Photoperiod Mediates the Effects of Temperature and Light Intensity on the Proliferation of Ulva prolifera

In order to study the complex effects of photoperiod, temperature, and light intensity on the spore maturation and release number of Ulva prolifera, we cultured thalli segment (2–3 mm) under three different photoperiods (L:D = 12:12, 14:10 and 10:14), temperature (15°C (LT), 25°C (MT) and 30°C (HT)) and light intensity (100, 200 and 400 µmol m−2 s−1, noted as LL, ML and HL, respectively) conditions. Then the maturation time, spore release number and chlorophyll fluorescence were analyzed. The results suggested that: 1) The spore maturation time was accelerated by higher temperature or higher light intensity from 62 h to 36 h, and changes in day length accelerated the spore maturation to a certain extent as compared with 12:12 light/dark cycle; 2) Higher light intensity significantly decreased the chlorophyll fluorescence (Fv′/Fm′, NPQ, rETRmax and a) of the mature reproductive segment under 30°C with 12:12 light/dark cycle. But when in the other photoperiods (10:14 and 14:10 conditions), the inhibitory effects of high light intensity were alleviated significantly; 3) The optimum condition for the spore maturation and release was 12:12 light/dark cycle, 25°C, 400 µmol m−2 s−1, with both shorter and longer photoperiod reducing the spore release number; 4) Higher light intensity significantly increased the spore release number under 25°C, but these effects were alleviated by 30°C treatment. This study is the first attempt to elucidate the coincidence effects of photoperiod, temperature and light intensity on the reproduction of Ulva, which would help to reveal the mechanism of the rapid proliferation of green tide.