Cafileria is a genus of marine microscopic protists. It is monotypic, comprising the single species Cafileria marina, described in 2019 from Norway. It is part of a clade of heterotrophic flagellates that consume bacteria, known as Bicosoecida, a basal lineage of Stramenopiles. Due to its small size it is described as a nanoflagellate. It is the only organism where direct connections between mitochondria and the cell nucleus have been observed. Another peculiarity of C. marina is the change in shape of the Golgi apparatus during the cell cycle.
Discovery
Cells of Cafileria marina were sampled from part of an algal mat community in a rock surface from Kvernesfjord, Norway. Their morphology, ultrastructure, flagellar apparatus and mitochondrial genome were investigated. The results, along with the formal taxonomic description of Cafileria marina, were published in 2019 by Czech researchers Dagmar Jirsová, Zoltán Füssy, Jitka Richtová, Ansgar Gruber and Miroslav Oborník.
The hapantotype of C. marina was deposited under the name IP CAS Pro 59 in the slide collection at the Biological Centre of the Czech Academy of Sciences in České Budějovice, Czech Republic.
Etymology
Cafileria is named after , the Czech name for a rendering plant where the biomass of animal origin is transformed for the production of lipids, glue and fertilizers. In a parallel manner, Cafileria feasts on bacteria and recycles organic materials that are part of their biofilm habitat. The species epithet marina is due to the marine origin of the species.
Cell structure
External appearance
Cells of C. marina are rounded on the right side and flattened on the left side, resembling the shape of a “D”. The cell body is 3–4 µm wide and 5–6 µm long, making it a nanoflagellate by size. The cellular surface is smooth, without any features (no lorica, cell wall, etc.) visible by light or scanning electron microscopy. Like other bicosoecids, they have two smooth flagella (anterior and posterior), with an equal length of around 1.5–2 times the length of the cell body. The flagella are in a sub-apical position and emerge from a dent on the ventral side.
Organelles
C. marina cells localize their nucleus and mitochondria with tubular cristae (as is common in Stramenopiles) in the anterior part of the cell. A peculiar phenomenon in C. marina is that, in young cultures (≤ 2 weeks old), the nucleus and mitochondria are tightly connected through junctions. Although clustering of mitochondria near the nucleus is seen in mammalian tissues, this is the first time that a full connection between these compartments has been described. Various functions for this peculiar connection have been hypothesized: (i) enabling the exchange of ATP/ADP between the two organelles, thereby providing the high energy needed by the nucleus, (ii) facilitating the transport of necessary nuclear tRNA that the mitochondria cannot produce, (iii) transporting mRNA to be translated in the mitochondria, (iv) equally segregating mitochondria to the daughter cells after mitosis, (v) or simply a more efficient use of the limited space in a small cell size.
The Golgi apparatus is in the anterior part of the cell, with its 4–5 cisternae aligned parallel to the nuclear envelope. During the cell cycle, the shape of the Golgi cisternae changes from flat-stacked to rounded: the flat cisternae curve inside and create hollowed rounded shapes. A similar phenomenon happens in mammalian cells, in association with changes in sphingomyelin metabolism, but in the case of Cafileria the mechanism responsible is unknown.
Several small vesicles are scattered across the cytosol, while food vacuoles are considerably larger and are localized in the posterior part of the cell, occupying almost one third of its volume. Some of the food vacuoles can contain intact or partially digested bacteria.
Flagellar apparatus
C. marina has its two flagella attached to four roots made of microtubules. There are two basal bodies, in the anterior (front) part of the cell, at a 45° angle to each other, connected to each other through a striated fiber. The flagella each have an axoneme structure with two central microtubules and a circle of nine microtubules around them. The four roots (named R1, R2, R3 and R4) have 8, 3, 1, and 1 microtubules respectively, an arrangement unique to C. marina.