Some dinoflagellates, such as Symbiodinium, are able to form symbiotic relationships with larger marine organisms. An important aspect of dinoflagellate symbiosis involves the exchange of lipids, namely sterols, from the symbiont to the host. Much research has explored the lipid biochemistry of the symbiotic relationship between cnidarians and Symbiodinium dinoflagellates. However, no research has addressed the sterol biochemistry of the symbiosis between radiolarians and dinoflagellates such as Zooxanthella nutricula. To this end, we have provided the first sterol characterization of Z. nutricula isolated from a spumellarian polycystine radiolarian. Fifteen sterols and one steroidal ketone were observed where the major sterol identified was C₂₇ 22-dehydrocholesterol, which does not tend to be a dominant sterol among dinoflagellates, including closely related peridinioid species in the genus Heterocapsa. However, C₃₀ dinosterol and dinostanol were major sterols in both Z. nutricula and Heterocapsa spp., thus indicating common sterols between closely related taxa. Major sterols of the distantly related genus Symbiodinium, a symbiont of foraminifera and cnidarians, have included C₂₇ cholesterol and C₃₀ gorgosterol, whereas in Z. nutricula these sterols were minor and absent, respectively. Our results indicate potentially different sterol pools available to cnidarian and radiolarian symbiont hosts during their respective relationships with symbiotic dinoflagellates.

一些鞭毛藻，例如Symbiodinium，能够与较大的海洋生物形成共生关系。甲藻共生的一个重要方面涉及脂质，即甾醇，从共生体到宿主的交换。许多研究探索了刺胞动物和Symbiodinium dinoflagellates之间共生关系的脂质生物化学。然而，没有研究涉及放射虫和鞭毛虫（如Zooxanthella nutricula）之间共生的甾醇生物化学。为此，我们提供了Z. nutricula的第一个甾醇表征从 spumellarian 多胱氨酸放射虫中分离出来。观察到 15 种甾醇和 1 种甾体酮，其中鉴定的主要甾醇是 C ₂₇ 22-脱氢胆固醇，其在鞭毛藻中往往不是占优势的甾醇，包括与Heterocapsa属密切相关的peridinioid 物种。然而，C ₃₀ dinosterol 和dinostanol 是Z. nutricula和Heterocapsa spp.中的主要甾醇，因此表明密切相关的分类群之间存在常见甾醇。远亲Symbiodinium属（有孔虫和刺胞动物的共生体）的主要甾醇包括 C ₂₇胆固醇和 C ₃₀戈尔甾醇，而在Z. nutricula 中，这些甾醇分别是次要的和不存在的。我们的结果表明，刺胞动物和放射虫共生宿主在与共生甲藻的关系中可能存在不同的甾醇池。