Photosynthetic Characteristics of Three Cohabitated Macroalgae in the Daya Bay, and Their Responses to Temperature Rises

Biochemical compositions and photosynthetic characteristics of three naturally cohabitated macroalgae, <i>Ulva fasciata</i>, <i>Sargassum hemiphyllum</i> and <i>Grateloupia livida</i>, were comparably explored in the field conditions in Daya Bay, northern South China Sea, as well as their responses to temperature rise. Chlorophyll <i>a</i> (Chl <i>a</i>) and carotenoids contents of <i>U. fasciata</i> 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 <i>S. hemiphyllum</i> and <i>G. livida</i>; 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 (F_V/F_M) 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 F_V/F_M of <i>U. fasciata</i> (varying from 0.78 to 0.32) was 38% higher than that of <i>G. livida</i> throughout the day, and that of <i>S. hemiphyllum</i> was intermediate. The superoxide dismutase (SOD) and catalase (CAT) activities in <i>U. fasciata</i> were lower than that in <i>S. hemiphyllum</i> and <i>G. livida</i>. Moreover, the rises in temperature species-specifically mediated the damage (<i>k</i>) caused by stressful high light and the corresponding repair (<i>r</i>) to photosynthetic apparatus, making the <i>r</i>/<i>k</i> ratio increase with the rising temperature in <i>U. fasciata</i>, unchanged in <i>S. hemiphyllum</i> but decreased in <i>G. livida</i>. Our results indicate that <i>U. fasciata</i> may compete with <i>S. hemiphyllum</i> or <i>G. livida</i> and dominate the macroalgae community under aggravatedly warming future in the Daya Bay.