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.

摘要线粒体如何驻留在宿主的细胞质中，这一问题已经争论了 50 年。尽管目前的数据表明，最后的真核生物共同祖先拥有线粒体并且是一个复杂的细胞，但在真核进化中线粒体还是复杂性优先仍然存在争议。在自体模型中（复杂性优先），吞噬作用的起源构成了真核生物起源的限制步骤，线粒体作为未消化的生长底物出现较晚。在基于共生的模型中（线粒体优先），宿主是古细菌，线粒体的起源是真核生物起源的限制步骤，线粒体提供细菌基因，内化生物能膜上合成 ATP，线粒体衍生的囊泡作为种子真核生物内膜系统。宏基因组研究正在揭示新的宿主相关古菌谱系，据报道它们具有复杂性或吞噬性，尽管缺乏此类细胞的图像。在这里，我们回顾了真核生物吞噬作用的生理学和组成部分，批判性地检验了吞噬宿主的概念。从 ATP 的供需来看，缺乏线粒体的吞噬古菌发酵罐必须摄取约其体重 34 倍的原核猎物，才能获得足够的 ATP 来支持一次细胞分裂。它在质膜上缺乏化学渗透 ATP 合成，因为同一膜中的吞噬作用和化学渗透作用是不相容的。它可以靠氨基酸发酵生存，因为原核生物主要是蛋白质。相对于涉及化学渗透的典型古菌氨基酸发酵，其 ATP 产量会受到损害。相比之下，吞噬作用对于携带线粒体的细胞具有巨大的生理益处。