Microbe Profiles: Didinium
Carnivorous Pond-Dwellers Used as Models for Predator-Prey Dynamics
About Didinium
Didinium is a genus of eukaryotic ciliate. Like human cells, the DNA of a Didinium cell is wound up and compressed, and then packaged into a nucleus. As ciliates, their bodies are adorned with thousands of small, hair-like fibers that allow them to move quickly enough to ensnare and capture prey.
Habitat
Didinia are typically found in still, fresh waters. However, some species show a preference for marine habitats. A species by the ominous name Didinium gargantua was first isolated from the extreme waters of the Arctic Ocean in 1910.
Size
Often described as “barrel-shaped”, Didinia are typically around 100 micrometers long — too small to be viewed by the naked eye.
Taxonomy
Didinium is a genus-level classification that falls somewhere under the group Eukarya. That means that their cells contain inner, membrane-bound compartments. This is the same important distinction separates the cells of humans and plants from the cells of Bacteria and Archaea. There are at least 10 described species within Didinium, but Didinium nasutum seems to be the most thoroughly studied.
Diet
Didinia are carnivorous microbes that feed on Paramecia, dinoflagellates, or occasionally green-algae. Many of the studies conducted on species of Didinium focus on their ability to capture and consume prey. Most species of Didinium have a strong preference for Paramecia, which they hunt and capture before consumption. To do this, they use special dart-like organelles called ‘trichocysts’, and a sucking apparatus called a proboscis to ensnare and consume their prey. A study conducted in 1980 described how hungry Didinia automatically “snare” paramecia upon collision, but did not display that behavior when the Didinium had already eaten when it collided with a glass plane. Didinia have to eat — and hunt, every few hours, but if food sources are scarce, they can encyst themselves until food becomes more available.
Cellular Organization and Reproduction
Another unique thing about Didinia is the organization of their genetic material. In the image above, the hook-shaped organelle represents the nucleus of the cell, a stark contrast from the typical, spherical nucleus used in most diagrams of the eukaryotic cell. The genes within the nuclei are made up of DNA, which gets translated into RNA. This RNA serves as a template for gene products through specific triplet sequences (called codons). Codons “tell” ribosomes (in a chemical sort of way) which amino acids should be linked together and, importantly, when the chain should end.
Typically, eukaryotic cells have three stop codons: UAG, UGA, and UAA. But in Didinium and other ciliates, some evidence suggests variants of this universal genetic code may be used instead. In some organisms, only one of these codons seems to function as a stop, while others seem to encode amino acids (more information about this possibility can be found in the citations/additional resources). The nuclei of the cell changes in structure throughout the reproduction process which, in Didinium and all other ciliates, can occur sexually or through binary fission (cell splitting) depending on the conditions.
What we can learn
Didinium is a fascinating taxon that can offer many insights into biology. Not only is the taxa a wonderful demonstration of biodiversity and the kind of life that can exist, but it also serves as a model for the study of predator-prey relationships at the micro- and macroscopic levels.
Learn more and view the resources used in the making of this post here.
Check out my last post — about the many charms of dinoflagellates — below.
