LOW TEMPERATURE TREATMENT ON THE GROWTH, OIL ACCUMULATION, PHOTOSYNTHESIS, AND ANTIOXIDANT ENZYME ACTIVITY OF XANTHONEMA HORMIDIOIDES

The filamentous oleaginous microalga Xanthonema hormidioides was used as the experimental material, and a control experiment was carried out between the low-temperature (5℃) treated algal inoculum and the room-temperature (25℃) algal inoculum. We examined the physiological and biochemical indexes of the two treated inocula at different temperatures (5℃, 10℃, 15℃, 20℃, 25℃) and under two nitrogen concentrations (18 and 3 mmol/L). Additionally, we explored the impact of low temperature (5℃) on the photosynthetic activity and antioxidant enzyme system of the two algal inoculum. The results showed that the biomass of Xanthonema hormidioides subjected to low-temperature treatment surpassed that of the room-temperature algal inoculum at all temperatures, with the highest biomass of 10.1 g/L recorded at 15℃. The highest content of lipid and fatty acids was obtained at 20℃, accounting for 64.20% and 49.05% of dry weight, respectively (3 mmol/L). The protein content peaked at 15℃, reaching 43.5% (18 mmol/L), while total carbohydrates levels remained relatively consistent, ranging between 10% and 25%. At 5℃, the utilization efficiency of light (α) and the maximum electron transfer rate (ETR_max) of Xanthonema hormidioides from the room temperature were significantly inhibited. The chlorophyll a content, F_v/F_m, and F_v/F_o values of low-temperature treated Xanthonema hormidioides were higher than those of the room temperature experimental group from the 9th day, with the highest values reaching 9.98 mg/g, 2.99, and 0.75 (3 mmol/L), respectively. In contrast to the room temperature experimental group, the ETR_max and tolerance to strong light (IK) were enhanced in Xanthonema hormidioides to low-temperature treatment at 5℃. In addition, the maximum activities of superoxide dismutase (SOD) and catalase (CAT) in Xanthonema hormidioides under low temperature treatment reached 65.6 and 0.56 U/mg protein, respectively, significantly surpassing the values of Xanthonema hormidioides at normal temperature (P<0.05). The results indicate that low temperature treatment is more conducive to the growth and metabolites accumulation of Xanthonema hormidioides, and the adaptability to low temperature, photosynthetic efficiency, and resistance to oxidative stress.