BACTERIVORY BY THE CILIATE EUPLOTES IN DIFFERENT STATES OF HUNGER

The feeding of the marine ciliate Euplotes mutabilis was studied using bacteria (Vibrio natriegens) doubly labelled with H-3-thymidine and C -14-leucine. In the presence of abundant bacteria (30 x 10(6) bacteria ml(-1)), an average Euplotes cell (initially without food vacuoles) w ith a protein content of 12 ng consumed 16 x 10(3) bacteria in the fir st hour and 27 x 10(3) bacteria over four hours, accumulating about 60 % of the bacterial protein into ciliate macromolecules. Euplotes which had been starved or under-fed to reduce cell protein biomass to 7 or 9 ng consumed significantly fewer bacteria, but the gross growth effic iency for protein did not change. The rate of consumption of bacteria by large Euplotes of protein content 15 ng was initially less than tha t of 12 ng cells, and it decreased markedly before the end of a 4-hour experiment. Recently divided cells ingested bacteria rapidly, but sho wed a reduced gross growth efficiency of about 40%. At low bacterial c oncentrations (6 x 10(6) bacteria ml(-1)) the rates of ingestion were markedly reduced to between 1/2 and 1/3 of maximal levels; the smalles t cells could not sustain feeding activity at the low prey concentrati on and gross growth efficiency fell from 43 to 20% during a 4-hour exp eriment. The strategy adopted by Euplotes in response to local fluctua tions in food supply involves rapid consumption with high growth effic iency in times of plenty, but slow shrinkage without cell division to survive in times of shortage.