
Aphelidium

Aphelidium species are endoparasites of freshwater green algae. "Aphelidium" belongs to the phylum Aphelida, and is part of the Opisthosporidia, a sister clade to Fungi. The cells of "Aphelidium" are much smaller than the cells of its green algae host, which is protected by a robust cell wall. "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 biofuels using algaculture.
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 but have amoebic 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 analysis provided the needed resolution for "Aphelidium’s" position in the phylogenetic tree, placing it with Cryptomycota ("Rozella" and the like), and Microsporidia to form the ARM branch. The ARM branch, also known as the Opisthosporidia, forms a monophyletic sister group to fungi. 
Ecology and life cycle.
All known examples of "Aphelidium" are obligate parasites of freshwater phytoplankton. 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 contacting its host, a green alga. The singular flagellum 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 before digesting it internally in a central digestive vacuole. As the parasite expands within the host cell, it develops into a multinucleate plasmodium 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 cells which develop into zoospores, using the cell wall of the host alga as a sporangium. Finally, the uniflagellate zoospores erupt the husk of the host cell via the same puncture made by the infection tube of the parent "Aphelidium" to seek new green alga hosts.
Practical importance.
Microalgae are candidates for biofuel production, thanks to their fast generation times and low space requirements. In the high-density environment of algal agriculture, parasites can quickly propagate and devastate the algae population, which makes algal endoparasites such as "Aphelidium" important targets for study.