''Mesodinium rubrum'' (or ''Myrionecta rubra'') is a species of

ciliates The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagellum, eukaryotic flagella, but are in general shorter and present in much larger numbers, with a ...

. It constitutes a

plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...

community and is found throughout the year, most abundantly in spring and fall, in coastal areas. Although discovered in 1908, its scientific importance came into light in the late 1960s when it attracted scientists by the recurrent red colouration it caused by forming massive blooms, that cause red tides in the oceans. Unlike typical protozoans, ''M. rubrum'' can make its own nutrition by

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

. The unusual

autotrophic An autotroph or primary producer is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) using carbon from simple substances such as carbon dioxide,Morris, J. et al. (2019). "Biology: How Life Works", ...

property was discovered in 2006 when genetic sequencing revealed that the photosynthesising organelles,

plastids The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracellular endosymbiotic cyanobac ...

, were derived from the principal food of the ciliate, the photosynthetic algae called

cryptomonads The cryptomonads (or cryptophytes) 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 anterio ...

(or cryptophytes). It is, thus, both autotrophic and

heterotrophic A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

. This nature also indicates that it is an example of

endosymbiosis An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within" ...

, supporting the

endosymbiotic theory Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibl ...

, as well as the concept of stealing of cell organelles called kleptoplastidy. Moreover, ''M. rubrum'' represents additional endosymbiosis by transferring its plastids to its predators, the dinoflagellate planktons belonging to the genus ''Dinophysis''. In 2009, a new species of

Gram-negative bacteria Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...

called ''Maritalea myrionectae'' was discovered from a

cell culture Cell culture or tissue culture is the process by which cells are grown under controlled conditions, generally outside of their natural environment. The term "tissue culture" was coined by American pathologist Montrose Thomas Burrows. This te ...

of ''M. rubrum''.

Description

''M. rubrum'' is a free-living marine ciliate. It is reddish in colour and form dark-red mass during blooming. Its body is almost spherical, looking like a miniature

sunflower The common sunflower (''Helianthus annuus'') is a large annual forb of the genus ''Helianthus'' grown as a crop for its edible oily seeds. Apart from cooking oil production, it is also used as livestock forage (as a meal or a silage plant), as ...

with its radiating hair-like

cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...

on its body surface. It measures up to 100 μm in length and 75 μm in width. The body is superficially divided into two lobes due to formation of a constriction at the centre. The constriction gives rise to a larger anterior lobe and a smaller posterior lobe. The cilia arise from the constriction. Using the cilia it can jump about 10-20 times its body length in one movement. Its nucleus is prominently situated at the centre, and is surrounded by organelles mostly derived from algae. For example, its cytoplasm contains numerous plastids,

mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...

and other nuclei. These organelles are properly separated such that the mitochondria are fully enclosed in a vacuole membrane and two

endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...

membranes of the ciliate. This indicates that the ciliate is primarily a heterotroph, but after acquiring algal plastid, it transforms into an autotroph.

The endosymbiont

Genetic analysis showed that in the American coastal areas, the primary food of ''M. rubrum'' is the algae most closely related to the free-living '' Geminigera cryophila''. But in Japanese coasts, the major algal species is '' Teleaulax amphioxeia''. When these plastid-containing algae are ingested by the ciliate, they are not digested. The plastids remain functional and provide nutrition to the ciliate by photosynthesis. In order for the plastids to be normally active, they still require enzymes, which are synthesised by the sequestered algal nuclei. The single nucleus can survive and remain genetically active up to 30 days in the cytoplasm of the ciliate. As the retention time of the prey nuclei is short, an average ''M. rubrum'' cell may contain eight algal plastids per single prey nucleus and the nuclei need to be replaced by continuous feeding on fresh algae. Thus, the algal organelles are not permanently integrated.

References

External links

''Myrionecta rubra'' at PhytopediaWorld Register of Marine Species
{{Taxonbar, from=Q4044730 Mesodinium Endosymbiotic events Species described in 1908 Marine biology