Investigation of biopesticide potential of extracts obtained from microalgae by fermentation

Today, the excessive use of chemical pesticides causes environmental pollution, deterioration in soil and water quality, and serious risks to human health. Promoting sustainable agricultural practices and developing alternative biotechnological solutions are of critical importance for ecosystem health and food security. In this context, microalgae and cyanobacteria stand out as environmentally friendly biopesticide candidates due to their ability to produce bioactive compounds. In this thesis study, the bioactivity of green extracts obtained through microbial fermentation using the cyanobacterium Aliinostoc sp. AQUAMEB-58 (#23) strain was evaluated as an alternative to the negative effects of chemical pesticides on the environment and human health. Within the scope of this approach, which aims to contribute to sustainable agricultural practices, the antioxidant, antibacterial, and antifungal activities of the fermented extracts were compared with those of extracts prepared using classical methods. First, the effect of the autoclaving pretreatment was evaluated, and it was observed that the antioxidant and antibacterial activities of the autoclaved extracts were higher. Accordingly, autoclaving of the biomass prior to fermentation was standardised during the optimisation phase. In liquid fermentation studies conducted with Saccharomyces cerevisiae and Lactobacillus plantarum microorganisms, three basic parameters, namely algal concentration, microorganism inoculation rate, and fermentation time, were optimised using the Box-Behnken experimental design via Design Expert 7.0 software within the scope of the Response Surface Methodology (RSM). The suitability of the established model was statistically evaluated using analysis of variance (ANOVA). The optimal conditions for Saccharomyces cerevisiae were determined to be 39.44 mg/mL algal biomass concentration, 6.82% inoculation, and 4.57 days; for Lactobacillus plantarum, they were 31.52 mg/mL algal biomass concentration, 9.96% inoculation rate, and 2.61 days of fermentation time. Thanks to Lactobacillus plantarum's ability to produce lactic acid and lower pH in an anaerobic environment, cell wall permeability increased, and higher biological activity was observed in a shorter time. In terms of antioxidant activity, the DPPH radical scavenging capacity of fermented extracts was found to be 177% (Lactobacillus plantarum.) and 277% (Saccharomyces cerevisiae) higher than that of classical extracts. This is explained by yeast fermentation breaking down proteins into peptides, thereby creating small molecules with antioxidant effects. In antibacterial tests against Clavibacter michiganensis and Pseudomonas syringae pv. tomato pathogens, the MIC values of fermented extracts were found to be lower than those of classical extracts. Antifungal activity was only observed at a significant level in methanol extracts. This indicates that the effectiveness of fermentation may vary depending on the target microorganism. This study provides a unique contribution to the literature by statistically demonstrating the first comprehensive fermentation optimisation with Aliinostoc sp. strains. The high bioactivity of fermented extracts is considered an important step towards the production of environmentally friendly biopesticides obtained without the use of chemical solvents. Future studies are recommended to evaluate different microalgae species and characterise them at the metabolite level.