Allelopathic effects of Padina sp. and Sargassum sp. as biological control agents of harmful algal blooms (HABs) in the Persian Gulf and Gulf of Oman.

Harmful algal blooms (HABs) pose serious threats to coastal ecosystems, public health, and local economies. This study explores the potential of two native macroalgal genera, Padina sp. and Sargassum sp., collected from the Persian Gulf (Bandar Abbas) and the Gulf of Oman (Chabahar), as biological control agents against Cochlodinium polykrikoides. Species identification was achieved through morphological observations and DNA barcoding (rbcL and psbA), ensuring accurate classification despite overlapping traits. Methanolic extracts of Padina (sp. A and sp. C) and Sargassum (sp. B and sp. D) strains inhibited C. polykrikoides, with Padina sp. A showing the strongest activity (LC₅₀ = 122.2 µg/mL) and Sargassum sp. D the weakest (LC₅₀ = 371.2 µg/mL). Fractionation and identification of the most active extracts using thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC) revealed the presence of terpenoid and phenolic functional groups in the active fractions of Padina sp. A and Sargassum sp. B, respectively. These extracts exhibited negligible effects on non-target microalgae (Chlorella vulgaris, Isochrysis galbana) at concentrations > 1 mg/mL, shrimp larvae (Litopenaeus vannamei), and human endothelial cells (HUVEC), highlighting their selectivity and safety. These findings demonstrate that Padina and Sargassum produce bioactive compounds capable of selectively suppressing harmful microalgae, underscoring their promise as environmentally compatible agents for managing HABs in marine and aquaculture systems.