LAUSR

The dinoflagellate Noctiluca scintillans exists in two forms, green and red Noctiluca, with and without the photosynthetic endosymbiont Pedinomonas noctilucae, respectively. In Southeast Asia, green Noctiluca has a northern boundary of distribution in the Luzon Strait and the Vietnamese coast, approximately corresponding to the 25°C isotherm. We examined whether this boundary arose from the growth dependence of Noctiluca on temperature and/or that of P. noctilucae. We analyzed the effect of temperature on growth by examining one and four unialgal cultures of red and green Noctiluca, respectively, and two monocultures of the endosymbiont. Growth dependence on temperature was markedly different between host cells of green Noctiluca and monocultures of the endosymbiont, although some variations in growing temperature were observed among strains of both Noctiluca and the endosymbiont. Green Noctiluca grew at a temperature range of 20–31°C, whereas P. noctilucae did not grow at low temperatures ≤25°C. Growth rates of green Noctiluca were considerably higher when fed compared to monocultures. Our findings demonstrate that the effect of temperature on P. noctilucae growth is a crucial factor controlling the geographical distribution of green Noctiluca in the South China Sea. An implication of this finding is discussed in the context of global warming.