Antibacterial activity of Caulerpa racemosa against pathogenic bacteria promoting ice-ice disease in the red alga Gracilaria verrucosa

Increasing ocean temperatures associated with climate change have triggered the occurrence of diseases in marine resources such as macroalgae or seaweed. Ice-ice is one of the most devastating diseases affecting economically important seaweeds such as Gracilaria and Eucheuma. In this study, we investigate the bacterial composition of diseased and healthy Gracilaria verrucosa, a red seaweed cultured in brackish water ponds in Takalar, Indonesia. Morphologic and phenotypic characteristics showed that the isolates from diseased Gracilaria belong to various genera: Vibrio, Chromobacterium, Flavobacterium, Pseudomonas, and Achromobacter. Several bacteria were also isolated from healthy Gracilaria including Corynebacterium, Serratia, Shigella, Micrococcus, Proteus, and Flavobacterium. Using Koch's postulates, bacterial pathogenicity was established by bath exposure of naive G. verrucosa to each of the bacteria isolated from diseased Gracilaria resulting in symptom characteristic of ice-ice disease. The antibacterial property of the green seaweed Caulerpa racemosa against the pathogenic bacteria was assessed using extracts that were prepared with solvents of various polarities such as hexane, chloroform, ethyl acetate, methanol, and methanol-water. The highest antibacterial activity was observed in methanol extracts Caulerpa while extracts using the other solvents showed moderate to low activities. These findings demonstrate the potential of Caulerpa to inactivate bacterial pathogens associated with ice-ice disease.