Many organisms have formed symbiotic relationships with nitrogen (N)-fixing bacteria to overcome N limitation. Diatoms in the family Rhopalodiaceae host unicellular, N-fixing cyanobacterial endosymbionts called spheroid bodies (SBs). Although this relationship is relatively young, SBs share many key features with older endosymbionts, including coordinated cell division and genome reduction. Unlike free-living relatives that fix N exclusively at night, SBs fix N largely during the day; however, how SB metabolism is regulated and coordinated with the host is not yet understood. We compared four SB genomes, including those from two new host species (Rhopalodia gibba and Epithemia adnata), to build a genome-wide phylogeny which provides a better understanding of SB evolutionary origins. Contrary to models of endosymbiotic genome reduction, the SB chromosome is unusually stable for an endosymbiont genome, likely due to the early loss of all mobile elements. Transcriptomic data for the R. gibba SB and host organelles addressed whether and how the allocation of transcriptional resources depends on light and nitrogen availability. Whereas allocation to the SB was high under all conditions, relative expression of chloroplast photosynthesis genes increased in the absence of nitrate, but this pattern was suppressed by nitrate addition. SB expression of catabolism genes was generally greater during daytime rather than at night, although the magnitude of diurnal changes in expression was modest compared to free-living cyanobacteria. We conclude that SB daytime catabolism likely supports N-fixation by linking the process to host photosynthetic carbon fixation.

中文翻译：

---

固氮硅藻内共生体中的昼夜转录变异减少

许多生物体与固氮（N）细菌形成共生关系以克服氮限制。 Rhopalodiaceae 科的硅藻寄生着单细胞、固氮蓝藻内共生体，称为球状体 (SB)。尽管这种关系相对较新，但 SB 与较古老的内共生体有许多共同的关键特征，包括协调细胞分裂和基因组减少。与自由生活的亲戚只在晚上修复氮不同，SB 大部分在白天修复氮；然而，SB代谢如何与宿主进行调节和协调尚不清楚。我们比较了四种 SB 基因组，包括来自两个新宿主物种（Rhopalodia gibba 和 Epithemia adnata）的基因组，以构建全基因组系统发育，从而更好地了解 SB 进化起源。与内共生基因组减少模型相反，SB染色体对于内共生基因组来说异常稳定，这可能是由于所有移动元件的早期丢失。 R. gibba SB 和宿主细胞器的转录组数据揭示了转录资源的分配是否以及如何依赖于光和氮的可用性。尽管在所有条件下SB的分配都很高，但在没有硝酸盐的情况下，叶绿体光合作用基因的相对表达增加，但这种模式被硝酸盐的添加所抑制。分解代谢基因的 SB 表达通常在白天比晚上更高，尽管与自由生活的蓝藻相比，表达的昼夜变化幅度不大。我们得出的结论是，SB 白天分解代谢可能通过将这一过程与宿主光合碳固定联系起来来支持固氮。