| Summary: | This study aimed at the characterization of the antioxidant power of polyphenol extracts (PE) obtained from the algae <i>Cystoseira foeniculacea</i> (CYS) (Phaeophyta) and from the halophyte <i>Halocnemum strobilaceum</i> (HAL), growing in the solar saltworks of western Sicily (Italy), and at the evaluation of their anti-microfouling properties, in order to correlate these activities to defense strategies in extreme environmental conditions. The antioxidant properties were assessed in the PE based on the total antioxidant activity test and the reducing power test; the anti-microfouling properties of the two PE were evaluated by measuring the growth inhibition of marine fish and shellfish pathogen bacteria as well as marine surface fouling bacteria and microalgae exposed to the fractions. Similar polyphenol content (CYS 5.88 ± 0.75 and HAL 6.03 ± 0.25 mg gallic acid equivalents (GAE) g<sup>−1</sup> dried weight, DW) and similar reducing power percentage (93.91 ± 4.34 and 90.03 ± 6.19) were recorded for both species, even if they exhibited a different total antioxidant power (measured by the percentage of inhibition of the radical 2,2 diphenyl-1-picrylhydrazyl DPPH), with CYS (79.30) more active than HAL (59.90). Both PE showed anti-microfouling properties, being inhibitors of adhesion and growth of marine fish and shellfish pathogen bacteria (<i>V. aestuarianus</i>, <i>V. carchariae</i>, <i>V. harveyi</i>, <i>P. elyakovii</i>, <i>H. aquamarina</i>) and fouling bacteria (<i>V. natriegens</i>, <i>V. proteolyticus</i>, <i>P. iirgensii</i>, <i>R. litoralis</i>) with minimum inhibitory concentrations comparable to the commercial antifouling products used as a positive control (SEA-NINE™ 211N). Only CYS was a significant inhibitor of the microalgae strains tested, being able to reduce <i>E. gayraliae</i> and <i>C. closterium</i> growth (MIC 10 µg·mL<sup>−1</sup>) and the adhesion of all three strains tested (<i>E. gayraliae</i>, <i>C. closterium</i> and <i>P. purpureum</i>), suggesting its promise for use as an antifouling (AF) product.
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