Seaweeds grow in relative abundance along the Irish coastline with about 40,000 tonnes of natural seaweed harvested in Ireland annually, estimated to be worth €18 million. In the last three decades, the discovery of metabolites with biological activities from macroalgae has increased significantly. The vast array of phytochemicals produced by seaweeds include those that exhibit antifungal and antibacterial properties. Such properties have been exploited by researchers in the search for novel antimicrobial compounds. However, there is also a significant requirement for the discovery of compounds active against phytopathogens arising from a multitude of concerns including; climate change, emergence of resistance against current treatments, controlling of invasive species and the negative effects associated with the use of synthetic pesticides. It is estimated that plant diseases can affect 30% of the crop harvest if not managed correctly and efficiently. These needs have driven the main objectives of the present study: (1) to screen seaweeds collected from the South-East coast of Ireland against fungal and bacterial plant pathogens, (2) to compare activity to commercially available plant protection products and (3) to isolate and identify the antimicrobial compounds from the most promising crude extracts. The initial antifungal screen using the poisoned food technique, found the methanol extract of the green seaweed, Ulva lactuca exhibited the strongest antifungal activity against the destructive root rot fungus Heterobasidion annosum sensu stricto. Additionally, the poisoned food technique was reported to be the optimum test method for this particular extract and fungal strain. The efficacy of this active seaweed extract was compared to the chemical Plant Protection Product urea; the main chemical method currently in use for controlling this worldwide pathogen. The protectant activity of both the extracts and urea were evaluated on disks of Picea sitchensis (Sitka spruce), a species sensitive to infection from H. annosum s.s. The data demonstrated that the extracts exhibited strong protectant activity at 20 mg/mL after 18 h and 24 h soaking periods. Highlighting a potential biopesticide product for use in forestry which is particularly relevant as result of the predicted pressures associated with climate change making trees more susceptible to disease and encouraging the introduction of invasive species. Crude seaweed extracts were also tested against nine quarantine bacterial plant pathogens using the disk diffusion assay. The seaweeds were found to exhibit a broad spectrum of activity, with methanol found to be the optimum solvent for extracting antimicrobial compounds from the different seaweed species. Polysiphonia lanosa showed activity against the majority of the tested organisms; particularly the methanol extracts which proved the most potent with an inhibition zone of 15.83 ± 0.41 mm exhibited against Xanthomonas arboricola. The minimum inhibitory concentration of the most effective extract, the methanol extract of P. lanosa, was determined to be 6.25 mg/mL with the same concentration also found to exhibit antibiofilm activity (>80% inhibition) against Xanthomonas fragariae in a dose response manner. Unfortunately, the crude methanol extract of P. lanosa was found to be phytotoxic to the model plant species Arabidopsis thaliana at and below its minimum inhibitory concentration (6.5 mg/mL). The crude extracts, therefore, contained a number of compounds that were toxic to A. thaliana in vivo and required isolation and purification of the bioactive(s) to allow for a more accurate phytotoxicity study. Investigation into conventional and non-conventional extraction techniques demonstrated that solvent extraction and Soxhlet extraction achieved similar yields of extracts with the same degree of activity compared to ultrasound-assisted extraction. The structure of the antibacterial compound present in P. lanosa could not be identified completely due to the fact that this was a semi-pure fraction, but it was concluded to be a bromophenol through its distinctive constituents such as OH groups, aromatics and the presence of a halogenated compound believed to be a bromine. Antioxidant activity was also exhibited by the extract with phenolic compounds widely reported to be responsible for such activity. Further purification is required to identify the complete structure of the bromophenol bioactive(s). However, activity of the semi-pure fraction was significantly higher compared to its crude extract, demonstrating the increased concentration of the active compounds and the possible antagonistic effects of other compounds present in the P. lanosa extract. In conclusion, this study demonstrated that seaweeds represent a promising source of pesticidal compounds that are active against a range of fungal and bacterial plant pathogens, with potential biopesticide applications. A growing interest in biopesticides signifies a shift in the reliance on synthetic pesticides to a lower impact alternative. This is the first report of seaweed extracts exhibiting activity against H. annosum s.s., and P. lanosa against the bacterial pathogen X. fragariae.
|Publication status||Unpublished - 2019|
- Seaweed extracts, Biopesticides