Oxidative Stress-Mediated Development of Symbiosis in Green Paramecia

Abstract

Single cells of Paramecium bursaria (often referred to as green paramecia) harbor in its cytoplasm several hundred cells of green algae, which are morphologically and genetically almost identical to Chlorella species. P. bursaria serves as an excellent experimental model for studying the nature of endosymbiosis in which one species propagates inside the cells of other species under precise control through chemical communication between the symbiotic partners. This chapter focuses on the impact of oxidative stress due to algal photosynthesis on the behavior of hosting ciliate and the fate of symbiotic system. In fact, some of alga-derived chemicals including the members of reactive oxygen species (ROS) are apparently threatening the host cells, besides their beneficial supply with nutrients. When the ROS production by the symbiotic algae was experimentally promoted, the excretion of algae from the host cells was highly stimulated. This suggests a scenario in which the host cell bodies in P. bursaria positively eliminate the algae that are the sources of photochemical ROS production. By doing so, the host cells possibly survive by avoiding the risk of internal oxidation when the levels of oxidative stress were high enough to damage the host cells. This type of loose symbiosis between algal cells and the ciliate may support a coevolution process leading to highly tolerant host cell species that are capable of optimizing energy gain from the oxidative symbiosis. We present also a case of symbiosis distortion, which causes unregulated growth of algae.