Antifouling activity exhibited by pyrrolo compound isolated from marine sponge associated bacterium Halobacillus kuroshimensis SNSAB01 against barnacles

The increasing use of natural products from marine organisms and their synthetic analogs as antifouling agents is replacing chemical biocides due to their ecological compatibility. This study evaluated the antifouling potential of pyrrolo compound (PC) from the sponge-associated bacterium Halobacillus kuroshimensis against the barnacle Amphibalanus reticulatus larval survival, metamorphosis and settlement in the laboratory. The successful concentrations were also subjected to field based static immersion tests to study the efficacy of the PC extract on macrofouling settlement. Results showed decreased naupliar survival and affected larval metamorphosis in a dose-dependent manner, with varying sensitivity among developmental stages. Higher PC concentrations significantly reduced cyprid yield and settlement, with over 50% of cyprid larvae failing to settle at concentrations of 1300 μg/ml and above. Field experiments confirmed a steady decline in barnacle settlement on PC-treated coupons, with less than 50% settlement observed at concentrations of 500–2000 μg/ml over 50 days. The LC₅₀/EC₅₀ ratio for nauplius N-II was 1.11, with higher ratios observed in later stages and for the cypris (1.66), suggesting moderate toxicity and a narrow margin of safety. These ratios suggest barnacle settlement inhibition occurring through a toxic mechanism. The results demonstrate the compound's ability to disrupt barnacle settlement, but also raise concerns about its environmental impact (long-term usage), particularly the potential risks to non-target species before using the compound in real-world antifouling applications. Despite higher concentrations used compared to previous reports, PC shows potential as an effective natural antifouling compound.