Ichthyotoxic red tide is a problem that the world is facing and needs to solve. The use of antialgal compounds from marine macroalgae to suppress ichthyotoxic red tide is considered a promising biological control method. Antialgal substances were screened and isolated from
Bangia fusco-purpurea,
Gelidium amansii,
Gloiopeltis furcate,
Hizikia fusifarme,
Laminaria japonica,
Palmaria palmata, and
Sargassum sp. to obtain new materials for the development of algaecides against ichthyotoxic red tide microalgae using bioactivity-guided isolation methods. The fractions of seven macroalgae exhibited selective inhibitory activities against
Amphidinium carterae and
Karenia mikimotoi, of which the ethyl acetate fractions had the strongest and broadest antialgal activities for the two tested red tide microalgae. Their inhibitory effects on
A.
carterae and
K.
mikimotoi were even stronger than that of potassium dichromate, such as ethyl acetate fractions of
B.
purpurea,
H.
fusifarme, and
Sargassum sp. Thin-layer chromatography and ultraviolet spectroscopy were further carried out to screen the ethyl acetate fraction of
Sargassum sp. Finally, a new glycolipid derivative, 2-
O-eicosanoyl-3-
O-(6-amino-6-deoxy)-
β-D-glucopyranosyl-glycerol, was isolated and identified from
Sargassum sp., and it was isolated for the first time from marine macroalgae. The significant antialgal effects of 2-
O-eicosanoyl-3-
O-(6-amino-6-deoxy)-
β-D-glucopyranosyl-glycerol on
A.
carterae and
K.
mikimotoi were determined.
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