For billions of years, the color of the oceans has changed according to the chemical and biological transformations of the earth. Formerly dominated by greenish colors due to dissolved iron, they took their current blue shine with the progressive oxygenation of the waters. But this fragile balance is changing. Under the effect of global warming, the distribution of phytoplankton is evolving, already resulting in visible changes from space. If this trend continues, the green oceans could become a reality by the end of the century, revealing a major ecological upheaval.
Formerly green oceans
Three billion years ago, the Earth's oceans were very different from those we know today. They were rich in dissolved iron, especially in the form of iron hydroxide, a chemical compound which absorbed blue light and let the green wavelengths pass. This peculiarity gave the oceans a much greater shade than currently, a phenomenon confirmed by the research carried out by scientists from the University of Nagoya, relayed by Popular Mechanics.
Cyanobacteria, first life forms capable of photosynthesis, have prospered in these waters rich in iron. Unlike terrestrial plants that only use chlorophyll to capture light, these microorganisms had other pigments, phycobilines, allowing them to absorb specific wavelengths, especially red and green.
This adaptation phenomenon was highlighted by the team of researcher Taro Matsuo. His work has shown how these bacteria can develop in conditions close to those of the Archaean.
Over the millennia, cyanobacteria has profoundly transformed the oceans. By releasing oxygen, they favored the oxidation of dissolved iron. This process caused the precipitation of the metal at the bottom of the seas, forming ferric deposits. Even today, these training courses are visible in certain old rocks. They bear witness to the gradual transition from green oceans to a marine chemistry closer to the current one. 600 million years ago, dissolved iron was too rare to maintain this shade. The oceans then took the blue color that we know.
Back to the green oceans?
Global warming is currently upsetting the balance of the oceans and could lead to a similar transformation, although by different mechanisms. MIT researchers, in a study published in Nature Communications, have shown that the increase in temperatures modifies the distribution and density of phytoplankton, these microscopic organisms which, such as cyanobacteria, use photosynthesis to develop.
The model designed by the MIT provides that more than 50% of the oceans will change the shade by 2100 due to warming. In certain subtropical areas, the decrease in phytoplankton will strengthen the dominance of blue, a sign of a reduced marine life. Conversely, near the equator and poles, certain species will proliferate, intensifying the green shade of water. This phenomenon is already underway. A study conducted in 2023 showed that 56% of the oceans have changed color in twenty years. The researchers consider this signal as a key marker of ecological disturbances.
The satellites already make it possible to observe these changes by capturing the subtle variations in light reflected by the ocean. This tool is now essential to follow ecological developments on a large scale.
A transformation already under way
The current evolution of the color of the oceans is not limited to a simple question of aesthetics. Behind these chromatic changes hides a deep transformation of marine ecosystems. Phytoplankton is the basis of the oceanic food chain, nourishing a wide variety of organisms ranging from zooplankton to large marine mammals. A change in its distribution or its diversity therefore has major repercussions on the entire trophic network.
The models developed by MIT researchers show that certain phytoplankton species adapted to warmer waters could gradually replace those that are currently thriving. This substitution would modify the chemical composition of water and the availability of nutrients, impacting the growth of fish and other marine organisms which depend on these microalgues.
Beyond the biological effects, this phenomenon could also influence the climate itself. Phytoplankton plays a key role in the absorption of atmospheric carbon dioxide, participating in the regulation of the terrestrial climate. An upheaval of these communities could therefore have consequences on the carbon cycle, potentially amplifying global warming.
Researchers are concerned about the speed with which these changes occur. Unlike past transformations, which have spread over millions of years, the human impact considerably accelerates the process. In just two decades, the oceans have already started to transform, and these modifications could become even more visible in the decades to come.
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