N-REMINERALIZATION IN PLANKTONIC PROTOZOA

Ammonium excretion rates of two marine planktonic protozoa (an oligotrichous ciliate, Strombidium sulcatum, and an aplastidic flagellate, Pseudobodo sp.) both isolated from oligotrophic Mediterranean waters were quantified with heat-killed bacteria as the food source for the protozoans. Ammonium excretion rates ranged from 0.25 to 2 mug N mg DW-1 h-1 for the ciliate and from 2.8 to 8.5 for the phagotrophic flagellate. Maximum excretion rates took place during the early exponential growth phase, suggesting possible covariation of excretion rates with either protozoan body size or their growth rates or metabolic state. Comparison of these excretion and regeneration rates with those obtained in situ for oligotrophic waters (approximately 0-37 nmol liter 1 h-1) indicates that protozoa may account for >50% of the nutrient recycling in these waters