Temperature as a determinant of symbiodiniaceae health: physiological, antioxidant, and metabolic responses under co-exposure to microplastics

Microplastics (MPs) and elevated temperatures are major environmental stressors threatening Symbiodiniaceae health. However, comparative studies under in vitro conditions remain scarce. Here, we investigated the responses of the thermally sensitive Breviolum minutum and the thermally tolerant Durusdinium trenchii to MPs, heat stress, and their combined exposure by evaluating photosynthetic efficiency, antioxidant responses, and metabolomic profiles. MPs alone had minimal effects on D. trenchii but significantly inhibited B. minutum growth after 72 h, potentially due to reduced zeatin biosynthesis. Both heat stress and combined exposure suppressed pigment content and growth, with temperature acting as the dominant stressor. Antioxidant responses indicated oxidative stress in B. minutum but not in D. trenchii. Metabolomic analyses revealed that D. trenchii responds to heat stress by enhancing amino acid biosynthesis, increasing fatty acid saturation via downregulation of the unsaturated fatty acid pathway, maintaining transmembrane transport through upregulation of ABC transporters, and suppressing the tricarboxylic acid (TCA) cycle to support oxidative balance under thermal stress. Our findings highlight the central role of temperature in Symbiodiniaceae responses and demonstrate the superior metabolic plasticity of D. trenchii in coping with elevated temperatures.