Darkness and low nighttime temperature modulate the growth and photosynthetic performance of Ulva prolifera under lower salinity

In order to understand how darkness/irradiance and low nighttime temperature might alter physiology of Ulva prolifera under lower salinity conditions, we analyzed the growth rates, water content, superoxide dismutase (SOD) activity, total soluble proteins (SPs) and carbohydrates content at the end of dark and light period under three temperature levels (25-25 degrees C treatment: 25 degrees C for day and night; 15-15 degrees C treatment: 15 degrees C for day and night; 25-15 degrees C treatment: 25 degrees C for day with 15 degrees C for night) and two salinity conditions (15, 25), meanwhile, the pigment content (chlorophyll a and b), chlorophyll fluorescence and photosynthetic oxygen evolution also were determined during light phase. We found that the U. prolifera showed higher growth rate and SOD activity during dark phase at 25 degrees C, but this dark-induced increase could not be observed at 15 degrees C. The reasons for this increase varied, however, maybe not included water content and SPs for no significant difference in water content observed under all the treatments, as well as lower SPs content for dark period aside that at 15 degrees C and salinity 15. Compared to other two temperature treatments, the thalli grown at 25-15 degrees C showed higher growth rate and the photosynthetic oxygen evolution rate in light phase under salinity 15 conditions, although the maximum relative electron transport rate (rETR(max)) showed higher value under 25 degrees C treatment. These results indicate that the darkness and the lower nighttime temperature maybe responsible reason for the rapid growth of these green tide algae.