A double defense protects photosynthesis from extreme heat

Photosynthesis is fundamental to life on Earth, allowing plants and microalgae to convert sunlight and carbon dioxide into organic compounds. However, this essential process is highly vulnerable to heat. As global temperatures rise and heatwaves become more frequent, declining photosynthetic efficiency threatens agricultural productivity and the large-scale cultivation of microalgae for biofuels and other valuable products. Photosystem II (PSII), the molecular complex responsible for converting light energy and releasing oxygen during photosynthesis, is particularly sensitive to high temperatures. Heat not only damages PSII itself but also disrupts the cellular machinery needed to repair it. Understanding how these processes are connected and finding ways to overcome them have remained major challenges.

An international research team led by Professor Yoshitaka Nishiyama of the Graduate School of Science and Engineering at Saitama University, together with Dr. Pornpan Napaumpaiporn of the University of Turku, who was a Ph.D. student at Saitama University at the time of the research, and Associate Professor Anchalee Sirikhachornkit of Kasetsart University, investigated the molecular mechanisms underlying heat-induced damage to PSII. Using the model cyanobacterium Synechocystis sp. PCC 6803, the research team developed genetically engineered strains with enhanced antioxidant defenses and a more oxidation-resistant protein-synthesis system. By combining these two protective mechanisms, the team successfully improved the heat tolerance of PSII while maintaining photosynthetic activity and cell survival under severe heat stress. The findings were published online on November 11, 2026, in the international journal Plant and Cell Physiology.

The researchers first discovered that raising the temperature from the normal growth temperature of 32℃ to 46℃ increased ROS production approximately fourfold in wild-type cells. Further experiments revealed that ROS generated during photosynthetic electron transport directly accelerated heat-induced damage to PSII while also interfering with its repair. To overcome these effects, the team engineered cells to produce higher levels of two ROS-scavenging enzymes, superoxide dismutase (SodB) and catalase (VktA), together with a modified version of the translation elongation factor EF-Tu (C82S) that is resistant to oxidative damage.

This combined approach proved more effective than strengthening either protective system alone. Enhanced antioxidant defenses reduced ROS accumulation and protected PSII from structural damage, while the modified EF-Tu helped maintain the synthesis of D1, a protein essential for PSII repair. At 46℃, the doubly enhanced strain maintained D1 protein synthesis at approximately 1.6 times the level observed in wild-type cells and showed substantially less loss of photosynthetic activity. Remarkably, during prolonged cultivation at 40℃, the engineered cells continued to survive and proliferate, whereas wild-type cells became bleached and died.

These findings provide new insight into why photosynthesis fails under extreme heat and demonstrate that protecting PSII requires more than preventing damage alone. By simultaneously reducing oxidative damage and preserving the cellular capacity for repair, the study establishes a molecular strategy for improving photosynthetic thermotolerance. This coordinated approach could open new directions in the engineering of photosynthetic organisms capable of functioning under increasingly challenging environmental conditions.

Looking ahead, this strategy could contribute to more reliable and cost-effective production of biofuels and other valuable biological materials using microalgae, particularly in outdoor cultivation systems exposed to high summer temperatures. Improving heat tolerance may help maintain productivity while reducing the energy required for cooling. Because the molecular machinery involved in photosynthesis and protein synthesis is highly conserved across photosynthetic organisms, the findings may also provide a foundation for developing heat-tolerant crops, such as rice, wheat, and soybeans. Further research could help translate these advances into more resilient food and biomass production systems in a warming world.