment whereas without the activity of these organisms the levels would be around 500-600 ppmv.

Finally, how does anthropogenic climate change affect phytoplankton?

Talking about phytoplankton, knowing about its existence and the important role it plays on the planet is key for all of us to take care of it. Due to climate change, the temperature of the planet is increasing and consequently that of the ocean.

Oxygen production by phytoplankton varies, among other factors, with water temperature and can therefore be affected by global warming. Regarding the study conducted by Sekerci and Petrovskii in 2018, whether the trend does not change, the natural environment may undergo an abrupt change caused by a decrease in phytoplankton activity, resulting in a depletion of the oxygen produced. Furthermore, the respiration of marine species also plays an important role in the decrease of oxygen concentration in the ocean. The consumption of this gas increases as the ocean warms.

Because of this oxygen reduction, a catastrophic situation for marine life might occur. Mass extinction of animal species could be the result of this global ecological disaster that should not be underestimated.

Therefore, the identification of potential threats to phytoplankton is a vitally important issue. To quote scientist Galen A. McKinley “… we want people to understand that there will come a time when the ocean will limit the effectiveness of mitigation actions …”. Thus, we underline the importance of protecting the environment around us and even the unknown marine environment, without which the existence of a species like ours would be conditioned.