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Marine and freshwater cyanobacteria protect their photosynthesis system from overexposure

Marine and freshwater cyanobacteria protect themselves from over-excitation in strong light by activating the Orange Carotenoid Protein (OCP). The picture depicts three bacteria, one in full sunlight (top, reddish), one in shade (bottom, orange hue) and one in the middle. (Image courtesy of Itay Goldshmid, Macromolecular Research Laboratory)

While we are all familiar with photosynthesis from the plant world, not everyone knows that it is also essential for all living things in the animal kingdom. Even those organisms that do not photosynthesize, do consume its primary product: glucose.

Due to the importance of this process, organisms have developed mechanisms that protect them from overexposure to sunlight. Just as photographic film can be overexposed, natural photosynthetic systems are also liable to damage from overexposure, leading to the organism's death.

Prof. Noam Adir and PhD student Dvir Harris of the Schulich Faculty of Chemistry have now revealed one such defense mechanism. Their study, conducted in collaboration with Dr Diana Kirilovsky's laboratory at I2BC-CEA, France, was published in the prestigious scientific journal PNAS in March 2016.

The defense mechanism was deciphered in cyanobacteria (once known as “blue-green algae”). The main player in this mechanism is Orange Carotenoid Protein (OCP). OCP modifies its structure and color in response to intense light, blocking the flow of energy that reaches the center of the photosynthetic reaction.

“In this study we discovered how the OCP blocks the energy,” explains Adir, who is the current dean. “In effect, the protein acts as a biological switch. In response to strong light, part of the protein penetrates into the phycobilisome (PBS) - the protein complex that functions as a light-harvesting antenna in the cyanobacteria - and changes the PBS structure, thereby diverting the flow of energy to the reaction centers. According to experiments done by the Kirilovsky lab, this defense mechanism blocks more than 90 percent of the sun’s radiation. As soon as the radiation diminishes, the protein returns to its normal state and the flow of energy resumes.”