Nassula is a genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of unicellular ciliate

Ciliate

The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella...

 protists, belonging to the class Nassophorea

Nassophorea

The Nassophorea are a class of ciliate protozoa. Members are free-living, usually in freshwater but also in marine and soil environments. The mouth is anterior ventral and leads to a curved cytopharynx supported by a prominent palisade of rods or nematodesmata, forming a structure called a cyrtos...

. Like other members of the class, Nassula possesses a basket-like feeding apparatus (nasse, or cyrtos) made up of cytopharyngeal rods (nematodesmata), which are themselves composed of closely packed microtubules. Nassula use this structure to ingest filamentous cyanobacteria, drawing individual strands of blue-green algae through the cytopharynx and into the body of the cell, where they are digested. As the algae are broken down, they can take on a variety of bright colours, which give Nassula a distinctive, variegated appearance under the microscope.

Appearance and characteristics

The body is ovoid to elongate, and uniformly ciliated, with a single macronucleus and a partial hypostomial frange (synhymenium) running from the left side of the cell to the oral aperture. It has been proposed that the elongated species Nassula elegans is actually a form taken by the species Nassula ornata when it is deprived of food. When food is scarce, members of the genus have the ability to become dormant by forming a microbial cyst

Microbial cyst

A microbial cyst is a resting or dormant stage of a microorganism, usually a bacterium or a protist, that helps the organism to survive in unfavorable environmental conditions. It can be thought of as a state of suspended animation in which the metabolic processes of the cell are slowed down and...

. Excystment can be induced by exposure to a medium inoculated with wild bacteria.

Reproduction

Nassula reproduce asexually, by binary fission. During reproduction, the cell is divided transversally. As in most ciliates (with the exception of the Karyorelictea

Karyorelictea

All ciliate possess multiple dimorphic nuclei which have separated the functions of protein synthesis, preformed by the macronuclei or somatic-nuclei, and heritable DNA replication, performed by the micronuclei or germ-line nuclei...

) the macronucleus

Macronucleus

A macronucleus is the larger type of nucleus in ciliates. Macronuclei are polyploid and undergo direct division without mitosis. It controls the non-reproductive cell functions, the everyday tasks, such as metabolism...

 splits, during division, and the micronuclei undergo mitosis.

Nassula also reproduce through conjugation, during which compatible mating individuals come together and transfer genetic material across a cytoplasmic link. In conjugation, the micronuclei of each cell undergo meiosis

Meiosis

Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....

, and haploid micronuclei are then exchanged from one cell to the other.