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Secondary endosymbiosis - a process in which an organism having an endosymbionate becomes an endosymbiona of another organism.

The hypothesis of secondary endosymbiosis is used to describe the evolution of photosynthetic protists from the evolutionary line Archaeplastidae (including glaucophytes, algae and greens with terrestrial plants). According to her, chloroplasts originated from the primary endosymbiosis of cyanobacteria within the early eukaryotes of the Archaeplastida line, and then some of the resulting algae became endosymbions of other, previously unsuitable for photosynthesis of organisms. This phenomenon occurred independently of each other in several lines of development. Details remain investigated.

According to some studies, secondary endosymbiosis occurred three times - the green herbs (or their ancestors) became the two endosymbiontes, and the hosts developed the eugenin line (Excavat supergroup) and Chlorarachniophyta (Rhizaria subspecies amphibus), once dwarfed and their hosts The representatives of the supergroup Chromalveolata, whose descendants form the following groups, are: Stramenopile (diatoms, chrysotile, brown), cryptomonads, haptophytes and alveolata (eg ferns). The common pathway of endosymbiosis leading to follicles and other yellow-brown algae is, however, undermined.

The degree of integration of endosymbiont varies. Typically, chloroplasts of this origin have 4 membranes (two originating from the original endosymbiont, ie cyanobacteria, also present in normal chloroplasts, one from the primary host and one from the secondary host, attached to its endoplasmic reticulum. Two groups - cryptomonad and Chlorarachniophyta between one of the cyanogenic membranes and the membrane of the primary host present a residual but functional nucleus with a genome different from that of the primary host, called the nucleomorph.

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