<![CDATA[Caulerpa racemosa is an economically valuable green seaweed with potential as a functional food source because it contains bioactive compounds, including antioxidants. This study aimed to analyze the effect of different seedling weights of C. racemosa on antioxidant content in a cultivation system using recycled plastic bottle containers. The study was conducted for 14 days in Gerupuk Hamlet, Sengkol Village, Pujut District, Central Lombok Regency, using an experimental method with a Completely Randomized Design (CRD) consisting of three seedling weight treatments, namely P1 80 g, P2 90 g, and P3 100 g, each with two replications. The observed parameters included growth, biomass retention, antioxidant activity, chlorophyll, proximate composition, and water quality. The results showed that the weight of C. racemosa increased on day 7, namely P1 90.5 g, P2 115 g, and P3 130 g, but decreased on day 14 to 62.5–67 g. The highest biomass retention was found in P1 at 78.13%, while the lowest was found in P3 at 62.5%. Antioxidant activity increased in all treatments, with the highest value in P3 at 55.71%, followed by P1 at 55.10% and P2 at 52.15%. The highest final chlorophyll content was also found in P3 at 10.79 mg/L. The ANOVA test on the biomass retention parameter showed that different seedling weights did not have a significant effect on the biomass retention of C. racemosa. The decrease in biomass was presumed to be influenced by the limited space in the 600 mL bottles, the absence of substrate, suboptimal water circulation, and undetected phosphate. Therefore, the use of recycled plastic bottles still allowed C. racemosa to survive and increase antioxidant activity, but it was not yet optimal for supporting biomass growth until the end of the cultivation period.]]>
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