Modelling and optimization of phlorotannin extraction from Stoechospermum marginatum using response surface methodology and evaluating its activity against marine microfouling bacteria

The detrimental effects of synthetic antifouling ingredients has underscored the necessity for the development of eco-friendly antifouling compounds. Phlorotannins are secondary metabolites biosynthesized exclusively in brown seaweeds with a potential as an antifouling compound. This study optimized the extraction of phlorotannins from Stoechospermum marginatum using Box-Behnken design of response surface methodology and evaluated its potential for antifouling activity against marine microfouling bacteria isolated from the hull of fishing boat. Based on biofilm formation index, 13 bacterial isolates identified by 16S rRNA sequencing were found to have the ability to form biofilm in the initial stage in biofouling. The conditions that produced the highest phlorotannin content of 2.75 +/- 0.02 mg of phloroglucinol equivalents/g are as follows: ethanol concentration of 78.6%, solvent-solid ratio of 39.5 mL/g and temperature of 43.5 degrees C. The crude extract and purified phlorotannin fraction subsequently tested for antifouling activity, both showing promising antibacterial activity. Among them, ethyl acetate fraction of purified phlorotannin exhibited the highest antibacterial activity against all tested bacteria with a minimum inhibitory concentration and minimum bactericidal concentration ranging from 16 to 128 mu g/mL and 32 to 256 mu g/mL, respectively. Gram-positive bacteria were more susceptible to purified phlorotannins than gram-negative bacteria. This study demonstrated the ability of phlorotannin to inhibit the biofilm formation and to eradicate the mature biofilms in a concentration-dependent manner. Analysis of the structural damage of bacterial cells by scanning electron microscopy further confirms the antifouling activity. Therefore, phlorotannins from S. marginatum are a promising sustainable and biocompatible antifouling alternative to the synthetic antifouling compounds.