Directed Evolution of an Alginate Lyase from Flammeovirga sp. for Seaweed Fertilizer Production from the Brown Seaweed Laminaria japonica

An alginate lyase (FsAly7) from Flammeovirga sp. was engineered by directed evolution to improve its optimum temperature and thermostability. The optimum temperature of the positive mutant mFsAly7 (FsAly7-Ser43Pro) was increased by 5 °C, and the thermal inactivation half-lives at 40 and 45 °C were 4.4 and 5.6 times higher than those of FsAly7, respectively. mFsAly7 was expressed in high levels in Pichia pastoris with the highest yield of 3125.5 U·mL^-1. On the property-improved enzyme, a bioconversion strategy was developed to prepare the Laminaria japonica hydrolysate, and the highest AOS yield (29.5 mg·mL^-1) was 1.5 times higher than that of FsAly7. Moreover, the application potential of the L. japonica hydrolysate as an organic fertilizer was evaluated. The addition of L. japonica hydrolysate improved the sprouting rate, fresh mass, seedling height, and stem diameter of the buckwheat sprouts. Therefore, mFsAly7 may be a good candidate for the production of seaweed fertilizers from L. japonica.