Five species of seagrasses and six species of seaweeds in 11 genera were collected from the Gulf of Mannar, on the southeastern coast of India and were screened for antioxidant activity by in vitro methods. Methanolic and water extracts of the seagrass Halophila ovalis had the highest antioxidant activity, with IC50 values 78.95±1.15 and 85.40±3.40 μg ml-1 (means±SE), respectively, using the NO radical scavenging method. With the H2O2 scavenging method, both methanolic and water extracts of many of the seagrasses and seaweeds had higher antioxidant activity, with the IC50 values lower than that of the standard tocopherol. Except for vitamin B3, the rest of the antioxidant-potent phytoconstituents estimated in these marine organisms were found to be strongly positively correlated with the antioxidant activity.

Of the six genera of seagrasses recorded in India, four were present throughout a 1-year study period at a station off Hare Island, Tuticorin (8°45′ N, 78°12′ E), in the Gulf of Mannar region. Phytochemical analysis of these seagrass extracts revealed the presence of pharmaceutically potent secondary metabolites and appreciable quantities of primary and some secondary metabolites.

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