A small flagellate (strain no.MMDL4071 )was isolated from the phycosphere of dinoflagellate Alexandri-um tamarense.The morphological observation demonstrated that the cells of the flagellate were spherical,measured 3 μm and 2 unequal flagella arisen from the anterior face to the cell.The ultrastructure observation by scanning e-lectron microscope showed that the cells of the flagellate contain nucleus,chloroplast,chloroplast endoplasmic re-ticulum,Golgi body,mitochondria,and "9 +2"microtubule structure flagella.Control experiment proved that the flagellate was heterotrophic species,and the binary fission process was captured.Combine with the morphological, ecological characteristics and phylogenetic analysis,this heterotrophic flagellate was identified as Ochromonas sp.%从塔玛亚历山大藻（Alexandrium tamarense）藻际中分离出一株微型鞭毛藻 MMDL4071，经光学显微镜形态学观察发现，该鞭毛藻体长约为3μm，椭球型，头部端具不等长的两条鞭毛；扫描电镜观察揭示了该鞭毛藻的细胞核、色素体、色素体-内质网膜、高尔基体、线粒体以及“9＋2”微管结构的鞭毛等亚显微结构．对照培养实验表明该鞭毛藻属于依赖摄食细菌的异养型藻类，并观察到了该藻二分裂的增殖过程．通过对18S rDNA 及 ITS 序列扩增、比对，构建了系统发育树．综合形态学、生态学特征以及系统发育树分析的结果，该藻被确定为棕鞭藻属的一个未定种（Ochromonas sp．）．
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