Evolutionary and Molecular Aspects of Plastid Endosymbioses

Dear Colleagues,

Plastids—photosynthetic organelles of eukaryotes—have been instrumental in our understanding how eukaryotes are evolving as multi-compartmented cellular entities. As postulated by Merezhkowski already in 1905, plastids ultimately arose as a result of an intracellular symbiosis (endosymbiosis) relationship of eukaryotic cells with cyanobacteria. As radical as it seemed at that time, the theory of symbiogenesis became widely accepted with the accumulation of morphological and molecular data. In fact, with the increasing number of single-celled eukaryotes that have been found to possess plastids, we are excited by how many shapes plastids take up, in terms of both morphology and biology.

Plastids that we call primary are descendants of symbionts arising from prokaryote-to-eukaryote endosymbioses with cyanobacteria. However, plastids are mildly promiscuous organelles and, through higher-order (eukaryote-to-eukaryote) endosymbiosis, they have been horizontally transferred across highly divergent eukaryotic lineages, resulting in an explosion of diversity of phototrophic eukaryotes. Through photosynthesis, plastids shifted the metabolic possibilities of their hosts, algae and plants, to a new level. The virtually infinite source of energy of light allowed them to turn inorganic molecules into biocompounds and thrive in and colonize marine, freshwater, and dry land environments of many kinds. Eukaryotic phototrophs are thus responsible for a major part of primary production on Earth.

As a result of disparate origins, plastids show high diversity in terms of biological functions and evolutionary trajectories. The genetic and metabolic integration of a phototrophic endosymbiont into a host cell results in unique evolutionary arrangements of metabolic pathways and cellular interactions. Besides photosynthesis, plastids frequently constitute a metabolic hub for carbon, nitrogen, and sulfur metabolism, as well as for the biosynthesis of cofactors and vitamins. Compound exchanges and the subtle balance of the metabolic flows between the plastid and other cell compartments is crucial for algae to succeed in competitions for resources in the long run. Surprisingly, organisms lose photosynthesis at least as frequently as they gain it. Plastid endosymbioses appear to be the most efficient evolutionary processes in nature as they cause trophic switches from heterotrophy to photoautotrophy and back again, always bringing bursts of new genetic material with high innovative potential.

With the advance of high-throughput sequencing, the diversity of life is being quickly unveiled by gathering data on a range of levels from environmental samples to single cells. Only now has it become apparent how widespread plastids are. Signs of plastid metabolism have been found in disparate inconspicuous microeukaryotes; thus, these organisms often profoundly change our view of the tree of life. Some of these long-overlooked organisms host photosynthetic plastids, some of them possess cryptic non-photosynthetic plastids; either way, they are important to a better understanding of plastid evolution, as they fill in the huge gaps between the phototrophic crown groups. Furthermore, we have to realize that plastids are not the only endosymbionts making a difference. In fact, more or less intimate relationships are formed among eukaryotes and prokaryotes in other branches of the tree of life and in environments that lack access to light. In fact, it appears that metabolic innovations similar to the acquisition of plastids are tightly linked to evolutionary success.

We compiled this Special Issue of Biomolecules to reflect the many facets of the biology of plastids. We hope this issue will provide valuable insights into the achievements and future prospects of this developing research field.

Prof. Dr. Miroslav Oborník
Dr. Zoltán Füssy
Guest Editors

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• chloroplasts
• endosymbiosis
• horizontal gene transfer
• photosynthesis
• essential plastid pathways
• plastid metabolism
• plastid dependence
• protein translocation
• transporters
• non-photosynthetic algae
• cryptic plastids
• next-generation sequencing
• single-cell sequencing
• alveolates
• apicomplexans
• chromerids
• diatoms
• euglenophytes
• Paulinella