New insecticides and antimicrobials derived from Sargassum wightii and Halimeda gracillis seaweeds: Toxicity against mosquito vectors and antibiofilm activity against microbial pathogens

Abstract

Phytochemicals derived from seaweeds offer a novel source of compounds to develop new insecticides and antimicrobials. In the present study, two seaweed species, the brown Sargassum wightii and the green Halimeda gracilis, were investigated. Extracts were obtained using five solvents of varying polarity (acetone, chloroform, methanol, ethanol, and water). The total phenolic content, total flavonoid content and the antioxidant activities of the extracts were estimated. The ethanol extract from S. wightii exhibited more potent antioxidant activity compared to that of the H. gracilis extracts. The ethanol extract from S. wightii possessed higher values of total phenolics, flavonoids, and antioxidant activity compared to those of the H. gracillis ethanol extract. The total phenolic and flavonoid content for ethanol extracts of S. wightii and H. gracillis was 5.5 mg gallic acid equivalent (GAE)/g, 3.8 mg GAE/g, 10 mg quercetin equivalent (QE)/g, and 8.3 mg QE/g, respectively. The total antioxidant activity of the seaweed extracts was 47 mg ascorbic acid equivalent (AAE)/g for S. wightii and 35.9 mg AAE/g for H. gracilis. Additionally, the bioactive compounds present in these seaweeds were characterized by FTIR spectroscopy and GC–MS analysis, and the antimicrobial activity of these extracts was then tested against the biofilm formation capabilities of three pathogenic Gram-negative bacteria. The ethanol extracts from S. wightii and H. gracillis presented the highest inhibitory effect with up to a 40–75% significant reduction in biofilm formation by Gram-negative bacteria. Finally, their insecticidal potential was assessed against late third instar larvae of malaria vectors (Anopheles stephensi), dengue and Zika virus vectors (Aedes aegypti), and the Japanese encephalitis vector (Culex tritaeniorhynchus). LC50 values estimated for the S. wightii ethanol extract were lower than 50 ppm against all tested mosquito species. Light microscopy revealed that morphological changes were triggered upon exposure to seaweed extracts. Overall, the present study sheds light on the relevant bioactivity of phytochemicals obtained from two widespread seaweeds, suggesting the possibility that these seaweeds can be used to develop novel and biodegradable pesticides and also antimicrobial drugs that can be used to inhibit the development of microbial biofilms.