Role of Core Microbiome Shifts in Octocoral Litophyton Under Diurnal Temperature Fluctuations

Climate change is projected to raise sea surface temperatures and intensify diurnal temperature fluctuations (DTF), threatening the survival of both scleractinian corals and octocorals. Litophyton, a common octocoral in Taiwan's shallow reefs, is frequently exposed to large DTF and summer heat stress, making it a suitable model to study thermal resilience. Coral-associated bacterial communities are known to shift under thermal stress, and key bacterial taxa may play crucial roles in host acclimation. This study aimed to address two questions: (1) Can higher DTF mitigate cumulative heat stress in octocorals? (2) If so, what physiological and microbial community changes accompany this effect? To answer these questions, we conducted tank experiments under constant warming and two short-term DTF regimes (± 5 °C and ± 7 °C; baseline 25-27.8 °C), along with a no-fluctuation control. We measured physiological stress indicators, including superoxide dismutase (SOD) and catalase (CAT) activities, and monitored bacterial community dynamics. Our results show that DTF helped maintain stable photosynthetic efficiency (Fv/Fm) compared to constant warming. Notably, significant differences in ROS activity were only observed in the ± 5 °C group, rather than in the larger ± 7 °C group, indicating a measurable alleviation of thermal stress and greater plasticity in Litophyton coping with temperature changes. Moreover, 29.4% more significantly abundant in the ± 7 °C group compared to the control in the core microbiome Endozoicomonas preceded detectable physiological changes in the host, suggesting a potential role in early stress mitigation. These findings deepen our understanding of octocoral holobiont resilience under fluctuating thermal regimes and highlight Endozoicomonas diversity as a potential indicator of Litophyton health.