Growth phase and elemental stoichiometry of bacterial prey influences ciliate grazing selectivity

Protozoa are known to selectively graze bacteria and can differentiate prey based on size and viability, but less is known about the effects of prey cellular composition on predator selectivity. We measured the effect of growth phase and elemental stoichiometry of Escherichia coli on grazing by two ciliates, Euplotes vannus and Cyclidium glaucoma. Bacterial cells of a single strain were transformed with green and red fluorescent protein and harvested from culture at differing growth stages. Cells in exponential growth phase had low carbon:phosphorus (39) and nitrogen:phosphorus (9) ratios, while cells from stationary phase had high carbon:phosphorus of 104 and nitrogen:phosphorus of 26. When offered an equal mixture of both types of bacteria, Cyclidium grazed stationary phase, high carbon:phosphorus, high nitrogen:phosphorus cells to 22% of initial abundance within 135 min, while Euplotes reduced these cells to 33%. Neither ciliate species decreased the abundance of the exponential phase cells, lower carbon:phosphorus and nitrogen:phosphorus, relative to control treatments. Because protozoa have higher nitrogen:phosphorus and carbon:phosphorus ratios than their prokaryotic prey, this study raises the possibility that it may be advantageous for protozoa to preferentially consume more slowly growing bacteria.