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Marine algae have unique defense strategies against microbial infection. However, their mechanisms of immunity remain to be elucidated and little is known about the similarity of the immune systems of marine algae and terrestrial higher plants. Here, we suggest a possible mechanism underlying algal immunity, which involves hexose oxidase (HOX)-dependent production of hydrogen peroxide (H 2 O 2 ). We examined crude extracts from five different red algal species for their ability to prevent bacterial growth. The extract from one of these algae,  Ptilophora subcostata , was particularly active and prevented the growth of gram-positive and -negative bacteria, which was completely inhibited by treatment with catalase. The extract did not affect the growth of either a yeast or a filamentous fungus. We partially purified from  P .  subcostata  an enzyme involved in its antibacterial activity, which shared 50% homology with the HOX of red seaweed  Chondrus crispus . In-gel carbohydrate oxidase assays revealed that  P .  subcostata  extract had the ability to produce H 2 O 2  in a hexose-dependent manner and this activity was highest in the presence of galactose. In addition,  Bacillus subtilis  growth was strongly suppressed near  P .  subcostata  algal fronds on GYP agar plates. These results suggest that HOX plays a role in  P .  subcostata  resistance to bacterial attack by mediating H 2 O 2  production in the marine environment.

Hexose Oxidase-Mediated Hydrogen Peroxide as a Mechanism for the Antibacterial Activity in the Red Seaweed Ptilophora subcostata