Energetic and carbon allocation strategies shape coral holobiont responses to combined thermal and nutrient stress

Climate change and coastal eutrophication increasingly threaten coral reefs, yet their combined impact on coral holobionts remains poorly understood. This study examined the physiological response and carbon budget of two holobionts ( Galaxea fascicularis in symbiosis with Cladocopium ; Heteroxenia fuscescens in symbiosis with Durusdinium ) exposed to nitrate–phosphate enrichment and thermal stress (30 °C). In G. fascicularis , individual stressors severely reduced photosynthate translocation (−90%) due to a significant increase in symbiont respiration, and this species suffered significant bleaching (86% symbiont loss) after the heat-stress phase. Conversely, H. fuscescens was resilient to individual stressors, showing no bleaching and increased carbon translocation under nutrient enrichment or thermal stress alone. Both species exhibited an “energy saving” response following heat stress exposure, significantly increasing lipid and carbohydrate stores. Combined stressors temporarily boosted photosynthetic rates and carbon translocation in G. fascicularis , before a collapse in these parameters after the stress. In contrast, under combined stress H. fuscescens suffered severe bleaching but maintained high rates of carbon translocation to the host and accumulated substantial energy reserves. These findings suggest that different strategies in carbon allocation dictate competitive success under environmental stress: while G. fascicularis prioritizes symbiont maintenance, H. fuscescens maintains or enhances translocation to preserve host metabolism. This study highlights the importance of assessing holobiont carbon budgets and energy reserves to predict coral resilience in a changing ocean.