Fishman Yelena, Zlotkin Eliahu, Sher Daniel
Department of Cell and Animal Biology, Silberman Institute of Life Sciences, The Hebrew University, Jerusalem, Israel.
PLoS One. 2008 Jul 2;3(7):e2603. doi: 10.1371/journal.pone.0002603.
Algal-cnidarian symbiosis is one of the main factors contributing to the success of cnidarians, and is crucial for the maintenance of coral reefs. While loss of the symbionts (such as in coral bleaching) may cause the death of the cnidarian host, over-proliferation of the algae may also harm the host. Thus, there is a need for the host to regulate the population density of its symbionts. In the green hydra, Chlorohydra viridissima, the density of symbiotic algae may be controlled through host modulation of the algal cell cycle. Alternatively, Chlorohydra may actively expel their endosymbionts, although this phenomenon has only been observed under experimentally contrived stress conditions.
We show, using light and electron microscopy, that Chlorohydra actively expel endosymbiotic algal cells during predatory feeding on Artemia. This expulsion occurs as part of the apocrine mode of secretion from the endodermal digestive cells, but may also occur via an independent exocytotic mechanism.
Our results demonstrate, for the first time, active expulsion of endosymbiotic algae from cnidarians under natural conditions. We suggest this phenomenon may represent a mechanism whereby cnidarians can expel excess symbiotic algae when an alternative form of nutrition is available in the form of prey.
藻类与刺胞动物的共生关系是促成刺胞动物成功的主要因素之一，对珊瑚礁的维持至关重要。虽然共生体的丧失（如在珊瑚白化过程中）可能导致刺胞动物宿主死亡，但藻类的过度增殖也可能损害宿主。因此，宿主需要调节其共生体的种群密度。在绿水螅（Chlorohydra viridissima）中，共生藻类的密度可能通过宿主对藻类细胞周期的调节来控制。或者，绿水螅可能会主动排出其内共生体，不过这种现象仅在实验设定的应激条件下被观察到。
我们通过光学显微镜和电子显微镜观察发现，绿水螅在捕食卤虫时会主动排出内共生藻类细胞。这种排出是作为内胚层消化细胞顶浆分泌模式的一部分发生的，但也可能通过独立的胞吐机制发生。
我们的结果首次证明了在自然条件下刺胞动物能主动排出内共生藻类。我们认为这种现象可能代表了一种机制，即当有猎物这种替代营养形式时，刺胞动物可以排出多余的共生藻类。
