The invention discloses an 
alginate lyase (Alg509) derived from marine 
bacteria and a 
gene thereof, and also disclosed a method for 
recombinant expression and preparation of the 
alginate lyase. According to the method, the alg509 
gene is cloned into an E. coli 
expression vector, and the vector is transformed into an E. coli host strain to obtain a recombinant 
engineering strain which can heterologously express the 
enzyme. The 
alginate lyase Alg509 disclosed in the invention has high 
enzyme activity, the 
specific enzyme activity can reach up to 48000 U / mg and above, the optimum reaction pH is 10, the optimum 
reaction temperature is 55 DEG C, and the 
enzyme activity has no dependence on various 
metal ions. The enzyme is active to 
sodium alginate, poly-
guluronic acid (polyG) and ploymannuronic acid (polyM), and can completely degrade 
sodium alginate to produce alginate oligomers such as alginate 
disaccharide, alginate 
trisaccharide, alginate tetrasccharide, etc. The enzyme exhibits strongbasophilia, has certain tolerance to high pH, has certain potential of industrial applications, and can be widely applied in the fields of 
agriculture, food, 
feed additive, 
medicine and the like.