PHLOROTANNINS AND RELATED-COMPOUNDS AS LARVAL SETTLEMENT INHIBITORS OF THE TUBE-BUILDING POLYCHAETE HYDROIDS ELEGANS

This study examined the inhibition of Hydroides elegans larval settlem ent by brown algal phlorotannins as well as 2 related compounds, tanni c acid and phloroglucinol. Two mechanisms have been suggested to expla in the antifouling effects of natural compounds: natural compounds may target macrofoulers directly, or they may regulate the growth of micr ofoulers, such as bacteria, which in turn affects larval settlement. W e hypothesized that phlorotannins, phloroglucinol and tannic acid inhi bit H. elegans larval settlement through both mechanisms. In this stud y, we investigated (1) the LC50 Of the 3 compounds on H. elegans larva e, (2) the EC50 for inhibition of H. elegans larval settlement by the 3 compounds, (3) antibacterial activity of the 3 compounds, and (4) se ttlement of H., elegans larvae on both monospecies and multispecies ba cterial films. Twelve different strains of marine bacteria were isolat ed for antibiosis assays and larval settlement assays. Our results ind icated that phlorotannins, tannic acid, and phloroglucinol were inhibi tory to H. elegans larval settlement and to the growth of certain mari ne bacteria. Assays of larval settlement on bacterial films revealed t hat H., elegans larvae settled differentially on different species of bacteria. Changes in either bacterial species or ratio of bacterial sp ecies in multispecies bacterial films also affected H., elegans larval settlement. An integration of results from the antibiosis assays toge ther with those from the assays of larval settlement on bacterial film s revealed that phlorotannins and the related compounds were inhibitor y to some of the bacterial species that induced high levels of H. eleg ans: larval settlement; however, some of the bacteria that induced low levels of H., elegans larval settlement were resistant to the compoun ds. Therefore, we speculate that the bacteria which induct high level of H. elegans larval settlement may be lacking in a biofilm that is de veloped under the influence of the compounds, and the niches become av ailable to the phenolic-resistant bacteria that cause low levels of H. , elegans larval settlement. Consequently, the phenolic-treated biofil m may become unfavorable to H., elegans larval settlement.