Investigation of prey ingestion and digestion in the ciliate predator Tetrahymena pyriformis and the impact on growth
This study evaluated the effect of four prey states on ingestion, digestive vacuole (DV) formation, digestion and growth in the ciliate Tetrahymena pyriformis. The prey included several strains of live RFP-expressing heterotrophic bacteria (Live), heat-killed-DTAF-stained heterotrophic bacteria (Dead), live autotrophic Synechococcus species (Pico) together with fluorescently-labelled microspheres (FLMs). Prey biovolume did not affect ingestion rates over the 5 minute period tested, therefore the effect of the different prey states could be directly compared. Live heterotrophic bacteria were able to quickly upregulate DV formation rate; something not demonstrated by any other prey. All prey, except FLMs, showed controlled vacuole filling over a 20 minute period whereby new DVs contained an equivalent number of prey to those in older vacuoles. Live heterotrophic and autotrophic prey yielded a significantly lower maximum number of prey/vacuole compared to Dead cells, and whereas significant differences in prey/vacuole between bacterial species were discerned for Dead species, no such differences were evident for Live species. It was hypothesised that ingestion of prey is receptor-mediated and that the predominant pathway into the ciliate for Live prey is the lectin-receptor-based route, and that for inert prey is the non-specific-receptor-mediated route, with a yet unidentified route for Dead prey. Using pulse-chase experiments, the digestion of Live and Dead heterotrophic bacteria were found to be equivalent, i.e. ca. 50% of ingested cells within a DV were digested. There was however a very strong relationship (for both prey states) between biovolume of prey in the vacuole and the total biovolume of prey digested in that vacuole implying that larger prey cells might lead to higher ciliate specific growth rates. This was not found to be the case. No relationship between ciliate specific growth rate was discerned with prey biovolume, or even digestion rates, as % prey/vacuole/min over the digestion period (where there is a linear decline in prey due to digestion). The digestion period (DP) was considerably shorted with Dead cells and it was hypothesised that, due to an incomplete suite of ligands, DVs did not go through the complete sequence of fusion events with lysosomes. Digestion of prey had little effect on ciliate growth rates. However there was no obvious relationship between growth rate and any parameter tested. This suggests that digestion and growth are too far removed with regards to cellular processes. In conclusion, the findings suggest that there are two prey recognition systems in T. pyriformis, one which recognised prey particles and leads to ingestion, and a second, present in the DV, which recognises the type of prey enclosed within it and leads to digestion behavioural changes.
Networked Digital Library of Theses and Dissertations