Tiny sea creatures rescue phytoplankton

Phytoplankton masses in the Barents Sea -

• At the base of the marine food chain, phytoplankton suffer from climate change, according to a study published by our partner The Conversation.

• Fortunately, tiny organisms restore the nitrogen levels necessary for its survival.

• The analysis of this phenomenon was carried out by Mar Benavides, scientific researcher at the Institute for Research for Development

The ocean absorbs about a third of the CO₂ that humans emit into the atmosphere, which mitigates climate change and makes life on Earth possible.

Much of this CO2 is consumed by phytoplankton, small sea creatures that use light to produce photosynthesis, like plants and trees on earth.

At the same time, they transform CO2 into organic material that produces about half of the oxygen we breathe.

Phytoplankton is the basis of the marine food chain, and its productivity not only affects CO2 levels, but also fisheries and the global economy.

So why is phytoplankton still unknown to most of us, if it is so important?

Try to find it the next time you visit an aquarium, you might have a hard time!

Most species of phytoplankton are 100 times smaller than the ants in your garden, which means you will need a very powerful magnifying glass (a microscope) to be able to study them.

From our coasts to the middle of the ocean, phytoplankton is very widespread but its knowledge requires a certain practice of the sea.

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Samaritans of the Ocean

However, phytoplankton needs a key ingredient to be active: nitrogen.

Just like fertilizers or legumes are needed to grow crops on land, nitrogen provides the nutritional value that phytoplankton need to thrive in the ocean.

But it can be difficult to get enough nitrogen from the marine environment.

Coasts receive nitrogen through rivers or upwelling nitrogen-rich deep waters, but most of the ocean is too far away to benefit from these sources.

To complicate matters, the surface of the tropical ocean is warm, making it very difficult to mix with the deep, nutrient-rich waters.

These “oceanic deserts” are large expanses of clear blue water and represent [approximately 60% of the world's ocean surface].

How is life possible there without nitrogen?

Fortunately, other tiny creatures called "diazotrophs" also exist in these areas.

What the color of the oceans reveals about climate change https://t.co/EvWTjSrASw pic.twitter.com/CRU3fRp1d0

- The Conversation France (@FR_Conversation) April 1, 2019

Diazotrophs come to the rescue by performing a Herculean service: transforming inert nitrogen in the air into juicy nitrogenous forms available for phytoplankton.

This transformation implies a great energy investment of the diazotrophs, to end up giving this nitrogen to the community.

The diazotrophs are thus the true Samaritans of the ocean.

However, their crucial mission is threatened by climate change.

Pollution, acidification, loss of oxygen and warming are some of the negative effects of our economic development and ever-increasing population growth.

Climate change is already affecting the amount of nitrogen reaching the ocean by modifying the flow of currents, increasing the nitrogen load from agriculture by rivers or atmospheric inputs from industrial activities.

But how will climate change affect the activity and diversity of diazotrophs?

Difficult to say since we do not know their number or the extent of their diversity.

Only five species have been studied in the ocean, and climate change simulation experiments have only been tested on two of them.

Global circumnavigation expeditions have revealed that diazotrophic bacteria are much more diverse than we thought.

Experiments to test the response of diazotrophic cells to climate change © Mar Benavides

An essential CO₂ sink

Our scientific project Notion will look at the future of phytoplankton through a diazotrophic lens.

In the laboratory, we will recreate the conditions of climate change and observe how diazotrophs react to them.

We will answer questions such as: Does the additional CO2 present in the water due to climate change affect their growth?

Do diazotrophs give even more "fertilizing" nitrogen to other organisms in a world where the CO2 content is higher?

Global models of ocean circulation and distribution of phytoplankton species already exist, but they need to be supplemented with experimental data in order to predict what our ocean will look like in the future.

The Notion project will integrate new global and experimental databases in order to integrate the missing information within the models.

We will thus transform biology into mathematics, using the response behavior of diazotrophs as trends that can be projected into different scenarios of future climate change.

With these tools, we hope to better understand how the ocean reacts to climate change.

Such knowledge will be essential for the sustainable use of oceanic expanses and their resources, as well as for evaluating its capacity to remain an indispensable sink of CO2.

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This analysis was written by Mar Benavides, research scientist at the Institute for Research for Development.

The original article was published on The Conversation website.

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