Effects of nutrient depletion on ejectisomes of cryptophytes
Rhiel, Erhard; Ammermann, Silke; Hillebrand, Helmut
Ejectisomes are extrusive organelles found in cryptophytes with a potential role in grazer avoidance/deterrence. In order to test whether ejectisome concentrations in the cells vary with nutrient supply, two Cryptomonas and a Chroomonas species were subjected to nutrient starvation (i. e., depletion of N, P, or S), and compared to nutrient-replete controls. Since changes in pigmentation were observed after two weeks, cells were harvested by centrifugation after three weeks, lysed, and analysed further after adjustment to equal protein concentrations. For all three strains, absorption spectroscopy revealed significant reductions of proteins functioning in photosynthesis (i. e., biliproteins and chlorophyll bearing polypeptides) under N-starvation. Depletion of P and S caused significant losses of these proteins for the strain Cryptomonas S2, too. Sodium dodecyl sulfate polyacrylamide gel electrophoresis followed by Western immunoblotting using an antiserum directed against reconstituted, ejectisome-like filaments revealed significant reductions in the amounts of ejectisins for N-starved cells in all three species and for P- and S-depleted cells of the Chroomonas species. The reduction in ejectisins was not significant for the two Cryptomonas strains when P- or S-starved. The maintenance of at least a certain amount of ejectisomes obviously obtains a similar priority as the maintenance of a minimal photosynthetic apparatus.