''Aphelidium'' species are

endoparasites Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

of freshwater

green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...

. ''Aphelidium'' belongs to the phylum Aphelida, and is part of the

Opisthosporidia Opisthosporidia is a superphylum of intracellular parasites with amoeboid vegetative stage, defined as a common group of eukaryotic groups Microsporidia, Cryptomycota (also known as Rozellida, Rozellomycota, or Rozellosporidia) and Aphelidea. ...

, a sister

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

Fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

.Karpov, S. A., Mamkaeva, M. A., Aleoshin, V. V., Nassonova, E., Lilje, O., Gleason, F. H.. 2014: Morphology, phylogeny, and ecology of the aphelids (Aphelidea, Opisthokonta) and proposal for the new superphylum Opisthosporidia. Front. Microbiol.. https://doi.org/10.3389/fmicb.2014.00112 The cells of ''Aphelidium'' are much smaller than the cells of its green algae host, which is protected by a robust

cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...

. ''Aphelidium'' have evolved a remarkable life cycle to defeat host's defenses. The infection process for ''Aphelidium'' is notable for its method of entry. An ''Aphelidium'' cyst attached to a potential host will raise its internal pressure by expanding the posterior vacuole before using the sudden release of this pressure to defeat the host cell wall and jet itself into the host. As parasites of green algae, ''Aphelidium'' have important implications for the production of

biofuel Biofuel is a fuel that is produced over a short time span from biomass, rather than by the very slow natural processes involved in the formation of fossil fuels, such as oil. According to the United States Energy Information Administration (E ...

s using algaculture.Ding, Y., Peng, X., Wang, Z., Wen, X., Geng, Y., Li, Y.. Isolation and Characterization of an Endoparasite from the Culture of Oleaginous Microalga ''Graesiella'' sp. WBG-1. Algal Research. 29: 371-379

History of research

''Aphelidium'' was first described in 1885 by Wilhelm Zopf, however the current phylogenetic organisation was not solidified until the 2010s. In the first half of the 20th century, ''Aphelidium'' was put in the Monadinea group, a group of organisms with life cycles resembling a

fungus A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...

but have

amoebic An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by extending and ret ...

trophic stages. Beginning from the 1950s ''Aphelidium'' were included in class Rhizopoda, and for the rest of the 20th century while studies in ''Aphelidium'' life cycles and ecology continued ''Aphelidium'' was not included in the updated phylogenetic classifications. Interest in classifying ''Aphelidium'' renewed in the 21st century when class Aphelidea was established by Gromov, which includes ''Aphelidium, Amoeboaphelidium,'' and ''Pseudaphelidium''.

rRNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosoma ...

analysis provided the needed resolution for ''Aphelidium’s'' position in the

phylogenetic tree A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...

, placing it with Cryptomycota (''Rozella'' and the like), and

Microsporidia Microsporidia are a group of spore-forming unicellular parasites. These spores contain an extrusion apparatus that has a coiled polar tube ending in an anchoring disc at the apical part of the spore. They were once considered protozoans or prot ...

to form the ARM branch. The ARM branch, also known as the

, forms a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

sister group to

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

Ecology and life cycle

All known examples of ''Aphelidium'' are

obligate parasite An obligate parasite or holoparasite is a parasitic organism that cannot complete its life-cycle without exploiting a suitable host. If an obligate parasite cannot obtain a host it will fail to reproduce. This is opposed to a facultative parasite, ...

s of

freshwater phytoplankton Freshwater phytoplankton is the phytoplankton occurring in freshwater ecosystems. It can be distinguished between limnoplankton (lake phytoplankton), heleoplankton (phytoplankton in ponds), and potamoplankton (river phytoplankton). They differ in s ...

. As parasites, the life cycle of ''Aphelidium'' is closely tied to its ecological relationship with its hosts. The cycle begins with the motile ''Aphelidium''

zoospore A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Diversity Flagella types Zoospores may possess one or mo ...

contacting its host, a

green alga The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...

. The singular

flagellum A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...

of the zoospore is lost as part of the attachment process. A pseudopodium extends from the zoospore and probes the host surface, seeking a weakness or gap in the cell wall. The attached zoospore first encysts then inserts an infection tube into the host cell, priming the ingress of the ''Aphelidium''. The cyst forms a posterior vacuole, which expands and raises the internal pressure of the cyst. Ultimately the pressure pushing against the chitin wall of the cyst punctures the cell wall of the host green alga at the point of insertion of the infection tube, and the ''Aphelidium'' enters its host abruptly, leaving the cyst cell wall behind. Once within the host, ''Aphelidium'' becomes an amoeboid that proceeds to consume the host from the inside out by phagocytizing host

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...

before digesting it internally in a central digestive vacuole.Schnepf, E., Hegewald, E., Soeder, J.. 1971: Elektronenmikroskopische Beobachtungen an Parasiten aus ''Scenedesmus''-Massenkulturen. Archiv für Mikrobiologie. 75(3): 209-299 As the parasite expands within the host cell, it develops into a multinucleate

plasmodium ''Plasmodium'' is a genus of unicellular eukaryotes that are obligate parasites of vertebrates and insects. The life cycles of ''Plasmodium'' species involve development in a blood-feeding insect host which then injects parasites into a vert ...

which grows to eventually replace the entirety of the host cytoplasm. Now all that remains of the green alga is its cell wall and the residual body, a clump consisting of host cell fragments indigestible to the parasite. The ''Aphelidium'' plasmodium then proceeds to divide into

uninucleate {{Short pages monitor