Symbiogenesis, or endosymbiotic theory, is an evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms, first articulated in 1905 and 1910 by the Russian botanist Konstantin Mereschkowski, and advanced and substantiated with microbiological evidence by Lynn Margulis in 1967. It holds that the organelles distinguishing eukaryote cells evolved through symbiosis of individual single-celled prokaryotes . The theory holds that mitochondria, plastids such as chloroplasts, and possibly other organelles of eukaryotic cells represent formerly free-living prokaryotes taken one inside the other in endosymbiosis. In more detail, mitochondria appear to be related to Rickettsiales proteobacteria, and chloroplasts to nitrogen-fixing filamentous cyanobacteria. Among the many lines of evidence supporting symbiogenesis are that new mitochondria and plastids are formed only through binary fission, and that cells cannot create new ones otherwise; that the transport proteins called porins are found in the outer membranes of mitochondria, chloroplasts and bacterial cell membranes; that cardiolipin is found only in the inner mitochondrial membrane and bacterial cell membranes; and that some mitochondria and plastids contain single circular DNA molecules similar to the chromosomes of bacteria.
The alveolates are a group of protists, considered a major clade and superphylum within Eukarya, and are also called Alveolata.
The cryptomonads are a group of algae, most of which have plastids. They are common in freshwater, and also occur in marine and brackish habitats. Each cell is around 10–50 μm in size and flattened in shape, with an anterior groove or pocket. At the edge of the pocket there are typically two slightly unequal flagella.
Nucleomorphs are small, vestigial eukaryotic nuclei found between the inner and outer pairs of membranes in certain plastids. They are thought to be vestiges of primitive red and green algal nuclei that were engulfed by a larger eukaryote. Because the nucleomorph lies between two sets of membranes, nucleomorphs support the endosymbiotic theory and are evidence that the plastids containing them are complex plastids. Having two sets of membranes indicate that the plastid, a prokaryote, was engulfed by a eukaryote, an alga, which was then engulfed by another eukaryote, the host cell, making the plastid an example of secondary endosymbiosis.
Chromalveolata was a eukaryote supergroup present in a major classification of 2005, then regarded as one of the six major groups within the eukaryotes. It was a refinement of the kingdom Chromista, first proposed by Thomas Cavalier-Smith in 1981. Chromalveolata was proposed to represent the organisms descended from a single secondary endosymbiosis involving a red alga and a bikont. The plastids in these organisms are those that contain chlorophyll c.
Viridiplantae are a clade of eukaryotic organisms made up of the green algae, which are primarily aquatic, and the land plants (embryophytes), which emerged within them. Green algae traditionally excludes the land plants, rendering them a paraphyletic group. Since the realization that the embryophytes emerged from within the green algae, some authors are starting to include them. They have cells with cellulose in their cell walls, and primary chloroplasts derived from endosymbiosis with cyanobacteria that contain chlorophylls a and b and lack phycobilins. More than 350,000 species of Viridiplantae exist.
The Archaeplastida are a major group of autotrophic eukaryotes, comprising the red algae (Rhodophyta), the green algae, and the land plants, together with a small group of freshwater unicellular algae called glaucophytes. All of these organisms have chloroplasts that are surrounded by two membranes, suggesting that they were acquired directly from endosymbiotic cyanobacteria. In all other groups besides the amoeboid Paulinella chromatophora, the chloroplasts are surrounded by three or four membranes, suggesting they were acquired secondarily from red or green algae.
An apicoplast is a derived non-photosynthetic plastid found in most Apicomplexa, including Toxoplasma gondii, Plasmodium falciparum and other Plasmodium spp., but not in others such as Cryptosporidium. It originated from an alga through secondary endosymbiosis. The apicoplast is surrounded by four membranes within the outermost part of the endomembrane system. The apicoplast hosts important metabolic pathways like Fatty acid synthesis; Isoprenoid precursor synthesis and parts of the Heme biosynthesis pathway
Ostreococcus is a genus of unicellular coccoid or spherically shaped green algae belonging to the class Mamiellophyceae. It includes prominent members of the global picoplankton community, which plays a central role in the oceanic carbon cycle.
The Prasinophytes or chlorophyta are a class of unicellular green algae. Prasinophytes mainly include marine planktonic species, as well as some freshwater representatives. The prasinophytes are morphologically diverse, including flagellates with one to eight flagella and non-motile (coccoid) unicells. The cells of many species are covered with organic body scales; others are naked. Well studied genera include Ostreococcus, considered to be the smallest free-living eukaryote, and Micromonas, both of which are found in marine waters worldwide. Prasinophytes have simple cellular structures, containing a single chloroplast and a single mitochondrion. The genomes are relatively small compared to other eukaryotes.
Paulinella is a genus of about nine species of freshwater amoeboids.
Picoeukaryotes are picoplanktonic eukaryotic organisms 3.0 µm or less in size. They are distributed throughout the world’s marine and freshwater ecosystems and constitute a significant contribution to autotrophic communities. Though the SI prefix pico- might imply an organism smaller than atomic size, the term was likely used to avoid confusion with existing size classifications of plankton.
Ostreococcus tauri is a unicellular species of marine green alga about 0.8 micrometres (μm) in diameter, the smallest free-living (non-symbiotic) eukaryote yet described. It has a very simple ultrastructure, and a compact genome.
Michael Melkonian is a German botanist and professor of botany at the University of Cologne.
Cryptista is a clade of algae-like eukaryotes. It is sometimes placed along with Haptista in the group Hacrobia, within the kingdom Chromista. However, in 2016, a broad phylogenomic study found that cryptists fall within the group Archaeplastida, while haptophytes are closely related to the SAR supergroup.
Corbihelia is a phylum of eukaryotes.
A plastid is a membrane bound organelle found in plants, algae, and other eukaryotic organisms that function in the production of pigment molecules. Most plastids are photosynthetic, thus leading to color production and energy storage or production. There are many types of plastids in plants alone, but all plastids can be separated based on the number of times they have undergone endosymbiotic events. Currently there are three types of plastids; primary, secondary and tertiary. Endosymbiosis is reputed to have led to the evolution of eukaryotic organisms today, although the timeline is highly debated.
