Endosymbiosis in microalgae with special attention to Noctiluca scintillans

Endosymbiosis in host cells has been described for many photosynthetic microalgae. There are two types of the endosymbiosis in terms of the mode of transmission of the symbiont: some hosts get the symbiont vertically in each generation, while others gain it horizontally. The relationship between host and symbiont is well investigated in some symbiotic systems such as the coral-algal symbiosis and the Paramecium bursaria-Chlorella symbiosis. In artificially aposymbiotic P. bursaria and some corals, both of which obtain symbionts horizontally, free living algal cells are engulfed by phagocytosis by the host. Then, some algal cells are digested, and others settle in as symbionts. In the microalgal symbioses examined to date, symbionts are enveloped by membranes within the host cell, and material exchange occurs through the membrane. One exception is found in the symbiosis of a heterotrophic dinoflagellate Noctiluca scintillans with a photosynthetic green flagellate Pedinomonas noctilucae. A large number of symbionts swim freely within the host cell without being enveloped by any membranes. While N. scintillans in temperate waters forms distinct reddish red tides, those in tropical Southeast Asia and adjacent areas have greenish discoloration due to the presence of the symbiont. For this reason, it is often called the "green Nocttluca". In addition to the lack of the membrane envelope, this symbiotic system is unique. First, P. noctilucae survives only within the host cell and not in seawater. Second, the symbiont is unlikely to be transferred vertically. Although the cell size of the green Noctiluca tends to be smaller than that of asymbiotic N. scintillans in temperate waters, the morphology, life cycle, bioluminescence and volume specific respiration of the green Noctiluca are similar to asymbiotic N. scintillans, and both are considered to be identical species. Growth rates under laboratory conditions and natural populations show that the symbiont of the green Noctiluca assures subsistence of the host under food-1imited conditions, although the symbiont does not give higher growth rates than in asymbiotic N. scintillans. Cellular net photosynthesis and cellular content of the symbiont vary considerably, implying that the symbiotic relationship between the host and the symbiont is dynamic.