Maintenance of algal endosymbionts in Paramecium bursaria: a simple model based on population dynamics

Abstract

Algal endosymbiosis is widely distributed in eukaryotes including many protists and metazoans, and plays important roles in aquatic ecosystems, combining phagotrophy and phototrophy. To maintain a stable symbiotic relationship, endosymbiont population size in the host must be properly regulated and maintained at a constant level; however, the mechanisms underlying the maintenance of algal endosymbionts are still largely unknown. Here we investigate the population dynamics of the unicellular ciliate Paramecium bursaria and its Chlorella-like algal endosymbiont under various experimental conditions in a simple culture system. Our results suggest that endosymbiont population size in P. bursaria was not regulated by active processes such as cell division coupling between the two organisms, or partitioning of the endosymbionts at host cell division. Regardless, endosymbiont population size was eventually adjusted to a nearly constant level once cells were grown with light and nutrients. To explain this apparent regulation of population size, we propose a simple mechanism based on the different growth properties (specifically the nutrient requirements) of the two organisms, and based from this develop a mathematical model to describe the population dynamics of host and endosymbiont. The proposed mechanism and model may provide a basis for understanding the maintenance of algal endosymbionts.

Did Gause Have a Yeast Infection?

“…Furthermore, Gause () cites Pringsheim (), who suggests that P. bursaria can grow on yeast, supporting the recent work of Iwai et al. (). However, further assessment of Pringsheim () provides reference to Bozler () who states that P. caudatum cannot grow on yeast alone.…”

“…Regardless of the above speculations, in contrast to our work, Gause (1935) and Gause et al (1936) indicate that both P. aurelia and P. bursaria can be sustained on yeast. Furthermore, Gause (1935) cites Pringsheim (1928), who suggests that P. bursaria can grow on yeast, supporting the recent work of Iwai et al (2016). However, further assessment of Pringsheim (1928) provides reference to Bozler (1924) who states that P. caudatum cannot grow on yeast alone.…”

“…Also, Iwai et al. () indicate that P. bursaria ingests and grows on S. cerevisiae BY4741, even in the dark when its algal symbionts cannot photosynthesise. Why might some ciliates grow on yeast while others do not?…”

“…However, T. pyriformis and C. striatum did grow, supporting work of others that some ciliates grow on yeast (Fabrea salina, Repak 1986; Euplotes sp., de Freitas Côrtes et al 2013). Also, Iwai et al (2016) indicate that P. bursaria ingests and grows on S. cerevisiae BY4741, even in the dark when its algal symbionts cannot photosynthesise. Why might some ciliates grow on yeast while others do not?…”

“…Second, it appears that the abundance of Paramecium and yeast, in the original experiments, were manipulated during the experiments. Further use of the Paramecium-yeast pair should be discouraged, with the exception of the potential use of P. bursaria, as outlined by Iwai et al (2016). Likewise, subsequent studies that have relied on these predator-prey pair data should be viewed with caution; purely theoretical conclusions may still hold, but any application to reality must surely be disregarded.…”