Diel changes in phagotrophy by Cryptomonas in Lake Biwa

Diel changes in bacterial ingestion by a mixotrophic flagellate, Cryptomona s sp., and heterotrophic nanoflagellates (HNF) were examined in situ at 4-h intervals for 2 d in the epilimnion and metalimnion of Lake Biwa using bac teria-sized fluorescent microspheres as a tracer food. Clearance rates of H NF for the microspheres ranged between 1.3 and 4.5 nl cell(-1) h(-1), but t he average rate did not differ between day and night. In contrast, clear di el changes were observed in the clearance rate of Cryptomonas sp. in the ep ilimnion from <0.5 nl cell(-1) h-l at midnight to >3 nl cell(-1) h(-1) at n oon. in the metalimnion where light intensity was lower, however, the clear ance rate of Cryptomonas sp. was always <0.5 nt cell(-1) h(-1) through the study period. Thus, bacterial ingestion of Cryptomonas sp. is not to acquir e supplementary energy or carbon at low phototrophic activities. During the study period, both inorganic phosphorus and nitrogen concentrations were l ess than or close to the detection limits (10 nM P and 1 <mu>M N) in the ep ilimnion, but much higher in the metalimnion. The results strongly support the idea that Cryptomonas sp. utilizes N and P from bacteria as substitutab le nutrients when photosynthesis takes place under conditions of nutrient d epletion. To assess the grazing effect of mixotrophic algae on bacterial po pulations, it is essential to consider diel changes in their phagotrophic m ode of nutrition that are induced by light regime and nutrient concentratio ns in ambient water.