Iridocytes, containing multiple stacks of proteinaceous platelets and crystalized guanine, alternating with thin cytoplasm sheets, are specialized cells that act as multilayer nano-reflectors. Convergence evolution led to their arising across a broad range of organisms, including giant clams of the Tridacninae subfamily – the only sessile and photosymbiotic organism, among animals known to possess iridocytes. Through the interference of light with their nanoscale architecture, iridocytes generate “structural colors,” which are reported to serve different purposes, from intra-species communication to camouflage. In giant clams, iridocytes were previously reported to promote a lateral- and forward scattering of photosynthetically productive radiation (PAR) into the clam tissue, as well as the back reflection of non-productive wavelengths. Hence, they are assumed to promote an increased efficiency in the use of available solar energy, while simultaneously preventing photodamage of the algal symbionts. We report the use of guanine crystals within Tridacna maxima giant clam iridocytes as a basis for photonic cooperation between the bivalve host and their photosynthetic symbionts. Our results suggest that, in addition to the previously described scattering processes, iridocytes absorb potentially damaging UV radiation (UVR) and, through successive emission, emit light at longer wavelengths, which is then absorbed by the photosynthetic pigments of the algal symbionts. Consequently, both, host and algal symbionts are sheltered from (potentially) damaging UVR, while the available solar energy within the PAR spectrum increases, thereby potentially enhancing photosynthetic and calcification rates in this large bivalve. Further, our results suggest that this photonic cooperation could be responsible for the broad repertoire of colors that characterizes the highly diverse mantle patterns found in T. maxima.

中文翻译：

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虹膜细胞介导巨蛤（Tridacninae）与其光合共生体之间的光合作

虹膜细胞含有多叠蛋白质血小板和结晶鸟嘌呤，与薄细胞质片交替，是充当多层纳米反射器的特化细胞。趋同进化导致它们出现在广泛的生物体中，包括砗磲亚科的巨蛤——已知具有虹膜细胞的动物中唯一的固着和光共生生物。通过光与其纳米级结构的干涉，虹膜细胞产生“结构颜色”，据报道这些颜色用于不同的目的，从物种内交流到伪装。在巨蛤中，之前有报道称虹膜细胞可促进光合生产辐射 (PAR) 向蛤组织中的横向和前向散射，以及非生产波长的后向反射。因此，它们被认为可以提高可用太阳能的使用效率，同时防止藻类共生体的光损伤。我们报告了在 Tridacna maxima 巨蛤虹彩细胞中使用鸟嘌呤晶体作为双壳类宿主与其光合共生体之间光子合作的基础。我们的结果表明，除了前面描述的散射过程之外，虹膜细胞还吸收潜在的有害紫外线辐射 (UVR)，并通过连续发射发出更长波长的光，然后被藻类共生体的光合色素吸收。因此，宿主和藻类共生体都免受（潜在）破坏性 UVR 的影响，而 PAR 光谱内的可用太阳能增加，从而有可能提高这种大型双壳类动物的光合作用和钙化率。此外，我们的结果表明，这种光子合作可能是造成在 T. maxima 中发现的高度多样化的地幔图案特征的广泛颜色库的原因。