Morphology, growth, and photosynthesis of Ulva prolifera O.F. Müller (Chlorophyta, Ulvophyceae) gametophytes, the dominant green tide species in the Southern Yellow Sea

Green tide with the main dominant species is Ulva prolifera O.F. Müller have been reported in the Southern Yellow Sea since 2007. This study investigated differences in the morphology, growth, chlorophyll fluorescence parameters, and chloroplast pigment content of different day-aged U. prolifera gametophytes. Gametes had two flagella and exhibited positive phototaxis. On the 7th day of culture, young gametophytes began to branch out. Changes in the length and width of the main branches, and the length of the first-level branches showed "S"-shaped growth within 0–30 days, while the fresh weight of single algae exhibited "J"-shaped growth. The average fresh and dry weight of U. prolifera was 12.32 and 1.38 g per thallus, respectively, when cultured from one gamete to 30 days. The average number of first-level branches of 10-day-aged thalli was 119 per blade, and the maximum branching level was one. The average number of first-level branches of 30-day-aged thalli was 243 per thallus, and the maximum branching level was five. Except for the NPQ and IK values of 0 d thalli compared with 10 d thalli, the IK value of 0 d compared with 20 d, the NPQ, ETRmax and IK value of 10 d compared with 20 d, and the Alpha value of 10 d compared with 30 d, the Fv / Fm, YII, NPQ, ETRmax, IK and Alpha values of the other 0 d, 10 d, 20 d and 30 d thalli showed significant or extremely significant differences. There was no significant difference in the Chl b / Car value between 0 d and 10 d, the Chl b value between 10 d and 20 d, the Chla / Car value between 10 d and 30 d, and the Car value between 20 d and 30 d was significantly different. The Chl a, Chl b, Car, Chl a / Chl b, Chl a / Car and Chl b / Car values of the other 0 d, 10 d, 20 d and 30 d were highly significantly different. These results not only verify that U. prolifera gametophytes can directly develop into new thalli by parthenogenesis, but also reveal the morphological and physiological characteristics of different day-aged gametophytes. In addition to presenting the typical morphology of different-day-aged gametophytes, this study also reports significant differences in growth, chlorophyll fluorescence parameters, and chloroplast pigment contents among them. This revealed the physiological mechanism of varying growth rates of different day-aged U. prolifera. These findings provide essential reference information for green tide prevention and outbreak mechanism research.