Aims Gracilaria lemaneiformis is a commercially important marine red alga from which agar is extracted for use in food chemistry and in production of tissue culture media. It has been transplanted from Qingdao, Shandong Province to Nan'ao Island, Guangdong Province and other places in South China. Understanding the relationship between its photosynthesis and environmental factors has theoretical and practical importance for growing the macroalga more rationally, understanding its biological functions and increasing its utilization.
Methods We cultured G. lemaneiformis seaweeds in Qing'ao Bay of Nan'ao Island, South Sea, China. The silky samples (immersed in seawater in plastic casks) were taken into a laboratory and provided oxygen by air pumps. We studied their physiological responses to six days of growth in seawater eutrophicated to different degrees by nitrogen and phosphorus alone and in combination.
Important findings Contents of phycobiliprotein and chlorophyll a and total antioxidative ability were decreased significantly in mild phosphorus eutrophic seawater (P 0.2 mg·L^-1) and in severe nigrogen+phosphorus eutrophic seawater (P 10 mg·L^-1, N 55 mg·L^-1). Chlorophyll fluorescence kinetic parameters (F_v/F_m, F_v/F_o, q_P, q_N, Φ_PSⅡ, q_o) also showed the harmful effect of such treatments to PSⅡ function. However, no obvious changes in these parameters were found with mild nitrogen (N 0.9 mg·L^-1) and mild nitrogen+phosphorus (P 0.2 mg·L^-1, N 0.9 mg·L^-1) eutrophic seawater. Gracilaria lemaneiformis did not accumulate more nitrogen or phosphorus in vivo in mild eutrophic seawater than in the control seawater. Because G. lemaneiformis is tolerant to mild N and mild N+P pollution, this species may be used to purify lightly eutrophicated seawater.