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Secondary endosymbiosis has occurred several times and has given rise to extremely diverse groups of algae and other eukaryotes. Some organisms can take opportunistic advantage of a similar process, where they engulf an alga and use the products of its photosynthesis, but once the prey item dies (or is lost) the host returns to a free living state.
The standard nucleomorph is the result of secondary endosymbiosis: a cyanobacterium first became the chloroplast of ancestral plants, which diverged into green and red algae among other groups; the algal cell is then captured by another eukaryote.
Endosymbiosis played key roles in the development of eukaryotes and plants. Roughly 2.2 billion years ago an archaeon absorbed a bacterium through phagocytosis , that eventually became the mitochondria that provide energy to almost all living eukaryotic cells.
Secondary endosymbiosis results in the engulfment of an organism that has already performed primary endosymbiosis. Thus, four plasma membranes are formed. The first originating from the cyanobacteria, the second from the eukaryote that engulfed the cyanobacteria, and the third from the eukaryote who engulfed the primary endosymbiotic eukaryote. [11]
The plastids (membranous organelles) in all extant photosynthetic species result from secondary endosymbiosis between a euglenid and a green alga. [6] Structure
Chromalveolata was proposed to represent the organisms descended from a single secondary endosymbiosis involving a red alga and a bikont. [4] The plastids in these organisms are those that contain chlorophyll c. However, the monophyly of the Chromalveolata has been rejected.
Evidence suggests that the apicoplast is a product of secondary endosymbiosis, [6] and that the apicoplast may be homologous to the secondary plastid of the closely related dinoflagellate algae. An ancient cyanobacterium was first engulfed by a eukaryotic cell but was not digested.
[note 1] Unlike red and green algae, glaucophytes have never been involved in secondary endosymbiosis events. [10] The cells of the Archaeplastida typically lack centrioles and have mitochondria with flat cristae. They usually have a cell wall that contains cellulose, and food is stored in the form of starch. However, these characteristics are ...