Investigation of the antifouling properties of extracts from marine microalgae
Student thesis: Doctoral Thesis
Biofouling, through undesirable colonisation of surfaces by marine organisms, is responsible for significant economic and environmental impacts that justify the use of antifouling (AF) coatings. Extracts from cultivable microalgae were investigated as a sustainable and eco-friendly source of AF compounds. A multi-functional bioassay was developed to test microalgal extracts against biofilm-forming bacteria and diatoms(behaviour, growth, adhesion, toxicity, species interactions). The AF activity of extracts from seven microalgae was investigated against microfoulers, macroalgae and barnacles,and, the impact of stresses was assessed. Inhibitory activities were recorded against a wide spectrum of organisms between 0.01 and 100 μg.mL-1, and complex dose responses with stimulatory effects were highlighted. The AF activity was improvedusing competition and dark stress on marine microalgae, and was repressed using predation on freshwater species. Replicate experiments conducted using dark stress demonstrated the stability of stress responses, when comparing AF activity and metabolic profiles, for Cylindrotheca closterium and Thalassiosira pseudonana. These strains, which showed high biomass and extract yields (5%-10%), were further studied for fractionation and field-tests. Fractions of intermediate polarity from C. closterium demonstrated the most potential in the laboratory. In the field tests, Cc5 still showed moderate activity against early formation of algal slime in Portsmouth, and Cc6 reduced invertebrates’ settlement in Toulon. The microalgal culture-associated bacteria studied showed high diversity, microalgal proximity, and multifaceted chemical interactions. A comparative study showed that microalgal and bacterial extracts exhibited different patterns of AF activity and distinct chemical signatures, confirming the microalgalorigin of extracts studied throughout. This work proposed a multi-functional AF screening of microalgal compounds for a better understanding of the mode of action, it demonstrated the tangible potential of microalgal extracts for AF applications, and it addressed the limits of an AF strategy based solely on chemical interactions -encouraging future use of combinatory approaches.
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