The photosynthetic and respiratory responses to temperature and nitrogen supply in the marine green macroalga Ulva conglobata (Chlorophyta)

Temperature effects on photosynthesis and respiration were investigated in the green macroalga, Ulva conglobata, collected from low rocky coast of Nanao Island, Shantou, China. Thalli were cultured at 15 and 25°C and at low nitrogen (LN) and high nitrogen (HN) availability. Dark respiration and light-saturating photosynthesis were measured as oxygen exchange; the characteristics of chlorophyll fluorescence were also assayed. The maximal photochemical yield (Fᵥ/Fₘ) and maximum relative electron transport rates (rETRₘₐₓ) remained stable with moderate fluctuations of temperature (15–30°C) in the short term. However, the values of Fᵥ/Fₘ and rETRₘₐₓ declined with the high temperature (≥ 35°C), and such a decline was more accentuated in 15°C- than 25°C-grown algae. Both the rates of photosynthesis and respiration were sensitive to measurement temperature, with the Q₁₀ values being higher in 25°C-grown algae (HN) than 15°C-grown algae. It appeared that 25°C-grown algae displayed an optimum temperature (Tₒₚₜ) of 30°C for photosynthesis, while 15°C-grown algae exhibited the Tₒₚₜ of a range of 20–30°C. When measured at their respective growth temperature, the rates of photosynthesis were significant higher in 25°C- than 15°C-grown algae, while the rates of respiration were identical between 25°C- and 15°C-grown algae. Our results demonstrated that respiration displayed full acclimation; whereas, photosynthesis exhibited partial acclimation to changing growth temperatures in U. conglobata. Consequently, the balance between respiration and gross photosynthesis was re-established by changing growth temperature, with the ratio being lowered with warmer growth temperature. The results also showed that HN availability in culture significantly increased pigments and soluble protein contents and enhanced photosynthesis and respiration. We suggested that the acclimation potential of metabolisms in U. conglobata favored carbon acquisition and net carbon balance with the increasing seawater temperature resulting from climate change and/or increasing N loading from coastal eutrophication.