Abstract
<title>Abstract</title> <p> Marine algae are recognized as important sources of bioactive compounds with diverse biological activities and biotechnological potential. This study investigated the antimicrobial, antioxidant and toxicity profile of extracts obtained from the red macroalgae <italic>Amansia multifida</italic> and <italic>Enantiocladia duperreyi</italic> , and the microalgae <italic>Chaetoceros muelleri</italic> and <italic>Thalassiosira pseudonana</italic> . Biomass was extracted using solvents of increasing polarity (hexane, ethyl acetate and ethanol), and the resulting extracts were evaluated by disk diffusion and broth microdilution assays against bacterial and fungal strains. Antioxidant capacity was determined using the 2,2-diphenyl-1-picrylhydrazyl method, while toxicity was assessed using the <italic>Artemia salina</italic> lethality assay. Several extracts exhibited antimicrobial properties, particularly hexane extracts, which exhibited a broader spectrum of inhibition. Minimum inhibitory concentration values ranged from 62.5 to 500 µg mL⁻¹. Five extracts demonstrated radical scavenging above 50%, with the ethyl acetate extract of <italic>E. duperreyi</italic> showing the highest performance. Toxicity assays indicated low to moderate effects, with median lethal concentration values ranging from 248 to 548.8 µg mL⁻¹. Differences in antimicrobial, antioxidant and toxicity profiles were observed between species and collection sites, as well as among extraction solvents. These findings reinforce the potential of marine macroalgae and microalgae as sources of bioactive compounds and contribute to the bioprospecting of marine biodiversity from the Brazilian coast. Further studies are warranted to characterize the compounds responsible for the observed biological activities. </p>