科研成果

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.

The heterogeneous reaction of NO2 on NaCl particles has been investigated with the new sample preparation and the mode of gas-solid free diffusion. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) is used to characterize the adsorbed products in situ, combined with Ion Chromatographic (IC), X-ray Photoelectron Spectroscopy (XPS) and Scan Electron Microscopy (SEM). Our results indicate that the reaction is not limited to the surface of the NaCl particles, but penetrated into the upper layers. Surface reactive sites determine the reaction. Kinetic measurements show that nitrate formation on sodium chloride is second order in NO2 concentration and reactive uptake coefficient is (1.54 +/- 0.70) x 10(-5)