''Micractinium'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of microscopic

green algae The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ...

in the family

Chlorellaceae Chlorellaceae are a family of green algae in the order Chlorellales. About 250 species are currently accepted in the family. Members of the family are distributed worldwide and are common in a variety of freshwater, terrestrial and marine enviro ...

. Species of the genus ''Micractinium'' are found as

phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...

, and are commonly found in freshwater habitats around the world. A few species are found as

endosymbiont An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...

s of

ciliate 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 ...

s. There is increasing interest in ''Micractinium'' due to its high growth rate and

lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...

production.

Description

Cells of ''Micractinium'' are spherical to ellipsoidal, and are solitary or stuck together in colonies. Colonies can contain up to 128, occasionally 256 cells, and are polyhedral or pyramidal in shape. Most species generally produce long narrow bristles that taper from base to tip. Cells have a single cup-shaped

chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

with a single

pyrenoid Pyrenoids are sub-cellular phase-separated micro-compartments found in chloroplasts of many algae,Giordano, M., Beardall, J., & Raven, J. A. (2005). CO2 concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution. ''An ...

. Bristles consist of

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

and lack cellulosic fibers; they are produced after the cell wall. ''Micractinium'' reproduces asexually via the formation of

autospore Autospores are a type of spores that are produced by algae to enable asexual reproduction and spread. They are non-motile and non-Flagellated cell, flagellated Aplanospore, aplanospores that are generated within a parent cell and have the same sha ...

s (usually four per cell), which are released from by a rupture in the parental

cell wall A cell wall is a structural layer that surrounds some Cell type, cell types, found immediately outside the cell membrane. It can be tough, flexible, and sometimes rigid. Primarily, it provides the cell with structural support, shape, protection, ...

. Autospores generally remain attached to each other to form colonies, but occasionally detach and develop into single cells; when this happens, the number and arrangement of spines can differ from the typical morphology. In one species, '' Micractinium pusillum'',

oogamy Oogamy is a form of anisogamy where the gametes differ in both size and form. In oogamy the large female gamete (also known as ovum) is immotile, while the small male gamete (also known as spermatozoon) is mobile. Oogamy is a common form of an ...

has been reported, but this needs reinvestigation.

Ecology

Members of the genus ''Micractinium'' are common in a variety of freshwater to

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...

habitats, and can tolerate a wide array of temperatures, ranging from 0°C in Antarctica to hot springs over 70°C. They play an important role in the ecosystem, as

primary producer Primary or primaries may refer to: Arts, entertainment, and media Music Groups and labels * Primary (band), from Australia * Primary (musician), hip hop musician and record producer from South Korea * Primary Music, Israeli record label Work ...

s and producers of oxygen. They also purify the water they inhabit via accumulation and mineralization of pollutants. Some species, such as ''Micractinium conductrix'' are obligate

s of ciliates; these species cannot produce grow without

vitamin B1 Thiamine, also known as thiamin and vitamin B1, is a vitamin – an essential micronutrient for humans and animals. It is found in food and commercially synthesized to be a dietary supplement or medication. Phosphorylated forms of thia ...

and

vitamin B12 Vitamin B12, also known as cobalamin, is a water-soluble vitamin involved in metabolism. One of eight B vitamins, it serves as a vital cofactor (biochemistry), cofactor in DNA synthesis and both fatty acid metabolism, fatty acid and amino a ...

and thus obtain them from their hosts.

Taxonomy

The genus ''Micractinium'' was formerly placed in the family Micractiniaceae, characterized by sexual reproduction without the production of zoospores, and colonies covered with bristles.

Molecular phylogenetic Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

studies showed that Micractiniaceae was

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

and that ''Micractinium'' was closely related to ''Chlorella''. Accordingly, ''Micractinium'' has been transferred to the family

. Both ''Chlorella'' and ''Micractinium''

sporopollenin 270px, SEM image of pollen grains Sporopollenin is a biological polymer found as a major component of the tough outer (exine) walls of plant spores and pollen grains. It is chemically very stable (one of the most inert among biopolymers) and is ...

in their

s, and have pyrenoids that are transversed by

thylakoid membrane Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thyl ...

s and covered in a starch sheath. Traditionally, ''Micractinium'' has been distinguished from ''Chlorella'' on the basis of forming colonies and bristles. However, ''Micractinium'' sometimes does not produce bristles and colonies—it appears to be an inducible defense against grazers such as

rotifer The rotifers (, from Latin 'wheel' and 'bearing'), sometimes called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic Coelom#Pseudocoelomates, pseudocoelomate animals. They were first describ ...

s. Additionally, some species of ''Micractinium'' do not produce bristles, such as those that are endosymbiotic within other organisms. The genus ''Diacanthos'', distinguished by having exactly two spines on opposite sides of the cell, is a synonym of ''Micractinium''.

Species

AlgaeBase AlgaeBase is a global species database of information on all groups of algae, both seaweed, marine and freshwater algae, freshwater, as well as sea-grass. History AlgaeBase began in March 1996, founded by Michael D. Guiry, Michael Guiry. Text ...

accepted the following species: *'' Micractinium appendiculatum'' Korshikov *'' Micractinium belenophorum'' (Korshikov) T.Proschold, C.Block, W.Luo & L.Kreinitz *'' Micractinium bornhemiense'' (W.Conrad) Korshikov *''

Micractinium conductrix ''Micractinium'' is a genus of microscopic green algae in the family Chlorellaceae. Species of the genus ''Micractinium'' are found as phytoplankton, and are commonly found in freshwater habitats around the world. A few species are found as endos ...

'' (K.Brandt) Pröschold & Darienko *'' Micractinium conococcoides'' T.Hortobagyi *'' Micractinium crassisetum'' Hortobágyi *'' Micractinium depressum'' C.-C.Jao & Ling *'' Micractinium elongatum'' (H.J.Carter) Hegewald & Schnepf *'' Micractinium extremum'' Hortobágyi *''

Micractinium inermum ''Micractinium'' is a genus of microscopic green algae in the family Chlorellaceae. Species of the genus ''Micractinium'' are found as phytoplankton, and are commonly found in freshwater habitats around the world. A few species are found as endos ...

'' R.Hoshina & Y.Fujiwara *'' Micractinium kostikovii'' E.Krivina & A.Temraleeva *'' Micractinium parvisetum'' Walton *'' Micractinium parvum'' Hindák *'' Micractinium pusillum'' Fresenius *'' Micractinium quadrisetum'' (Lemmermann) G.M.Smith *'' Micractinium simplicissimum'' H.Chae, H.-G. Choi & J.H.Kim *'' Micractinium singularis'' H.Chae, H.-G.Choi & J.H.Kim *'' Micractinium strigoniense'' T.Hortobagyi *'' Micractinium tetrahymenae'' Pröschold, Pitsch & Darienko *'' Micractinium valkanovii'' Vodenicarov *'' Micractinium variabile'' H.Chae, H.-G.Choi & J.H.Kim Species have traditionally been defined based on morphological characteristics, such as cell shape, colony shape, and number and size of bristles. However, ''Micractinium'' displays a large amount of

phenotypic plasticity Phenotypic plasticity refers to some of the changes in an organism's behavior, morphology and physiology in response to a unique environment. Fundamental to the way in which organisms cope with environmental variation, phenotypic plasticity encompa ...

, such as producing or not producing bristles; this makes species delimitation based on morphological alone unreliable. This means there is a high amount of

cryptic diversity In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...

within ''Micractinium''.

References

{{Taxonbar, from=Q6838982 Trebouxiophyceae genera Chlorellaceae Freshwater algae