The heatwaves weaken the effect of light on the growth, photosynthesis, and reproductive capacity of Ulva prolifera.

In recent years, green tides (GTs) dominated by Ulva prolifera have become frequent ecological disasters in China. These expansive algal blooms form thick floating mats that drastically reduce light penetration, creating heterogeneous light environments across water depths. Concurrently, the increasing frequency and intensity of marine heatwaves (MHWs) in coastal ecosystems have introduced additional stressors to marine primary producers. Here, we investigated the combined effects of a simulated heatwaves and light intensity on the growth, photosynthesis, and reproduction of U. prolifera. The results showed that the light treatment groups (LTGs) had a maximum growth rate of 45 %, stabilizing at 13-14 % later in cultivation. In contrast, heatwaves and light treatment groups (HWLGs) inhibit growth, with lower growth rates and even negative growth (-2 % to -18 %). HWLGs also led to lower Fv/Fm and higher NPQ compared with that of the LTGs. When comparing release rates of reproductive cells, the HWLGs had significantly lower rates than the LTGs. The low light (LL) group had the highest release rate of reproductive cells at 84 %, while the heatwave low light group (HWLL) had the lowest (30 %). This study indicates that heatwaves weaken the effect of light on the growth, photosynthesis, and reproductive capacity of U. prolifera. As GTs in the Yellow Sea and Bohai Sea continue to be an ecological threat, further research is needed to better understand the impact of varying light and heatwave conditions on U. prolifera reproduction and bloom dynamics.