Voyage through the plankton jungle

By Lisa Garnier

Half of all the oxygen we breathe comes from organisms living in the sea. The Tara Oceans Expedition will be setting out to study these gas-producing machines.
Oxygen, known as O2 to chemists, is a molecule we don’t think about every day and yet it is exactly what our blood draws from our lungs. It is the antithesis of carbon dioxide, that famous CO2 which is produced by all living cells.

The source of oxygen, this gentle molecule which sustains the vast majority of living things, is well known. Oxygen is produced on Earth by an incredible mechanism called ‘photosynthesis’, the art of turning carbon dioxide into sugar by exploiting solar energy and water. The waters which cover two-thirds of our planet are home to the most important living oxygen ‘factory’ of all: phytoplankton (second place goes to land-based plants). And that is why Tara Oceans is setting out to explore the seven seas: to identify the source of this gas-producing machine and what are the best conditions for its sustainability.
Because without it the future of life on Earth would be in peril. Until now satellites have been our only means of obtaining a comprehensive view of the organisms which are capable of photosynthesis. Indeed, satellites allow scientists to measure the intensity of the green colour of the chlorophyll (chlorophyll is the molecule which specializes in absorbing solar energy) so that, depending on the season, they can determine the most active photosynthetic regions on land and at sea. For example, in the northern hemisphere spring and autumn are the most active times on land, when trees are covered with leaves, and the oceans exhibit the same phenomenon: during fine weather phytoplankton multiply and produce oxygen.
But this method of observation is limited by the fact that satellites are not able to detect chlorophyll which is more than five metres below the surface of the sea. And yet phytoplankton can be found to a depth of 100 metres, depending on the region. So what could be happening in the remaining ninety-five metres of water in the upper layer of the world’s oceans? It’s something we really need to know. As with the surface of the planet, water also has a ‘climate’ comprising cold and warm currents, cyclones and anticyclones, and areas rich in phosphorus, in nitrogen, and in iron which condition the growth of sea plants, those phytoplankton which produce the world’s oxygen.
Moreover, we know how to distinguish satellite images of broadleaved forests from a meadow or a cultivated field because we have the requisite knowledge for defining the character of those environments. At sea, all the photosynthetic areas are rich in phytoplankton! They contain microscopic algae which produce 40% of all the ‘marine’ oxygen.

This phytoplankton contains diatoms and innumerable species of algae which each play a precise role in their respective ecosystems, in the same way that silver birches, ferns, moss and heather play an essential role in forest ecosystems. The other half of the oxygen produced at sea comes from tiny photosynthetic bacteria such as those of the prochlorococcus genus, and the number of species in this genus is enormous.
In other words, the data collected during the Tara Oceans Expedition will open our eyes to the underwater ecosystems which have hitherto been an enigma. Invisible ecosystems which process the two molecules that are essential to our existence: oxygen and carbon dioxide.