Selection and characterization a new strain (PS-M4) of Pyropia suborbiculata with fast growth based on ⁶⁰Co-γ ray irradiation

Compared to Pyropia haitanensis, a species of seaweed cultured in southern China, Pyropia suborbiculata exhibits stronger high-temperature resistance but its round shape is unsuitable for commercial culture. To address this, researchers have isolated several long-type mutants and strains from wild-type strain of P. suborbiculata using artificial mutagenesis. However, their growth rates are still relatively slow. In this study, the blades of a long-type strain (PS-5) were irradiated with ⁶⁰Co-γ rays to determine the optimal mutagenic dose of 1200 Gy. Subsequently, a new strain with fast growth, PS-M4, was selected from its progeny based on growth rate. After 60 days of culture at 23 ℃, the average length and wet weight of PS-M4 blades were 76.3 cm and 1.3 g, respectively, which were 58.2% and 38.9% higher than those of PS-5. Both PS-M4 and PS-5 maintained normal cellular morphology without any dead cells after being cultured for 30 days at 23 ℃ followed by 30 days at 30 ℃, while the traditional cultivated strain (Ph-WT10) of P. haitanensis showed more cell pigment dispersion and death. After 14 days of culture at 30 ℃, the survival rates of conchospore germlings of PS-M4 and PS-5 were 70.6% and 66.5%, respectively, with no significant difference between them, but 133.8% and 120.2% higher than that of Ph-WT10. At this time, over 80% of the conchospore germlings of both PS-M4 and PS-5 contained more than 9 cells, while all the conchospore germlings of Ph-WT10 had only 1-4 cells. After 45 days of culture at 23 ℃, the contents of chlorophyll a and total phycobiliprotein of PS-M4 blades were 6.6 and 61.7 mg g^-1, respectively, which were 12.3% and 14.9% higher than those of PS-5. The average thickness of PS-M4 was 30.5 μm, which was 16.6% thinner than that of PS-5. In terms of total released conchospores over 10 consecutive days, PS-M4 reached up to 293.6×10⁴ per shell, which was 3.3 times that of PS-5. In summary, the new strain PS-M4 of P. suborbiculata has shown significant improvements in blades growth, quality, and released conchospores, while its conchospore germlings and blades exhibited superior high-temperature resistance compared to traditional cultivated strain of P. haitanensis, making it a promising candidate for commercial cultivation.