Abstract
Environmental stress-associated tissue deterioration remains an important challenge in the cultivation of Kappaphycus alvarezii . This study evaluated the effects of immersion-based application of Origanum vulgare extract on the growth performance and pigmentation of K. alvarezii cultivated under field conditions. In vitro antibacterial assays showed inhibitory activity of the extract against Staphylococcus aureus (21 mm inhibition zone) and Escherichia coli (19 mm inhibition zone). An in vivo experiment was conducted using four immersion treatments: control, 5 min, 15 min, and 25 min exposure durations. Growth performance differed significantly among treatments, with the 15-min immersion treatment producing the highest final weight (259.67 ± 13.05 g). Pigmentation scores were also significantly higher in the 15- and 25-min treatments compared with the control. Regression analysis suggested that intermediate immersion durations between approximately 15 and 18 min may provide favorable growth responses, although this estimate should be considered preliminary because only four treatment durations were evaluated. Overall, immersion-based application of O. vulgare extract improved growth performance and maintained better thallus pigmentation under field conditions. These findings support the potential use of plant-derived bioactive treatments as functional approaches for improving thallus condition in K. alvarezii aquaculture systems. However, the present study did not directly quantify disease incidence, pathogen abundance, or tissue mortality; therefore, interpretations regarding ice-ice disease mitigation should be considered preliminary.