Responses of the germination and growth of Ulva prolifera parthenogametes, the causative species of green tides, to gradients of temperature and light

Responses of the germination and growth of Ulva prolifera parthenogametes to gradients of temperature and light were evaluated. Results showed that U. prolifera parthenogametes could not germinate at 5 °C and 35 °C, and at all temperatures combined with dark conditions, but had high germination rates at the temperature of 15–25 °C and photosynthetically active radiation (PAR) of 80–160 μmol m–2 s–1. There was a significant interaction between temperature and PAR on the growth rate of U. prolifera germlings germinated from parthenogametes (P < 0.001), which indicated that U. prolifera germlings achieved the highest growth rate at specific combinations of temperature and light. Growth rate of U. prolifera germlings germinated from parthenogametes was as high as 93.5–99.2 % d−1 at combined conditions of 22 °C and 26 °C with 100 μmol m−2·s−1 and 200 μmol m−2·s−1, respectively. Ulva prolifera parthenogametes survived over two months at the temperature of 3 °C, and germinated and grew when the temperature increased from 3 °C to 13 °C. Ulva prolifera thalli germinated from parthenogametes maintained a relatively better state under the condition of 30 °C and 10 μmol m−2·s−1 compared with thalli cultured at 30 °C combined with PAR of 100 μmol m−2·s−1 and 200 μmol m−2·s−1, respectively. These results suggest that U. prolifera parthenogametes may largely contribute to green tides due to their high germination and growth rates, and their ability to survive over stressful environments in the southern Yellow Sea.