Microheliella is a monotypic genus of protists containing the sole species M. maris, first described in 2012. It has a variety of unusual morphological characteristics which make its broader classification difficult. These include a centrosome with two concentric granular shells and axopodia much simpler in structure than in visually similar protists (other 'heliozoa').[1]
The generic name Microheliella, as well as the names of the family Microheliellidae and order Microhelida, derive, referring to the small cell size and the authors' wishes to retain the original informal name 'microheliozoan' given to this organism prior to its formal description. The specific epithet maris refers to seawater, its natural habitat.[1]
The genus Microheliella was described from an organism discovered contaminating a culture of the amoeba Cochliopodium from the Ebro Delta, Spain, collected in 2003 by Alexey Smirnov. This organism was subsequently transferred into a pure culture through serial dilution and maintained at Oxford for nine years in a growth medium of 50% artificial seawater, with naturally occurring bacteria as its food. Initially it was given the informal name 'marine microheliozoan', and a phylogenetic analysis using 18S and 28S rRNA sequences placed it close to centrohelids.[2] Later, the same organism was examined under electron microscopy and its Hsp90 gene was sequenced. Combining the newly obtained morphological and genetic data, it was formally described by Thomas Cavalier-Smith and Ema E. Chao in 2012 as the novel species Microheliella maris. Its type (biology) culture, CCAP 1945/1, is kept in the Culture Collection of Algae and Protozoa at Oban, in Scotland.[1]
To accommodate this genus, the monotypic family Microheliellidae and order Microhelida were created, and classified as part of the phylum Cryptista in accordance to phylogenetic analyses.[1] Later, phylogenomic analyses demonstrated its position as the sister clade to all Cryptista, in a clade known as Pancryptista.
Recent phylogenomic analyses suggest the microhelida are sister to the Cryptista, forming a clade called Pancryptista, which would be sister to the Archaeplastida.[3]