Comparative effects of indole derivatives as antifouling agents on the growth of two marine diatom species

Antifouling agents, used to prevent biofouling, need to be assessed for their impacts on marine organisms and environment before the application. Diatoms are one of the main components of fouling biofilms, which play important roles in the formation of biofouling. Particularly, diatoms are also the important ingredients of primary production and present interest as ecotoxicological models in marine environment. In this study, two benthic diatoms Nitzschia closterium f. minutissima and Navicula climacospheniae, widely distributed in fouling biofilm, were used as models for screening the activities of potential antifoulants. Nine indole derivatives were tested and CuSO4 was used as a reference. Indole derivatives showed significant anti-algal activities and the EC50 values of most indole derivatives were lower than that of CuSO4. Halogen substituent enhanced the anti-algal activities of compounds, and the most efficient compounds for N. closterium f. minutissima were gramine and 7-chloroindole with the EC50 values of 1.94 and 2.1 mg/L, while for N. climacospheniae, 7-chloroindole and 6-bromoindole were the most efficient and the EC50 values were 3.91 and 4.25 mg/L, respectively. In conclusion, indole derivatives would be one of the promising candidates as antifoulants and our results strengthened the need to perform antifouling activity assays and environment-friendly evaluations.