Biological Control of Harmful Algal Blooms Using Padina sp. and Sargassum sp. from the coastal waters of the Persian Gulf and Gulf of Oman

Harmful algal blooms (HABs) are increasingly recognized as serious threats to coastal ecosystems, public health, and local economies. This study investigates the potential of two native macroalgal genera, Padina sp. and Sargassum sp., collected from the coastal waters of the Persian Gulf (Bandar Abbas) and the Gulf of Oman (Chabahar), as biological control agents against the harmful species Cochlodinium polykrikoides . We identified the species using a combination of morphological observations and DNA barcoding (rbcL and psbA), which was necessary because some specimens displayed overlapping or variable traits that made morphology alone insufficient. After confirming the identities, we prepared methanolic extracts and tested their effects on harmful microalgae. Both species showed inhibitory activity, although the strength of the effect differed. Extracts from Padina sp. A were the most potent, with an LC₅₀ of 122.2 µg/mL, while Sargassum sp. B showed moderate but still significant inhibition (LC₅₀ = 182.6 µg/mL).Importantly, the extracts had minimal impact on non-target microalgae. Growth of Chlorella vulgaris and Isochrysis galbana remained largely unaffected, even at the highest concentrations tested. Toxicity assays further indicated no adverse effects on shrimp larvae ( Litopenaeus vannamei ) or human endothelial cells (HUVEC), suggesting a high degree of selectivity. These results suggest a notable degree of selectivity, which is critical for any candidate compound intended for use in aquaculture or environmental applications. Chemical profiling through TLC and HPLC provided further insight into the nature of the active constituents. Several fractions showed strong chromatographic signals characteristic of terpenoids, phenolic compounds, flavonoids, and various carbohydrate-associated metabolites. Although the specific structures require further elucidation, the chemical signatures align with secondary metabolite profiles commonly reported for Padina and Sargassum species. Taken together, the findings indicate that these macroalgae possess bioactive metabolites capable of suppressing harmful microalgal species while leaving beneficial microalgae and non-target organisms largely unharmed. This combination of efficacy and selectivity underscores their potential as environmentally compatible tools for mitigating harmful algal blooms in marine and aquaculture systems. Chemical analysis using TLC and HPLC revealed several active fractions, with signals characteristic of terpenoids, phenolic compounds, flavonoids, and carbohydrate-associated metabolites. While the exact structures remain to be determined, these chemical signatures are consistent with secondary metabolites commonly found in Padina and Sargassum . Overall, our findings indicate that these macroalgae produce bioactive compounds capable of suppressing harmful microalgae while leaving beneficial species and non-target organisms largely unharmed, highlighting their potential as environmentally compatible agents for managing harmful algal blooms in marine and aquaculture settings.