Contradictory role of light in photosynthesis unravelled

The relationship between photosynthesis and light has been compared with Janus, the Roman god who is depicted with two faces that look in opposite directions. This is because light is necessary for plants and algae to take up CO₂ but, at the same time, is detrimental to them. It causes damage in cells. The research team has revealed how this damage arises at the molecular level. Their findings have been published in Science Advances.

Essential biological process

Photosynthesis is one of the most important biological processes on earth. During photosynthesis, aquatic algae and terrestrial plants use solar energy to capture carbon from the air and convert it into sugars, which they subsequently use to grow. The conversion of light into chemically usable energy is essential for practically all living organisms on our planet. Plants serve as food for herbivores which, in turn, serve as food for predators. Furthermore, all organisms need oxygen, the by-product of photosynthesis, to survive. Finally, photosynthetic organisms are responsible for creating the fossil fuels humans use as energy sources.

Photoinhibition
This research focuses on the process of photoinhibition, which occurs under the influence of light. If the light intensity increases, photosynthesis becomes less efficient. Photoinhibition is a significant factor in the decline in productivity of field crops and climate change is expected to severely exacerbate this problem.

Unique enzyme
Wojciech Nawrocki, the first author of the research explains: “Photoinhibition is best detected under high light intensities although, in fact, it occurs continuously under all light conditions. We know that it results in physical damage to Photosystem II (PSII), the unique biological enzyme that is able to split water molecules into electrons, protons and oxygen. It so happens that light sometimes inactivates this enzyme. The enzyme must then be repaired in the cells, a process that requires a lot of energy.

One-celled model alga
In the lab, the researchers used a one-celled model alga to localise the site of damage inside the enzyme PSII. Jointly with colleagues in France and the United States, they used a great many spectroscopic, biochemical, computational and genetic methods to unravel the molecular details of photoinhibition. Supervising professor Roberta Croce: “Based on these experiments, we were able to conclude that the damage is accompanied by the formation of a pigment where some of the absorbed light energy is immediately discharged. We then demonstrated that this pigment is present in the centre of PSII. The researchers suggest that this pigment is a specific chlorophyll, named ChlD1, which is oxidised during photoinhibition.”

These conclusions contribute to a better understanding of the process of photoinhibition, a phenomenon that limits the quantity of biomass in crops. The knowledge can help in the development of strategies to reduce this damage and methods to make plants easier to measure by farmers.