Xinyao Liu, Miao Shi, Yonghong Liao, Yin Gao, Zhongkai Zhang, Donghui Wen, Weizhong Wu, Chencai An
National Laboratory of Protein Engineering and Plant Genetic Engineering, College of Life Sciences, Peking University, Beijing, 100871, China.
Microb Ecol. 2006 Apr;51(3):315-25. doi: 10.1007/s00248-006-9031-2. Epub 2006 Apr 6.
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 Celsius. 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.
异养型纳米鞭毛虫和纤毛虫的噬菌作用在水生环境中已得到广泛研究，但关于变形虫摄食的数据仍然稀少。从中国云南省昆明市滇池的水样中，我们分离出一种变形虫，命名为耐格里属Naegleria sp.菌株W2，它对某种蓝细菌具有强大的摄食作用。在分批实验中研究了该变形虫的食物选择机制和消化过程。捕食实验表明，丝状蓝细菌（如鱼腥藻属、柱孢藻属、胶刺藻属和席藻属）很容易被消耗，清除率范围为0.332至0.513纳升变形虫⁻¹ 小时⁻¹。紧密的丝状体（颤藻属）和聚集体（束丝藻属）无法被摄取；然而，在食物泡内观察到它们的超声破碎片段，这表明它们的形态阻止了它们被摄取。实时视频显微镜观察发现，单细胞色球藻科（如聚球藻属、隐球藻属和微囊藻属）在摄取后被排出，这表明食物选择发生在食物泡内。为了确定是味道还是毒素阻止它们被消化，对热杀死的细胞重新进行捕食测试。根据食物泡内容物体积估计变形虫对丝状蓝细菌的消化率和摄取率。通过“冷追踪”方法，我们发现稀释的变形虫细胞中食物泡内容物呈指数下降，消化率相对恒定，在28摄氏度时平均约为食物泡内容物的1.5%每分钟。摄取强烈依赖于变形虫的饱食状态，饥饿的变形虫比饱食的变形虫摄食速率更高。我们的结果表明，该变形虫的食物选择和食物处理机制与拦截摄食鞭毛虫相似；然而，丝状蓝细菌在吞噬性变形虫的摄食压力下无法获得避难所，这可能拓宽我们对原生生物摄食的认识。
