Bacterivory by heterotrophic nanoflagellates and ciliates has been widely studied in aquatic environments, but data on the grazing of amoebae, are still scarce. From the water samples of Dianchi Lake (Kunming, Yunnan Province, China), we isolated an amoeba, designated as Naegleria sp. strain W2, which had potent grazing effects on some kind of cyanobacteria. The food selection mechanism and the digestion process of the amoeba were investigated in batch experiments. Predation experiments showed that filamentous cyanobacteria (e.g., Anabaena, Cylindrospermum, Gloeotrichia, and Phormidium) were readily consumed, with clearance rates ranging from 0.332 to 0.513 nL amoeba(-1) h(-1). The tight threads (Oscilltoria) and aggregates (Aphanizomenon) could not be ingested; however, their sonicated fragments were observed inside food vacuoles, suggesting that their morphologies prevent them from being ingested. Live video microscopy noted that unicellular Chroococcaceae (e.g., Synechococcus, Aphanocapsa, and Microcystis) were excreted after ingestion, indicating that food selection takes place inside food vacuoles. To determine whether the tastes or the toxins prevented them from being digested, heat-killed cells were retested for predation. Digestion rates and ingestion rates of the amoebae for filamentous cyanobacteria were estimated from food vacuole content volume. Through a ``cold-chase'' method, we found that the food vacuole contents declined exponentially in diluted amoebae cells, and digestion rates were relatively constant, averaging about 1.5% food vacuole content min(-1) at 28 degrees C. Ingestion strongly depended on the satiation status of the amoebae, starved amoebae fed at higher rates compared with satiated amoebae. Our results suggest that the food selection and food processing mechanisms of the amoeba are similar to those of interception feeding flagellates; however, filamentous cyanobacteria cannot obtain a refuge under the grazing pressure of phagotrophic amoebae, which may widen our knowledge on the grazing of protists.

A helical filamentous cyanobactericidal bacterium was isolated from Dianchi Lake, a eutrophic freshwater lake in Kunming City of the Yunnan Province in China using a special solid medium. This species was designated strain PdY3. This bacterium was identified as a novel Saprospira sp. on the basis of its morphological characteristics and 16S rDNA sequence. Strain PdY3 showed apparent group behavior on the solid medium, forming orderly, bundle-like group structures. These bundles moved as groups. Individuals in a bundle responded to the bundle as a whole. PdY3 also showed group behavior and formed a three-dimensional reticular structure when co-cultured with Anabaena in liquid media. This helical bacterium lysed cyanobacteria through direct contact and its group behavior greatly accelerated the cyanobactericidal process. Our experiments showed that PdY3 caused lysis of 64% of Anabaena cells within 1 day and that its cyanobactericidal range was broad. These results underscore potential application of Saprospira on the control of blooms of cyanobacteria. PdY3 group behavior might allow a more efficient capture of bacterial prey.