Phagotrophy was investigated in the photosynthetic and ichthyotoxic Prymnesium patelliferum (Haptophyceae) using fluorescent microspheres, fluorescently labeled bacteria, and live bacteria cells. Ingestion rates were estimated both from prey uptake and disappearance experiments in phosphorus (P)-limited and -replete algal cultures. Prymnesium patelliferum was feeding preferentially on bacteria (bact) compared to fluorescent microspheres (FM). Large fluorescent microspheres (1.6 and 3 mum) were ingested at very low rates (< 0.1 FM alga(-1) h(-1)), and small microspheres (0.5 mum) were not ingested. Ingestion of bacteria (mean size 2 mum) was highest in P-limited P. patelliferum cultures (up to four bact alga(-1) h(-1)) compared to P-replete cultures (0-1.2 bact alga(-1) h(-1)). In addition, cellular P content of P-limited cells fed with bacteria became similar to those of P-replete cultures after 48 h, indicating a close relation between cellular P content and feeding behavior. Hemolytic activity of P. patelliferum was up to four times higher in P-limited cultures compared to P-replete cultures. During the transition from P-limiting to P-replete conditions, the addition of bacteria and/or the corresponding bacterial filtrate (< 0.2 mum) and/or PO43- to P-limited cultures resulted in a decrease (50%) of the hemolytic activity after 24 h in relation to controls (no addition of bacteria, filtrate. or P). No PO43- was detectable as a result of enriching cultures with bacterial cells or bacterial filtrates. These results indicate that P. patelliferum can use different sources of P (inorganic and dissolved, organic and particulate) and adapt its mode of nutrition in a short time. Furthermore, the decrease of hemolytic activity in the highly toxic P-limited cells also occurred rapidly following a recovery in cellular P status through mixotrophic feeding or uptake of inorganic phosphate, suggesting that toxicity in P. patelliferum cells can be minimized by nutrient manipulation.