Several foraminifers found in warm and low-nutrient ocean surface water have photosynthetic algae as endosymbionts (photosymbiosis). To understand the trophic interactions, we studied Globigerinoides sacculifer, a spinose planktic foraminifer that has a dinoflagellate endosymbiont. We controlled two nutritional factors, feeding and inorganic nutrients in the seawater. The growth of the host and the symbionts and the photophysiological parameters were monitored under four experimental conditions. The results demonstrated that the holobionts primarily relied on phagotrophy for growth. The foraminifers grew considerably, and the chlorophyll a content per foraminifer, which is an indicator of the symbiont population, increased in the fed groups, but not in the unfed groups. The nutrient-rich seawater used for some of the cultures made no difference in either the growth or photophysiology of the holobionts. These observations indicated that the symbionts mainly utilized metabolites from the hosts for photosynthesis rather than inorganic nutrients in the seawater. Additionally, we observed that the symbionts in the starved hosts maintained their photosynthetic capability for at least 12 days, and that the hosts maintained at least some symbionts until gametogenesis was achieved. This suggests that the hosts have to retain the symbionts as an energy source for reproduction. The symbionts may also play an indispensable role in the metabolic activities of the hosts including waste transport or essential compound synthesis. Overall, our results revealed a novel mode of photosymbiosis in planktic foraminifers which contrasts with that found in benthic photosymbiotic foraminifers and corals.

中文翻译：

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营养条件对培养的球果（Ghibigerinoides sacculifer）（根茎，有孔虫）的光合生物联盟的影响。

在温暖和低营养的海洋表层水中发现的几种有孔虫将光合藻类作为共生菌（光共生）。为了理解营养相互作用，我们研究了球形双孢子有孔菌，具有双鞭毛内共生体的球状有孔虫Globigerinoides sacculifer。我们控制了两个营养因素，海水中的营养和无机营养。在四个实验条件下监测宿主和共生体的生长以及光生理参数。结果表明，整体人主要依靠吞噬细胞生长。有孔虫大量生长，而叶绿素a饲喂组中每个有孔虫的含量（共生菌数量的指标）增加，但未饲喂组则没有。用于某些养殖的富含营养的海水在全卤虫的生长或光生理学上都没有影响。这些观察结果表明，共生体主要利用宿主体内的代谢产物进行光合作用，而不是利用海水中的无机养分。此外，我们观察到饥饿的宿主中的共生体保持其光合能力至少12天，并且宿主达到配子发生之前至少保持一些共生体。这表明宿主必须保留共生体作为繁殖的能量来源。共生体在宿主的代谢活动中也可能起着不可或缺的作用，包括废物运输或必要的化合物合成。总体而言，我们的研究结果揭示了浮游有孔虫中一种新型的光共生模式，与底栖生物共生有孔虫和珊瑚中发现的这种模式形成了鲜明的对比。