Understanding differential heat tolerance of the coral Orbicella faveolata between sites in the Florida Keys.. Orbicella faveolata, Durusdinium trenchii and Breviolum minutum

Ocean warming is causing widespread degradation of coral reefs. In the Florida Keys, many reefs have lost much of their coral cover, yet some inshore reefs have demonstrated higher resilience. To explore the hypothesis that exposure to higher, more variable temperatures contributes to inshore resilience, we subjected three inshore and four offshore genotypes of the coral Orbicella faveolata to 30, 31, 32, or 33C for 31 days and measured photochemical efficiency (Fv/Fm), the types and relative abundance of dinoflagellate endosymbionts, and gene expression of the host and symbiont. All inshore coral genotypes, regardless of symbiont type, were significantly more thermotolerant than offshore genotypes based on declines in Fv/Fm. The most heat-tolerant inshore genotype (In1) was dominated by Durusdinium trenchii; all other genotypes were Breviolum-dominated, suggesting local adaptation or acclimatization contributes to the heat tolerance of inshore genotypes. After 31 days of heat stress, all coral genotypes (except In2) had lost most of their Breviolum and became dominated by D. trenchii. Host genotype In1 presented unique expression patterns of genes involved in heat shock response, immunity, and protein degradation. There were few changes in the symbiont transcriptomes of inshore corals under heat stress, but significant changes in symbiont gene expression from the offshore colonies, including increases in ribosomal and photosynthetic proteins. These data show that the differential thermotolerance between inshore and offshore O. faveolata in the Florida Keys is associated with statistically significant differences in both host and symbiont gene expression that provide insights into the mechanisms underlying holobiont heat tolerance.

海洋变暖正引发全球范围内的珊瑚礁大面积退化。在佛罗里达群岛，多数珊瑚礁已丧失大量珊瑚覆盖，但部分近岸珊瑚礁表现出更强的抗逆性。为验证“更高且更具波动性的温度暴露可提升近岸珊瑚抗逆性”这一假说，我们将3株近岸、4株远岸的华丽圆脑珊瑚（Orbicella faveolata）基因型分别置于30、31、32或33℃环境中培养31天，并测定了其光化学效率（photochemical efficiency，Fv/Fm）、沟鞭藻内共生体（dinoflagellate endosymbionts）的类群组成与相对丰度，以及宿主与共生体的基因表达水平。基于Fv/Fm的下降幅度，所有近岸珊瑚基因型（无论其共生体类群）均比远岸基因型具有显著更强的耐热性。耐热性最强的近岸基因型（In1）以特氏盾形虫黄藻（Durusdinium trenchii）为优势共生体；其余所有基因型均以短膜虫黄藻属（Breviolum）物种为优势共生体，这表明本地适应或驯化作用可提升近岸珊瑚基因型的耐热性。经过31天的热胁迫处理后，除In2外的所有珊瑚基因型均丧失了大部分短膜虫黄藻属共生体，转而被特氏盾形虫黄藻所主导。宿主基因型In1呈现出参与热休克响应、免疫反应及蛋白质降解过程的基因的独特表达模式。热胁迫下近岸珊瑚的共生体转录组仅发生少量变化，但远岸珊瑚群落的共生体基因表达则出现显著变化，包括核糖体蛋白与光合蛋白的表达上调。本研究数据表明，佛罗里达群岛近岸与远岸的华丽圆脑珊瑚（O. faveolata）之间的耐热性差异，与宿主及共生体基因表达的统计学显著差异相关，这些差异为解析全共生体（holobiont）耐热性的潜在机制提供了重要见解。