Species-specific effects and the ecological role of Programmed cell Death in the microalgae Ankistrodesmus (Sphaeropleales, Selenastraceae)

Reports of programmed cell death (PCD) in phytoplankton raise questions about the ecological evolutionary role of cell death in these organisms. We induced PCD by nitrogen deprivation and unregulated cell death (non-PCD) in one strain of the green microalga Ankistrodesmus densus and investigated the effects of the cell death supernatants on phylogenetically related co-occurring organisms using growth rates and maximum biomass as proxies of fitness. PCD-released materials from A. densus CCMA-UFSCar-3 significantly increased growth rates of two conspecific strains compared to healthy culture (HC) supernatants and improved the maximum biomass of all A. densus strains compared to related species. Although growth rates of non-A. densus with PCD supernatants were not statistically different from HC treatment, biomass gain was significantly reduced. Thus, the organic substances released by PCD, possibly nitrogenous compounds, could promote conspecific growth. These results support the argument that PCD may differentiate species or subtypes and increases inclusive fitness in this model unicellular chlorophyte. Further research, however, is needed to identify the responsible molecules and how they interact with cells to provide the PCD benefits.

Methods
Methods and statistical analysis are described fully in the associated main manuscript. The README.docx file provides all the instructions needed to replicate data analysis.

有关浮游植物程序性细胞死亡（programmed cell death, PCD）的研究报道，引发了学界对这类生物细胞死亡的生态演化功能的诸多探讨。我们以一株绿色微藻致密针杆藻（Ankistrodesmus densus）为实验材料，通过氮饥饿诱导其发生程序性细胞死亡（PCD），并同步诱导产生非调控性细胞死亡（non-PCD）。随后以生长速率与最大生物量作为适合度的替代指标，探究了细胞死亡上清液对系统发育亲缘关系较近的共栖生物的影响。相较于健康培养（healthy culture, HC）上清液，来自致密针杆藻CCMA-UFSCar-3的PCD释放物可显著提升两株同物种菌株的生长速率；且相较于近缘物种，该上清液可提高所有致密针杆藻菌株的最大生物量。尽管用PCD上清液培养的非致密针杆藻菌株的生长速率与HC处理组无统计学差异，但其生物量积累量却显著降低。由此可见，PCD过程中释放的有机物（大概率为含氮化合物）可促进同物种个体的生长。本研究结果支持如下观点：在该模式单细胞绿藻中，PCD或可参与物种或亚型的分化，并提升其综合适合度。不过，未来仍需开展深入研究，以明确发挥关键作用的具体分子，以及其与细胞的互作机制，从而阐释PCD带来的适应性优势。
材料与方法
材料与方法及统计学分析的完整细节已在相关主论文中予以阐述。README.docx文件提供了重复数据分析所需的全部操作指引。