The acoel flatworm Symsagittifera roscoffensis lives in obligatory symbiosis with the microalgal chlorophyte Tetraselmis convolutae. Although this interaction has been studied for more than a century, little is known on the potential reciprocal benefits of both partners, a subject that is still controversial. In order to provide new insights into this question, we have compared the photophysiology of the free-living microalgae to the symbiotic form in the flatworm, both acclimated at different light irradiances. Photosynthesis – Irradiance curves showed that the free-living T. convolutae had greater photosynthetic performance (i.e., oxygen production rates, ability to harvest light) than their symbiotic form, regardless of the light acclimation. However, they were affected by photoinhibition under high irradiances, which did not happen for the symbiotic form. The resistance of symbiotic microalgae to photoinhibition were corroborated by pigment analyses, which evidenced the induction of photoprotective mechanisms such as xanthophyll cycle as well as lutein and β-carotene accumulation. These processes were induced even under low light acclimation and exacerbated upon high light acclimation, suggesting a global stress situation for the symbiotic microalgae. We hypothesize that the internal conditions in the sub-epidermal zone of the flatworm (e.g., osmotic and pH), as well as the phototaxis toward high light imposed by the worm in its environment, would be major reasons for this chronic stress situation. Overall, our study suggests that the relationship between S. roscoffensis and T. convolutae may be a farming strategy in favor of the flatworm rather than a symbiosis with mutual benefits.

中文翻译：

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一个人过得更好吗？扁虫Rossagittifera roscoffensis与微藻Tetraselmis convolutae之间的共生关系的新见解

co扁虫（Symsagittifera roscoffensis）与微藻叶绿体四叶藻（Tetraselmis convolutae）生活在强制共生中。尽管对这种相互作用的研究已经有一个多世纪的历史了，但对于双方的潜在互惠利益知之甚少，这个话题仍然存在争议。为了提供对该问题的新见解，我们将自由生存的微藻的光生理学与扁虫中的共生形式进行了比较，二者均适应于不同的光照强度。光合作用–辐照度曲线表明，自由生活的卷心菜不论光适应程度如何，其光合性能（即氧气产生率，光收集能力）都比其共生形式高。但是，它们在高辐照度下受到光抑制作用，而共生形式则没有。色素分析证实了共生微藻对光抑制的抗性，这证明了诱导了诸如叶黄素循环以及叶黄素和β-胡萝卜素积累等光保护机制。这些过程甚至在弱光适应下也被诱导，而在强光适应下会加剧，这表明该共生微藻处于整体胁迫状态。我们假设扁虫的表皮下区域的内部条件（例如渗透和pH），以及蠕虫在其环境中强光照射的趋光性，将是造成这种慢性压力情况的主要原因。总体而言，我们的研究表明罗氏链球菌和卷积螺旋藻可能是一种有利于扁虫的耕作策略，而不是互惠共生的共生关系。