Paramecium bursaria

Paramecium bursaria is a species of ciliate found in marine and brackish waters. It has a mutualistic endosymbiotic relationship with green algae called Zoochlorella. About 700 Chlorella cells live inside the protist's cytoplasm and provide it with food, while the Paramecium provides the algae with movement and protection. P. bursaria is 80–150 μm long, with a wide oral groove, two contractile vacuoles, and a single micronucleus as well as a single macronucleus. P. bursaria is the only species of Paramecium that forms symbiotic relationships with algae, and it is often used in biology classrooms both as an example of a protozoan and also as an example of symbiosis.

A transcriptome sequence is determined.

Moreover, a combination of long-read sequencing (PacBio) and short-read sequencing (Illumina) can be used to assemble a high-quality, near-complete macronuclear genome of P. bursaria, providing insights into the mechanism of endosymbiosis. The genome size of P. bursaria is much smaller than that of other ciliate species but is comparable to P. caudatum.

Additionally, the standard core genes in the assembled genome of P. bursaria (94.8%) are similar to those of other well-studied ciliate genomes, such as T. thermophila (89.9%), O. trifallax (93.1%), and P. tetraurelia (92.7%).

Paramecium bursaria harbors approximately 700 cells of zoochlorellae (green algae) from the genera Chlorella or Micractinium under its cell cortex, forming endosymbionts. The core principle of these endosymbionts is nutrition, where the host obtains nutrients through phagotrophy by engulfing cells or particles, including Chlorella, which are digested in the digestive vacuole (DV). However, some Chlorella cells can resist this digestive process and become enclosed in a distinct vacuole, known as the perialgal vacuole (PV), formed by the host. This vacuole prevents lysosomal fusion, allowing the algae to survive and establish symbiosis. The PV can be distinguished from the digestive vacuole (DV) by the number and distribution of particles, exhibiting minimal endocytosis or exocytosis activity.

Moreover, the mechanism of the symbiotic relationship between Paramecium bursaria and Chlorella, based on comparative genome analysis, suggests that the host provides glutamine and magnesium. Chlorella utilizes glutamine as a nitrogen source and magnesium to support chlorophyll-based photosynthesis. In return, Chlorella within the perialgal vacuole (PV) provides the host with photosynthetic products such as fructose, maltose, and oxygen.

It is also worth highlighting how the host regulates nutrient exchange in this photosynthetic symbiosis. Andrew et al. (2016) developed a mathematical model, using the framework of general symbiotic relationships, to describe the mechanism of this interaction. During vertical transmission of symbionts through cytokinesis, daughter cells receive an equal number of symbionts as their parent cells. However, during horizontal transmission, the daughter cells acquire only half the symbionts. The host's growth rate is influenced by its nutritional state, which depends on the efficiency of nutrient exchange with the symbionts. By managing this exchange, the host can optimize its growth and the overall benefits of the symbiosis.

Focusing on light levels as an environmental condition, the study investigated how the host regulates this mechanism. The results show that as light levels increase, there is a reduction in the symbiont population. This regulation is crucial for maintaining a stable symbiont population and preventing parasitism, which could occur if the symbionts were to overgrow and harm the host.