A model for heat and light stress in photosynthesis

Photosynthesis is influenced by both light and temperature. While light is essential, high light levels and high temperatures can damage chloroplasts. These two stressors often occur together and interact. To explore these interactions, we set up a model that describes heat and light stress in photosynthesis. The model mechanistically tracks damage from heat and excess excitation with minimal assumptions. Chlorophyll dynamics are explicitly represented, allowing direct comparisons with fluorescence metrics. Processes that span time scales from fractions of seconds (excitation) to hours (damage and repair) are connected through an explicit numerical method. We compared the model with data for aquatic microalgae, but the formulation is broadly applicable to other photosynthetic organisms. We excluded drought effects that are typical of terrestrial systems. Key assumptions include: 1. light excites chlorophyll independently of temperature, 2. excitation energy is partitioned among photochemistry, non-photochemical quenching, and uncontrolled loss, which includes fluorescence, 3. heat stress directly damages the photosynthetic machinery (“heat damage”), 4. light stress indirectly damages the photosynthetic machinery by inducing harmful reactive oxygen species (ROS) through overexcitation or excitation of damaged machinery. These ROS cause further damage (“ROS damage”), and 5. repair of ROS damage requires photosynthetic energy. The model captured core phenomena: distinct mechanisms of light versus heat stress, their mutual aggravation, bistability under severe ROS damage, and recovery under favorable conditions.