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Abstract
Vegetative growth of Noctiluca scintillans containing the photosynthetic endosymbiont Pedinomonas noctilucae was investigated in culture and in natural populations. Clonal cultures of N. scintillans containing the endosymbiont, isolated from the inner Gulf of Thailand and Manila Bay, were of two types: one requiring an external food supply and the other not requiring an external food supply. The latter strain was nevertheless able to feed on phytoplankton, indicating facultative phagotrophy. Under optimal growth conditions there was no significant difference in growth rate between N. scintillans possessing the endosymbiont (0.33 day−1) and the heterotrophic N. scintillans (0.28 day−1), without the endosymbiont, isolated from the Seto Inland Sea, Japan. These observations support the hypothesis that the symbiont does not enhance the growth of Noctiluca, but phagotrophy does promote faster growth. Symbiosis is considered to secure survival of the host under food limitation. Cell cycle analysis conducted in a natural population in Manila Bay revealed a diurnal rhythm in cell division, peaking during early morning. The in situ specific growth rate of 0.16 day−1 was within the range reported for the heterotrophic N. scintillans in temperate waters.