Rising nutrient nitrogen reverses the impact of temperature on photosynthesis and respiration of a macroalga Caulerpa lentillifera (Ulvophyceae, Caulerpaceae)

Both temperature and nutrient nitrogen are rising in worldwide aquatic ecosystems. To explore their interactive impacts on algal physiology, we measured the growth, cell components, photosynthesis, and dark respiration of a farmed green alga, Caulerpa lentillifera, under a matrix of temperatures (low, 22 °C; high, 27 °C) and nitrogen concentrations (low, 47 μmol L−1; medium, 188 μmol L−1; high, 750 μmol L−1). The relative growth rate (RGR) was less than 1.0% day−1 at low temperature, which was ~ eightfold higher at high temperature, with no significant effect of nitrogen. Pigment contents of chlorophyll a (Chl a) and carotenoids (Car) and soluble protein content increased with increasing nitrogen levels. High temperature reduced Chl a content under lower nitrogen and enhanced Car contents under higher nitrogen, but had a limited effect on proteins. Photosynthetic parameters, i.e., light-utilized efficiency (α) and maximum photosynthetic rate (Pmax), and dark respiration rate (Rd) increased with increasing nitrogen levels at low temperature. High temperature enhanced the α, Pmax, and Rd under low nitrogen, but reduced them under high nitrogen. Moreover, high temperature lowered both superoxide dismutase (SOD) and catalase (CAT) contents, indicating the beneficial effects on metabolism of C. lentillifera and thus the growth. In addition, our results indicate that the temperature-caused effects on photosynthesis and respiration of C. lentillifera are reversed by increased nitrogen levels.