doThey are marine organisms as tiny as they are essential; They are, on a functional level, comparable to plants. During the long polar day, these microalgae thrive in the Arctic Ocean, even under ice floes, while in the middle of the polar night they remain “on standby”, with a slower metabolism.
But what happens when “the immense and black night with blonde tears” beloved by Louis Aragon finally breaks, after months of darkness? Only at the end of this long darkness do these single-celled organisms restart their biomass production factory: photosynthesis.
This fascinating alchemy operates in the cellular crucible of almost all current plants. It transmutes two basic ingredients, carbon dioxide and water, into sugars and oxygen. The biomass produced in this way is the basis of the entire food chain and, therefore, of all forms of life. Surprisingly, it was a bacterial cell that first inherited, about 2.5 billion years ago, the biochemical tools for this process, undoubtedly developed much earlier. This box was then stolen by primitive plant cells, between 900 and 1.2 billion years ago.
Return to the contemporary world, in the latitudes of the Far North. What minimum lighting do these single-celled algae need, the researchers asked, to restart photosynthesis? Very little light is enough, they responded on September 4 in the magazine Nature Communications.
Heavy research artillery
Let’s compare this small dose of photons with the light used by terrestrial plants during a typical day of sun exposure. So it is equivalent to “a drop of water, instead of three liters a day”estimates Niels Fuchs, from the University of Hamburg, co-author of the study.
This ability of Arctic phytoplankton to exploit a sliver of light and restart their growth is all the more fascinating as photosynthesis takes place under sea ice. However, this layer of sea ice, in turn covered with snow, is so thick (between approximately one and three meters) that it only allows a very parsimonious light to pass through. “ “The ice is pretty clear, but the snow blocks almost all the sunlight.”says researcher Thomas Lacour, a microalgae specialist at the French Research Institute for the Exploitation of the Sea of Nantes.
To reveal the ability of these microscopic creatures to take advantage of such a weak dose of light, it was necessary to mobilize the heavy artillery of research. One of the most ambitious polar exploration campaigns, the Mosaic expedition, dedicated to the study of the Arctic climate and ecosystem and which brings together more than eighty institutions from twenty countries.
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