Effect of bacteria associated with the green alga Ulva reticulata on marine micro- and macrofouling

The green alga Ulva reticulata (Forsskal) is often free from biofouling in Hong Kong waters. An early study indicated that bioactive substances from this alga inhibit settlement of the polychaete Hydroides elegans (Haswell). It is also predicted that epibiotic bacteria protect this alga from micro- and macrofouling. In this study, bacterial strains from the surface of U. reticulata were isolated and their inhibitive activities on micro- and macrofouling assayed. The strains were identified by 16S rRNA analysis as belonging to the genera Alteromonas, Pseudoalteromonas and Vibrio. There was no significant effect of these strains or their extracts (aqueous and ethanol) on the growth of five Vibrio strains isolated from natural biofilm. Two bacterial strains (Alteromonas sp. and Vibrio sp. 3) were non-toxic to the benthic diatom Nitzschia paleacea (Grunow) while the other five strains caused a low level of mortality. No one bacterial strain was toxic to the larvae of H. elegans. Aqueous extract of one of the isolated bacterial species, i.e. Vibrio sp. 2, significantly (p < 0.00001) inhibited the settlement and metamorphosis of H. elegans larvae. The putative antifouling compounds have a molecular weight of > 100kD. On the other hand, biofilm of Pseudoalteromonas sp. 2 and aqueous extract of Vibrio sp. 2 suppressed the settlement of larvae induced by 3-isobutyl-1-methylxanthine (IBMX). Other epibiotic bacteria and their extracts had neither inhibitive nor inductive effects on larval settlement of H. elegans. The results indicate that the antifouling mechanism of U. reticulata may be dependent not only on materials from the macroalga itself but also on the epibiotic bacteria on the algal surface.