In vitro cultivation of three <Emphasis Type="Italic">Kappaphycus alvarezii</Emphasis> (Rhodophyta,Areschougiaceae) variants (green,red and brown) exposed to a commercial extract of the brown alga <Emphasis Type="Italic">Ascophyllum nodosum</Emphasis> (Fucaceae,Ochrophyta)

Diseases such as “ice-ice” and “goose bumps” are responsible for loss of almost 70% of infected Kappaphycus production. To improve the growth of K. alvarezii and reduce the impact of diseases, the effect of the commercial Ascophyllum nodosum extract was tested. Five grams of apical branches of each K. alvarezii variant (n = 6) were bathed at different extract concentrations (e.g., 5, 10, 15, 20, 25, and 30 g L⁻¹) for 1 hour. To test the effect of the extract on epiphytes apical tips were not cleaned. After 2 weeks, the epiphytes such as Cladophora sp. and Ulva sp. disappeared. Polysiphonia subtilissima survived but its appearance was weak. The highest K. alvarezii growth rates were observed at extract concentrations of 15 and 20 g L⁻¹ (p < 0.001), almost double the expected for a 45-day production cycle used at Sepetiba Bay, Brazil. These results show that the extract was efficient at improving the growth of K. alvarezii and reducing the presence of some epiphytes.