Induction of phosphorus deficiency and phosphatase activity by salinity (NaCl) stress in Gracilaria tenuistipitata (Gigartinales, Rhodophyta)

The role of phosphorus (P) deficiency in the induction of phosphatase activity by salinity stress has been investigated in a brackish red macroalga Gracilaria tenuistipitata var. liui Zhang et Xia by determining changes in the intracellular concentrations of P and the specific activities of cellular acid (ACP; EC 3.1.3.2) and alkaline (AP; EC 3.1.3.1) phosphatases in response to varying salinity (adjusted by increasing or decreasing NaCI concentration in artificial seawater). After 4 d exposure to salinity ranging from 12 to 75 practical salinity units (psu), the net photosynthetic rate, specific growth rate, and P concentrations were found to be highest at 25 psu and lower as salinity decreased or increased. Unlike the 25 psu treatment, net photosynthetic rate, specific growth rate, and P concentrations in algae grown at 12 or 75 psu was not recovered by NaH₂PO₄ (60 μM) supplementation, indicating that the availability of P was restricted under both hyposaline and hypersaline conditions. The specific activities of both ACP and AP were higher when exposed to low salinity, especially at 12 psu. The specific activities of both APC (r² = 0.75, p < 0.05) and AP (r² = 0.84, p < 0.05) were inversely related to the logarithm of salinity (12–75 psu). Time-course experiments showed that compared to a constant level at 25 psu, the intracellular concentrations of P decreased to a plateau 1 d after exposure to 12 psu, and concomitantly, the specific activities of both ACP and AP increased, the phosphatase activities showing an inverse relationship to P concentrations. When exposed to 55 psu, the specific activities of both ACP and AP showed a transient increase with a peak around 12 h, whereas the concentrations of P decreased rapidly to a plateau after 1 d. The pattern in changes of ACP- and AP-specific activities was different in hyposaline and hypersaline conditions. The present results suggest that the limitation of P may be associated with the induction of cellular phosphatase activities by hyposaline stress in Gracilaria tenuistipitata. The relationships between hypersaline stress and a transient increase of cellular phosphatase activity are discussed.