MEPS 159:219-227 (1997)  -  doi:10.3354/meps159219

This study examined the inhibition of Hydroides elegans larval settlement by brown algal phlorotannins as well as 2 related compounds, tannic acid and phloroglucinol. Two mechanisms have been suggested to explain the antifouling effects of natural compounds: natural compounds may target macrofoulers directly, or they may regulate the growth of microfoulers, such as bacteria, which in turn affects larval settlement. We hypothesized that phlorotannins, phloroglucinol and tannic acid inhibit H. elegans larval settlement through both mechanisms. In this study, we investigated (1) the LC₅₀ of the 3 compounds on H. elegans larvae, (2) the EC₅₀ for inhibition of H. elegans larval settlement by the 3 compounds, (3) antibacterial activity of the 3 compounds, and (4) settlement of H. elegans larvae on both monospecies and multispecies bacterial films. Twelve different strains of marine bacteria were isolated for antibiosis assays and larval settlement assays. Our results indicated that phlorotannins, tannic acid, and phloroglucinol were inhibitory to H. elegans larval settlement and to the growth of certain marine bacteria. Assays of larval settlement on bacterial films revealed that H. elegans larvae settled differentially on different species of bacteria. Changes in either bacterial species or ratio of bacterial species in multispecies bacterial films also affected H. elegans larval settlement. An integration of results from the antibiosis assays together with those from the assays of larval settlement on bacterial films revealed that phlorotannins and the related compounds were inhibitory to some of the bacterial species that induced high levels of H. elegans larval settlement; however, some of the bacteria that induced low levels of H. elegans larval settlement were resistant to the compounds. Therefore, we speculate that the bacteria which induce high level of H. elegans larval settlement may be lacking in a biofilm that is developed under the influence of the compounds, and the niches become available to the phenolic-resistant bacteria that cause low levels of H. elegans larval settlement. Consequently, the phenolic-treated biofilm may become unfavorable to H. elegans larval settlement.

Hydroides elegans · Natural compounds · Settlement inhibition · Phlorotannins · Bacterial films