Table_5_Symbiodinium Functional Diversity in the Coral Siderastrea siderea Is Influenced by Thermal Stress and Reef Environment, but Not Ocean Acidification.csv

Coral bleaching events are increasing in frequency, demanding examination of the physiological and molecular responses of scleractinian corals and their algal symbionts (Symbiodinium sp.) to stressors associated with bleaching. Here, we quantify the effects of long-term (95-day) thermal and CO2-acidification stress on photochemical efficiency of in hospite Symbiodinium within the coral Siderastrea siderea, along with corresponding coral color intensity, for corals from two reef zones (forereef, nearshore) on the Mesoamerican Barrier Reef System. We then explore the molecular responses of in hospite Symbiodinium to these stressors via genome-wide gene expression profiling. Elevated temperatures reduced symbiont photochemical efficiencies and were highly correlated with coral color loss. However, photochemical efficiencies of forereef symbionts were more negatively affected by thermal stress than nearshore symbionts, suggesting greater thermal tolerance and/or reduced photodamage in nearshore corals. At control temperatures, CO2-acidification had little effect on symbiont physiology, although forereef symbionts exhibited constitutively higher photochemical efficiencies than nearshore symbionts. Gene expression profiling revealed that S. siderea were dominated by Symbiodinium goreaui (C1), except under thermal stress, which caused shifts to thermotolerant Symbiodinium trenchii (D1a). Comparative transcriptomics of conserved genes across the host and symbiont revealed few differentially expressed S. goreaui genes when compared to S. siderea, highlighting the host's role in the coral's response to environmental stress. Although S. goreaui transcriptomes did not vary in response to acidification stress, their gene expression was strongly dependent on reef zone, with forereef S. goreaui exhibiting enrichment of genes associated with photosynthesis. This finding, coupled with constitutively higher forereef S. goreaui photochemical efficiencies, suggests that functional differences in genes associated with primary production exist between reef zones.

珊瑚白化事件的发生频率逐年攀升，亟需明确石珊瑚目珊瑚（scleractinian corals）及其藻类共生体——虫黄藻属（Symbiodinium sp.）对与白化相关的胁迫因子的生理与分子响应机制。本研究针对中美洲堡礁系统（Mesoamerican Barrier Reef System）两个礁区（外礁坡（forereef）、近岸礁（nearshore））的星孔珊瑚（Siderastrea siderea），量化了为期95天的长期热胁迫与CO₂酸化胁迫对宿主体内（in hospite）虫黄藻光化学效率的影响，同时同步测定了对应珊瑚的颜色强度。随后，我们通过全基因组基因表达谱分析，探究了宿主体内虫黄藻对上述胁迫的分子响应模式。温度升高会降低共生体的光化学效率，且该现象与珊瑚颜色褪变高度相关。但相较于近岸礁共生体，外礁坡共生体的光化学效率受热胁迫的负面影响更为显著，这提示近岸珊瑚具有更强的热耐受能力，或其体内光损伤程度更低。在对照温度条件下，CO₂酸化对共生体生理的影响微弱，但外礁坡共生体的光化学效率本就高于近岸礁共生体。基因表达谱分析显示，星孔珊瑚（S. siderea）的共生体主要为戈氏虫黄藻（Symbiodinium goreaui，C1型），仅在热胁迫条件下，共生体群落会转变为耐热的特伦奇虫黄藻（Symbiodinium trenchii，D1a型）。对宿主与共生体的保守基因进行比较转录组分析后发现，相较于星孔珊瑚，戈氏虫黄藻的差异表达基因数量极少，这凸显了宿主在珊瑚应对环境胁迫过程中的核心作用。尽管戈氏虫黄藻的转录组未随酸化胁迫发生显著变化，但其基因表达强烈依赖于礁区来源：外礁坡来源的戈氏虫黄藻显著富集了与光合作用相关的基因。结合外礁坡戈氏虫黄藻本就更高的光化学效率这一结果，本研究提示不同礁区的珊瑚共生体在与初级生产相关的基因功能上存在差异。