Effect of irradiation on the protein profile, protein content, and quality of agar from Gracilaria asiatica Zhang et Xia (Rhodophyta)

There are about 15 species of Gracilaria reported in Vietnam. Of these, Gracilaria asiatica Zhang et Xia is being cultivated on a large scale in Northern Vietnam, which has a subtropical climate. During the rainy season, from May to October, the growth of G. asiatica is drastically reduced or even ceases due to very low salinity and high temperature. Therefore, it is important to improve the tolerance of G. asiatica to a wide range of salinity and temperatures. This paper presents the results of research on strain improvement of G. asiatica using irradiation and selection media. Three irradiation doses of 20, 60, and 100 krad were tested against the control (with no irradiation). Afterward, the seaweed biomass was cultivated on a selected medium, ESS-1, containing NaCl in concentrations of 23‰ (C1) and 0‰ (C2). The results showed that a higher survival rate of G. asiatica was observed with the 20- and 60-krad doses. The protein content and composition of selected seaweeds were analyzed and compared with the control. SDS-PAGE showed no remarkable difference in the protein composition between the control and irradiated samples. However, the 67-kDa protein band of seaweed treated with 20 and 60 krad, then grown on ESS-1 medium with 23% NaCl, had a higher density than other samples. This protein was reported to play an important role in G. asiatica, by enhancing its tolerance to variable salinity and temperature. Although the organic and inorganic content of all samples remained almost the same, the content and quality of agar extracted from irradiated seaweeds were higher than those of the controls. Due to the high amount of 3.6 anhydro-α-L-galactose combined with low amounts of sulfate found in irradiated seaweeds, the freezing and melting points of extracted agar were lower. Eventually, this resulted in higher condensation and better quality of agar, such as in its gel-forming ability. The quality of fluid agar extracted from selected seaweeds improved as shown in the remarkable decrease in Ca2+, Mg2+, and total Fe ion content, thus lowering its melting point compared with the control.