Quantitative analysis of trichocysts in Paramecium bursaria following artificial removal and infection with the symbiotic Chlorella variabilis

The ciliate Paramecium bursaria possesses cell organelles called trichocysts that have defensive functions. Paramecium bursaria is capable of symbiosis with Chlorella variabilis, and the symbiotic algae are situated in close proximity to the trichocysts. To clarify the relationship between trichocysts in P. bursaria and the presence or absence of the intracellular symbiotic C. variabilis, this study compared the regeneration capacity of trichocysts in alga-free and algae-bearing P. bursaria. In addition, trichocyst protein abundance was measured when alga-free P. bursaria specimens were artificially infected with Chlorella. After completely removing trichocysts from P. bursaria cells by treatment with lysozyme and observing them after 24 h, the percentage of regenerating trichocysts in the entire cell was significantly higher in alga-free cells than that in algae-bearing cells. We also developed a simple method for the isolation of high-purity trichocysts to quantify trichocyst protein amounts. There was a significant difference in the trichocyst protein abundance of P. bursaria before and one week after mixing with Chlorella (i.e., after the establishment of symbiosis with algae). This study shows the importance of trichocysts in alga-free P. bursaria as well as their competition with symbiotic C. variabilis for attachment sites during the algal infection process.