Scientists have discovered how Posidonia Oceanica has adapted to the dim light of the sea floor.
In the Mediterranean, vast meadows of seaweed grow underwater in places where sunlight is weak. However, these plants continue to photosynthesise efficiently, produce oxygen, store carbon, and support marine life.A new study has revealed how one of these seaweeds changed photosynthesis to make the most of the limited light available underwater.The study, published in the journal Nature Communications, focused on Posidonia oceanica, a seaweed found only in the Mediterranean Sea. It grows in shallow coastal waters to depths of about 50 metres, where sunlight is much weaker and red light almost disappears because it is quickly absorbed by the water.Unlike seaweeds, seaweeds are flowering plants. Their ancestors lived on land before returning to the sea between 70 and 100 million years ago.Today, Posidonia Oceanica forms large underwater meadows that are among the most productive ecosystems in the world. These meadows provide shelter for marine species, stabilize the seafloor, and sequester large amounts of carbon for thousands of years, making them important natural carbon sinks.
Light harvesting system
The researchers measured how efficiently seaweeds perform photosynthesis and compared them with two land plants, Arabidopsis thaliana and maize.Posidonia Oceanica performed particularly well under low light. While land plants only reached peak photosynthesis under brighter light, seaweeds did so at much lower levels.The plant also has a larger light-harvesting system around one of its major photosynthesis structures – photosystem I. It is a protein inside plant cells that captures sunlight and converts it into chemical energy.
In Posidonia Oceanica, this system contains additional light-harvesting proteins.Normally, enlarging the light harvesting system slows down the movement of energy within the plant. But seaweed seems to have solved this problem in an unusual way.
faster Energy movement
Using cryogenic electron microscopy and ultrafast spectroscopy, the researchers examined the plant’s photostructures in great detail.They found that the seaweeds lost some special chlorophyll molecules known as low-energy forms of chlorophyll. In land plants, these molecules help absorb far-red light, which is useful in forests where leaves filter sunlight.
However, underwater there is almost no far-red light that can be absorbed.
The photos show how Posidonia Oceanica reshapes its leaf cells as light decreases with depth.
Without these forms of chlorophyll, energy moves through the photosynthetic system more quickly. The researchers found that this compensates for the plant’s larger light-capturing antenna, allowing it to collect more light without slowing down the whole process.
Small genetic changes
The scientists also identified several small changes in the proteins surrounding chlorophyll molecules.
The changes resulted in a slight change in the position of chlorophyll that eliminated the low-energy forms found in land plants.To confirm the results, the scientists reversed these genetic changes in proteins grown in the laboratory. When they restored the land plant version, the lost low-energy forms of chlorophyll returned, showing that the genetic changes identified were directly responsible for underwater adaptation.
Designed for low light
The researchers say the plant’s photosynthesis system appears to have been redesigned specifically for underwater conditions.The light-harvesting antenna is larger, allowing it to capture more photons. At the same time, the absence of low-energy forms of chlorophyll speeds up the energy transfer process, preventing delays that could reduce efficiency.The team also found that seaweeds permanently retain some light-harvesting proteins associated with Photosystem I.
In land plants, these proteins typically move between two photosystems depending on changing light conditions.In Mediterranean seagrasses, however, they remain attached, indicating a long-term adaptation rather than a temporary response.Understanding how seaweeds optimize light capture while maintaining rapid energy transfer could provide new ideas for developing crops that perform better under shaded conditions.