Biochemical compositions and photosynthetic characteristics of three naturally cohabitated macroalgae, Ulva fasciata, Sargassum hemiphyllum and Grateloupia livida, were comparably explored in the field conditions in Daya Bay, northern South China Sea, as well as their responses to temperature rise. Chlorophyll a (Chl a) and carotenoids contents of U. fasciata were 1.00 ± 0.15 and 0.57 ± 0.08 mg g−1 in fresh weight (FW), being about one- and two-fold higher than that of S. hemiphyllum and G. livida; and the carbohydrate content was 20.3 ± 0.07 mg g−1 FW, being about three- and one-fold higher, respectively. Throughout the day, the maximal photochemical quantum yield (FV/FM) of Photosystem II (PS II) of these three macroalgae species decreased from morning to noon, then increased to dusk and kept steady at nighttime. Consistently, the rapid light curve-derived light utilization efficiency (α) and maximum relative electron transfer rate (rETRmax) were lower at noon than that at morning- or night-time. The FV/FM of U. fasciata (varying from 0.78 to 0.32) was 38% higher than that of G. livida throughout the day, and that of S. hemiphyllum was intermediate. The superoxide dismutase (SOD) and catalase (CAT) activities in U. fasciata were lower than that in S. hemiphyllum and G. livida. Moreover, the rises in temperature species-specifically mediated the damage (k) caused by stressful high light and the corresponding repair (r) to photosynthetic apparatus, making the r/k ratio increase with the rising temperature in U. fasciata, unchanged in S. hemiphyllum but decreased in G. livida. Our results indicate that U. fasciata may compete with S. hemiphyllum or G. livida and dominate the macroalgae community under aggravatedly warming future in the Daya Bay.

With global warming, the intensity and frequency of extreme episodic weather events such as typhoons are rising in tropical and subtropical regions, disturbing the water column and shifting phytoplankton therein from deep to surface layers, and exposing them to high light as well as nutrients. To explore how phytoplankton respond to such environmental changes, we tracked the growth, cell compositions and physiology of small Thalassiosira pseudonana and large Thalassiosira punctigera from simulated ambient to upward-shifted light and nitrogen (N) conditions. Shifting to high levels of light caused a limited effect on the growth of small T. pseudonana, but reduced that of large T. punctigera by 36%, with supplemental N alleviating the light-caused growth reduction. The upward-shifted light reduced the cellular pigments contents in small T. pseudonana, but not in large T. punctigera. The upward-shifted light reduced the photosynthetic capability (FV/FM) of both species, as well as the light utilization efficiency (α) and maximal relative electron transport rate (rETRmax), but it enhanced their dark reparations. Moreover, the upward-shifted light did not affect the superoxide dismutase (SOD) activity of small T. pseudonana, but it did enhance that of large T. punctigera. In addition, the supplemental N showed a limited effect on cellular pigments and the dark respiration of T. pseudonana, but it reduced that of T. punctigera. Our results showed that the growth responses of Thalassiosira to upward-shifted light and nitrogen vary with species and possibly with cell size, indicating that the field species composition might change after the occurrence of extreme weather events.