In the face of global warming and unprecedented coral bleaching, a new avenue of research is focused on relatively rare algal symbionts and their ability to confer thermal tolerance to their host by association. Yet, thermal tolerance is just one of many physiological attributes inherent to the diversity of symbiodinians, a result of millions of years of competition and niche partitioning. Here, we revealed that competition among cocultured symbiodinians alters nutrient assimilation and compound production with species-specific responses. For Cladocopium goreaui, a species ubiquitous within stable coral associations, temperature stress increased sensitivity to competition eliciting a shift toward investment in cell replication, i.e., putative niche exploitation. Meanwhile, competition led Durusdinium trenchii, a thermally tolerant “background” symbiodinian, to divert resources from immediate growth to storage. As such, competition may be driving the dominance of C. goreaui outside of temperature stress, the destabilization of symbioses under thermal stress, the repopulation of coral tissues by D. trenchii following bleaching, and ultimately undermine the efficacy of symbiont turnover as an adaptive mechanism.

面对全球变暖和前所未有的珊瑚白化，一种新的研究途径集中在相对稀有的藻类共生体及其通过关联赋予宿主耐热性的能力。然而，耐热性只是共生生物多样性固有的许多生理属性之一，这是数百万年竞争和生态位划分的结果。在这里，我们揭示了共培养的共生体之间的竞争通过物种特异性反应改变了营养同化和化合物的产生。对于在稳定的珊瑚群落中普遍存在的Cladocopium goreaui物种，温度胁迫增加了对竞争的敏感性，从而引发了对细胞复制投资的转变，即推定的生态位开发。同时，竞争导致Durusdiniumtrenchii，一种耐热的“背景”共生体，将资源从直接生长转移到储存。因此，竞争可能正在推动C. goreaui在温度胁迫之外的主导地位，共生在热胁迫下的不稳定，珊瑚组织在漂白后由D.trenchii重新繁殖，并最终破坏共生体转换作为一种适应性机制的功效.