Terpenes from the Red Alga Sphaerococcus coronopifolius Inhibit the Settlement of Barnacles

In this study, we screened eight terpenes isolated from the organic extract of Sphaerococcus coronopifolius for their antifouling activity in order to find possible new sources of non-toxic or less toxic bioactive antifoulants. The anti-settlement activity (EC50) and the degree of toxicity (LC50) of S. coronopifolius metabolites was evaluated using larvae of the cirriped crustacean Amphibalanus (Balanus) amphitrite (cyprids and nauplii) as model organism. For five of eight tested metabolites EC50 was lower than 5 mg/L. The most promising results were observed for bromosphaerol (3), which expressed an EC50 value of 0.23 mg/L, in combination with low toxicity levels (LC50 > 100 mg/L). The therapeutic ratio—an index used to estimate whether settlement inhibition is due to toxicity or other mechanisms—is also calculated and discussed.