Newly discovered role of the heterotrophic nanoflagellate Katablepharis japonica, a predator of toxic or harmful dinoflagellates and raphidophytes. Newly discovered role of the heterotrophic nanoflagellate Katablepharis japonica, a predator of toxic or harmful dinoflagellates and raphidophytes

Heterotrophic nanoflagellates are ubiquitous and known to be major predators of bacteria, however, the feeding of free-living heterotrophic nanoflagellates on phytoplankton is poorly understood, although these two components usually co-exist. To investigate the feeding and ecological roles of major heterotrophic nanoflagellates Katablepharis spp., the feeding ability of Katablepharis japonica on bacteria and phytoplankton species and the type of the prey that K. japonica can feed on were explored. Furthermore, the growth of K. japonica and its ingestion rates of the dinoflagellate Akashiwo sanguinea—a suitable algal prey item—heterotrophic bacteria, and the cyanobacteria Synechococcus sp., as a function of prey concentration were determined. Among the prey tested, K. japonica ingested heterotrophic bacteria, Synechococcus sp., the prasinophyte Pyramimonas sp., the cryptophytes Rhodomonas salina and Teleaulax sp., the raphidophytes Heterosigma akashiwo and Chattonella ovata, and the dinoflagellates Heterocapsa rotundata, Amphidinium carterae, Prorocentrum donghaiense, Alexandrium minutum, Cochlodinium polykrikoides, Gymnodinium catenatum, and A. sanguinea, however, it did not feed on the dinoflagellates Alexandrium catenella, Gambierdiscus caribaeus, Heterocapsa triquetra, Lingulodinium polyedra, Prorocentrum cordatum, P. micans, and Scrippsiella acuminata and the diatom Skeletonema costatum. Many K. japonica cells attacked and ingested a prey cell together after pecking and rupturing the surface of the prey cell and then uptaking the materials that emerged from the ruptured cell surface. Cells of A. sanguinea supported positive growth of K. japonica, but neither heterotrophic bacteria nor Synechococcus sp. supported growth. The maximum specific growth rate of K. japonica on A. sanguinea was 1.01 d-1. In addition, the maximum ingestion rate of K. japonica for A. sanguinea was 0.13 ng C predator-1d-1 (0.06 cells predator-1d-1). The maximum ingestion rate of K. japonica for heterotrophic bacteria was 0.019 ng C predator-1d-1 (266 bacteria predator-1d-1), and the highest ingestion rate of K. japonica for Synechococcus sp. at the given prey concentrations of up to ca. 107 cells ml-1 was 0.01 ng C predator-1d-1 (48 Synechococcus predator-1d-1). The maximum daily carbon acquisition from A. sanguinea, heterotrophic bacteria, and Synechococcus sp. were 307, 43, and 22%, respectively, of the body carbon of the predator. Thus, low ingestion rates of K. japonica on heterotrophic bacteria and Synechococcus sp. may be responsible for the lack of growth. The results of the present study clearly show that K. japonica is a predator of diverse phytoplankton, including toxic or harmful algae, and may also affect the dynamics of red tides caused by these prey species

异养鞭毛虫（Heterotrophic nanoflagellates）广泛存在，且被证实为细菌的主要捕食类群。尽管自由生活的异养鞭毛虫与浮游植物（phytoplankton）通常共存，但二者间的摄食关系至今仍未得到充分研究。为探究主要类群扁胞藻属（Katablepharis spp.）的摄食模式与生态功能，本研究针对日本扁胞藻（Katablepharis japonica）对细菌及浮游植物物种的摄食能力，及其可摄食的猎物类型展开了系统探索。
此外，本研究还测定了日本扁胞藻的生长情况，及其分别以甲藻（dinoflagellate）赤潮异弯藻（Akashiwo sanguinea）——一种适宜的藻类猎物——异养细菌、蓝细菌（cyanobacteria）聚球藻属（Synechococcus sp.）为猎物时，摄食率随猎物浓度变化的规律。
在所测试的猎物类群中，日本扁胞藻可摄食异养细菌、聚球藻属、青绿藻（prasinophyte）锥囊藻属（Pyramimonas sp.）、隐藻（cryptophytes）盐生红胞藻（Rhodomonas salina）与泰勒藻属（Teleaulax sp.）、异弯藻纲（raphidophytes）褐胞藻（Heterosigma akashiwo）与卵形卡盾藻（Chattonella ovata），以及甲藻类的圆形旋沟藻（Heterocapsa rotundata）、卡特前沟藻（Amphidinium carterae）、东海原甲藻（Prorocentrum donghaiense）、微小亚历山大藻（Alexandrium minutum）、多环旋沟藻（Cochlodinium polykrikoides）、链状裸甲藻（Gymnodinium catenatum）与赤潮异弯藻；但该藻无法摄食甲藻类的链状亚历山大藻（Alexandrium catenella）、加勒比冈比亚藻（Gambierdiscus caribaeus）、三鳍旋沟藻（Heterocapsa triquetra）、多边舌甲藻（Lingulodinium polyedra）、心形原甲藻（Prorocentrum cordatum）、中肋原甲藻（Prorocentrum micans）、尖刺原多甲藻（Scrippsiella acuminata），以及硅藻（diatom）中肋骨条藻（Skeletonema costatum）。
日本扁胞藻通常会有多枚个体共同啄破猎物细胞表面，随后攻击并摄食单个猎物细胞，摄取破裂后释放的细胞内含物。赤潮异弯藻可支撑日本扁胞藻的正向生长，但异养细菌与聚球藻属均无法维持其生长。日本扁胞藻以赤潮异弯藻为猎物时的最大比生长速率为1.01 d⁻¹。此外，其对赤潮异弯藻的最大摄食率为0.13 ng C predator⁻¹d⁻¹（0.06 cells predator⁻¹d⁻¹）；对异养细菌的最大摄食率为0.019 ng C predator⁻¹d⁻¹（266 bacteria predator⁻¹d⁻¹）；在猎物浓度最高约为10⁷ cells ml⁻¹的条件下，日本扁胞藻对聚球藻属的最高摄食率为0.01 ng C predator⁻¹d⁻¹（48 Synechococcus predator⁻¹d⁻¹）。
日本扁胞藻从赤潮异弯藻、异养细菌与聚球藻属获取的每日最大碳摄入量，分别相当于其自身体碳的307%、43%与22%。由此可见，日本扁胞藻对异养细菌与聚球藻属的低摄食率，可能是其无法以此两类生物为食实现种群生长的核心原因。本研究结果清晰证实，日本扁胞藻是一类可捕食多种浮游植物（包括有毒或有害藻类）的捕食者，且可能对上述猎物物种引发的赤潮动态产生调控作用。