Physiological cost of Ulva lactuca under combined pressure of nutrient-oxytetracycline: One potential formation and maintenance mechanism of Ulva sp. green tide.

Environmental pollution is considered to lead to Ulva sp. green tides. Nevertheless, nutrients with high concentrations inhibit algae which may be damaged by antibiotics, such as OTC (oxytetracycline). Thus, Ulva sp. algae might pay a physiological cost under nutrient-OTC combined pressures. If this hypothesis is confirmed, Ulva sp. algae cannot easily form green tides, or green tides are difficult to maintain. To test this hypothesis, an uniform design experiment during which OTC, ammonia (NH-N) and phosphate (PO-P) were factors was set to simulate nutrient-OTC combined pressures, and Ulva lactuca was exposed to the pressures for 96 h. The TN (total nitrogen, C) or TP (total phosphorus, C) content in U. lactuca increased with increasing nutrient concentrations, as C = 21.206+ 1.227NH-N × PO-P (R = 0.282, p < 0.05) and C = 1.886+ 0.877PO-P (R = 0.689, p < 0.05), respectively. The increase in dry weight of U. lactuca (W) had a relationship with combined pressures, W = 0.011 - 0.036PO-P (R = 0.243, p < 0.05), i.e., the algal growth was inhibited by increasing PO-P concentration. The SOD (Superoxide dismutase) activity (A) was stimulated by OTC, as A = 127.868+9.587 OTC (R = 0.193, p < 0.05). The contents of Chl a and b (C and C) were negatively affected by OTC or PO-P with high concentration, as C = 0.566 - 0.024 OTC × PO-P (R = 0.179, p < 0.05) and C = 0.512-0.044PO-P (R = 0.180, p < 0.05). Thus, too high concentrations of PO-P or OTC may hinder the formation and maintenance of Ulva sp. green tides.