Driven factors of sexual reproduction in Noctiluca scintillans

Noctiluca scintillans is a larger size bioluminescence red-tide dinoflagellate (200-1000 μm in diameter), which reproduces by sexual or asexual processes (binary fission). Its process of sexual reproduction is well known but the ecological role and the mechanism of shifting from asexual to sexual reproduction in Noctiluca scintillans is rarely understood. It is believed that the sexual reproduction may occur as facing environmental stress. In this study, we try to find that what factors drives Noctiluca scintillans to undergo sexual reproduction and further hypothesize the ecology role of its sexual reproduction by cultivating Noctiluca scintillans with different variation of temperature, Noctiluca scintillans density, prey concentration, cultivation time, cultivation volume, light exposure time and physical vibration (simulated wave motion). We then observed that which condition Noctiluca scintillans had an increase sexual reproduction rate, by counting the gametocyte mother cells. The result indicated that the rise of sexual reproduction or gametocyte mother cells was only observed in the variation of prey concentration, typically occurred after the exponential phase of the population. Noticeable upsurge occurred when the prey was depleted, the gametocyte mother cells increased to near 10%, in which other conditions were only 1% or less, of the total Noctiluca scintillans population. This implies that the shift from trophont to gametocyte mother cells may be induced by the fluctuation of prey concentration over time. We suggest that the sexual reproduction may happen as a response to an after-bloom situation, the depletion of the local food source, to increase the diversity of the remaining population.