SUMMARY How mitochondria came to reside within the cytosol of their host has been debated for 50 years. Though current data indicate that the last eukaryote common ancestor possessed mitochondria and was a complex cell, whether mitochondria or complexity came first in eukaryotic evolution is still discussed. In autogenous models (complexity first), the origin of phagocytosis poses the limiting step at eukaryote origin, with mitochondria coming late as an undigested growth substrate. In symbiosis-based models (mitochondria first), the host was an archaeon, and the origin of mitochondria was the limiting step at eukaryote origin, with mitochondria providing bacterial genes, ATP synthesis on internalized bioenergetic membranes, and mitochondrion-derived vesicles as the seed of the eukaryote endomembrane system. Metagenomic studies are uncovering new host-related archaeal lineages that are reported as complex or phagocytosing, although images of such cells are lacking. Here we review the physiology and components of phagocytosis in eukaryotes, critically inspecting the concept of a phagotrophic host. From ATP supply and demand, a mitochondrion-lacking phagotrophic archaeal fermenter would have to ingest about 34 times its body weight in prokaryotic prey to obtain enough ATP to support one cell division. It would lack chemiosmotic ATP synthesis at the plasma membrane, because phagocytosis and chemiosmosis in the same membrane are incompatible. It would have lived from amino acid fermentations, because prokaryotes are mainly protein. Its ATP yield would have been impaired relative to typical archaeal amino acid fermentations, which involve chemiosmosis. In contrast, phagocytosis would have had great physiological benefit for a mitochondrion-bearing cell.

概括线粒体如何进入其宿主细胞质中的争论已有50年了。尽管目前的数据表明，最后一个真核生物的共同祖先具有线粒体并且是一个复杂的细胞，但仍然讨论线粒体或复杂性在真核生物进化中排在首位。在自体模型中（首先是复杂性），吞噬作用的起源是真核生物起源的限制步骤，线粒体作为未消化的生长底物而来较晚。在基于共生的模型中（首先是线粒体），宿主是古细菌，而线粒体的起源是真核生物起源的限制步骤，其中线粒体提供细菌基因，内在生物能膜上的ATP合成以及线粒体来源的囊泡作为种子真核细胞内膜系统。元基因组学研究发现了新的与宿主相关的古细菌谱系，这些谱系被报告为复杂或吞噬，尽管缺乏此类细胞的图像。在这里，我们审查了真核生物中吞噬作用的生理学和组成部分，批判性地检查了吞噬营养宿主的概念。根据ATP的供求关系，缺乏线粒体的吞噬营养古细菌发酵罐将不得不摄取其体重的约34倍的原核猎物，以获取足够的ATP来支持一个细胞分裂。由于在同一膜上的吞噬作用和化学渗透作用是不相容的，因此质膜上将缺乏化学渗透作用的ATP合成。因为原核生物主要是蛋白质，所以它本来可以通过氨基酸发酵而活下来的。相对于典型的古细菌氨基酸发酵（涉及化学渗透），其ATP产量将受到损害。