Effect of solid-state fermentation and ultrasonication processes on antimicrobial and antioxidant properties of algae extracts

Algal biomass (AB) is prospective source of valuable compounds, however, Baltic Sea macroalgae have some challenges, because of their high microbial and chemical contamination. These problems can be solved, by using appropriate technologies for AG pre-treatment. The aim of this study was to evaluate the influence of two pre-treatments, solid-state fermentation with the Lactiplantibacillus plantarum LUHS135 and ultrasonication, on the antioxidant and antimicrobial characteristics of macro- (Cladophora rupestris, Cladophora glomerata, Furcellaria lumbricalis, Ulva intestinalis) and Spirulina (Arthrospira platensis) extracts. Also, combinations of extracts and LUHS135 were developed and their characteristics were evaluated. The total phenolic compound content was determined from the calibration curve and expressed in mg of gallic acid equivalents; antioxidant activity was measured by a Trolox equivalent antioxidant capacity assay using the DPPH(•) (1,1-diphenyl-2-picrylhydrazyl), ABTS(•+) 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonic acid), FRAP (Ferric Reducing Ability of Plasma) discoloration methods. Antimicrobial activity was measured by using agar well diffusion assay and in a liquid medium. The highest DPPH(•) and ABTS(•+) was shown by C.rupestris and F.lumbricalis extract × LUHS135 combinations, the highest FRAP - by non-pretreated C.rupestris and F.lumbricalis extract × LUHS135 combinations. Ultrasonicated samples inhibited four out of seven tested pathogens. Finally, the tested pre-treatments showed good perspectives and can be recommended for AB valorization.