Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / CoordenaÃÃo de AperfeiÃoamento de NÃvel Superior / FundaÃÃo Cearense de Apoio ao Desenvolvimento Cientifico e TecnolÃgico / As macroalgas marinhas sÃo fontes de compostos com atividade antioxidante como compostos fenÃlicos, pigmentos carotenÃides e vitamina E. O objetivo deste trabalho foi avaliar a atividade antioxidante in vitro de extratos metanÃlicos (50%) de macroalgas cearenses, atravÃs da capacidade de sequestrar o DPPH, habilidade de quelaÃÃo do Ãon ferroso (FIC), poder de reduÃÃo do ferro (FRAP) e degradaÃÃo do β-caroteno. AlÃm disso, tambÃm foi determinado o conteÃdo de compostos fenÃlicos pelo mÃtodo de Folin-Ciocalteu, seguido de uma prospecÃÃo fitoquÃmica para indicar possivelmente as principais classes de compostos. Os teores de α- e β-caroteno, luteÃna e α- e -tocoferol foram quantificados por cromatografia lÃquida de alta eficiÃncia. De um modo geral, capacidade de sequestrar o DPPH, FRAP e conteÃdo de compostos fenÃlicos foram maiores nos extratos das algas pardas, seguidos dos das algas verdes e vermelhas. FIC mais elevada foi observada nos extratos das algas vermelhas, seguidas das pardas e verdes. No sistema modelo β-caroteno/Ãcido linolÃico as maiores atividades foram determinadas nas clorÃfitas e rodÃfitas, tendo variado de 64,8% a 95,3%, nas ocrÃfitas, inferiores a 40,5%, com exceÃÃo de Padina gymnospora com cerca de 92%. Os extratos algÃceos analisados apresentaram atividades antioxidantes semelhantes ou superiores aos controles positivos (quercetina, BHA, BHT, Ãcido ascÃrbico, α-tocoferol, β-caroteno e EDTA). FenÃis foram detectados apenas em algas pardas; antocianinas, antocianidinas, chaconas, auronas e leucoantocianidinas foram observadas apenas em algumas espÃcies do Filo Rhodophyta; as demais classes de compostos fenÃlicos investigadas foram observadas em pelo menos uma espÃcie dentro de cada Filo, com exceÃÃo de flavanonÃis que nÃo foram encontrados nos extratos de algas verdes. O conteÃdo de compostos fenÃlicos foi o principal responsÃvel pelas atividades de sequestro do DPPH, FRAP e FIC nas algas verdes e vermelhas. Nas pardas esses compostos sà influenciaram no FRAP. β-Caroteno e luteÃna foram quantificados em todos os extratos de algas verdes, vermelhas e pardas, sendo a luteÃna o carotenÃide majoritÃrio nas clorÃfitas e rodÃfitas. Com exceÃÃo das algas pardas que naturalmente nÃo possuem α-caroteno, apenas os extratos de cinco espÃcies de clorÃfitas e rodÃfitas nÃo apresentaram esse composto. Todos os extratos analisados apresentaram α-tocoferol, menos Ulva fasciata e U. lactuca. Extratos de onze espÃcies apresentaram δ-tocoferol. As atividades antioxidantes e os teores de compostos detectados nos extratos algÃceos foram distintos, mas todos eles apresentaram potencial antioxidante. / Seaweeds are sources of a wide variety of beneficial compounds for human. Many of these compounds have antioxidant activity, such as phenolic compounds, carotenoids, and vitamin E. The aim of this research was to evaluate the in vitro antioxidant activity of 50% methanolic extracts from seaweed collected in the coastline of Cearà State, Brazil. The methods used were: DPPH radical scavenging, ferrous ion chelation (FIC), ferric reducing antioxidant power (FRAP), and β-carotene bleaching. In addition to in vitro antioxidant activity, the total phenolic content (TPC) was measured by the Folin-Ciocalteu method, followed by a phytochemical prospecting to point out which are the main classes of compounds present in the algal extracts. The quantification of carotenoids (α- and β-carotene, and lutein) and vitamin E (α- and -tocopherol) was carried out by HPLC. In general, the extracts of brown algae showed the highest ability to scavenger the DPPH radical, the largest FRAP and the highest TPC, followed by extracts of green and red algae. The greatest FIC was observed in red alga extracts, followed by brown and green alga extracts. The high antioxidant activity in β-carotene/linoleic acid model system of green and red alga extracts ranged from 65% to 95%, however it represented less than 40% in brown alga extracts, exception to Padina gymnospora extract which presented activity up to 92%. The majority of the algal extracts analyzed in this study presented activity similar to or even greater than those observed in positive controls (quercetin, BHA, BHT, ascorbic acid, α-tocopherol, β-carotene and EDTA). Fenols were detected in brown algae only; anthocyanins, anthocyanidins, chacons, aurons and leucoanthocyanins were observed in some species of Rhodophyta Phylum. All the other classes of phenolic compounds were found in at least one species within each Phylum, exception to flavononols which have not been detected in green alga extracts. TPC was the main responsible for the ability to scavenger the DPPH radical, FIC and FRAP in green and red algae. On the other hand, in brown algae TCP was influenced only by FRAP. All extracts of green, red and brown algae exhibited the presence of β-carotene and lutein. The latter was the major carotenoid within Chlorophyta and Rhodophyta. Naturally absent in brown algae, α-carotene was not detected in five species of Chlorophyta and Rhodophyta algae. α-Tocopherol was determined in all species, except Ulva fasciata and U. lactuca extracts. The isomer δ-tocopherol was quantified in eleven out of twenty-three alga species. Antioxidant activity and levels of compounds in the algal extracts were different, but all of them showed antioxidant potential.