Data from: Affinity, life cycle, and intracellular complexity of organic-walled 2 microfossils from the Mesoproterozoic of Shanxi, China

Light microscope and scanning electron microscope observations on new material of unicellular microfossils Dictyosphaera macroreticulata and Shuiyousphaeridium macroreticulatum, from the Mesoproterozoic Ruyang Group in China, provide insights into the microorganisms’ biological affinity, life cycle and cellular complexity. Gigantosphaeridium fibratum n. gen. et sp., is described and is one of the largest Mesoproterozoic microfossils recorded. Phenotypic characters of vesicle ornamentation and excystment structures, properties of resistance and cell wall structure in Dictyosphaera and Shuiyousphaeridium are all diagnostic of microalgal cysts. The wide size ranges of the various morphotypes indicate growth phases compatible with the development of reproductive cysts. Conspecific biologically, each morphotype represents an asexual (resting cyst) or sexual (zygotic cyst) stage in the life cycle, respectively. We reconstruct this hypothetical life cycle and infer that the organism demonstrates a reproductive strategy of alternation of heteromorphic generations. Similarly in Gigantosphaeridium, a metabolically expensive vesicle with processes suggests its protective role as a zygotic cyst. In combination with all these characters and from the resemblance to extant green algae, we propose the placement of these ancient microorganisms in the stem group of Chloroplastida (Viridiplantae). A cell wall composed of primary and secondary layers in Dictyosphaera and Shuiyouisphaeridium required a high cellular complexity for their synthesis and the presence of an endomembrane system and the Golgi apparatus. The plastid was also present, accepting the organism was photosynthetic. The biota reveals a high degree of morphological and cell structural complexity, and provides an insight into ongoing eukaryotic evolution and the development of complex life cycles with sexual reproduction by 1200 Ma.对采自中国中元古界汝阳群的单细胞微体化石新标本——网纹网球囊藻（Dictyosphaera macroreticulata）与网纹水球囊藻（Shuiyousphaeridium macroreticulatum）开展的光学显微镜及扫描电子显微镜观测，为解析该类微生物的生物亲缘关系、生命周期与细胞复杂性提供了关键视角。本文还记述了新属新种纤维巨球囊藻（Gigantosphaeridium fibratum），它是目前已记录的最大中元古界微体化石之一。网球囊藻（Dictyosphaera）与水球囊藻（Shuiyouisphaeridium）的囊泡纹饰、脱囊结构等表型特征，以及抗逆特性与细胞壁结构，均符合微藻孢囊的典型诊断特征。各类形态型的宽尺寸分布范围表明其生长阶段与生殖孢囊的发育进程相匹配。从生物学种的视角来看，每种形态型分别对应无性（休眠孢囊）或有性（合子孢囊）发育阶段。我们重构了这一假想的生命周期，并推断该微生物展现出异形世代交替的生殖策略。同样，纤维巨球囊藻（Gigantosphaeridium fibratum）拥有具有凸起结构且代谢成本高昂的囊泡，这暗示其作为合子孢囊具备保护功能。综合上述所有特征，并结合其与现存绿藻的相似性，我们提议将这些古老微生物归类于叶绿体植物门（Chloroplastida，Viridiplantae）的干群。网球囊藻与水球囊藻的细胞壁由初生层与次生层构成，其合成过程需要高度复杂的细胞机制，同时表明该类生物已具备内膜系统与高尔基体（Golgi apparatus）。质体的存在也进一步证实该生物具备光合自养能力。该生物群展现出极高的形态与细胞结构复杂性，为我们理解距今1200 Ma的真核生物演化进程，以及具备有性生殖的复杂生命周期的演化发展提供了重要启示。