Galls on the marine red alga Prionitis lanceolata (Halymeniaceae):: Specific induction and subsequent development of an algal-bacterial symbiosis

Gall formation in Prionitis laceolata is associated with a specific eubacterium (Proteobacteria [alpha subclass], Rhodobacter grouping), which, typical of bacterial symbionts, has not yet been cultivated or isolated in pure culture. This investigation tested the hypothesis that P. laceolata gall formation was caused by the associated eubacterium using a species-specific rDNA probe (S-S-P.l.sym-0949-a-A-25) to identify and assay for symbiont presence during consecutive laboratory induction trials. Gall induction was quantified and whole-cell in situ hybridization used to determine the relative percentage of symbiotic eubacteria in inoculation homogenates. In situ hybridization of symbionts in sections allowed localization and monitoring of this microbe during gall development. Induction trial results indicate a significant correlation between bacterial symbiont presence and gall initiation (P = 0.00005). The gall bacterium comprised the majority of the eubacteria hybridized in laboratory induction homogenates (85-97%?), in galls induced in the laboratory and in three algal populations in nature. The evidence presented here demonstrates the causative role of the identified eubacterium in gall induction and formation. This investigation is significant in the application of molecular methods towards understanding the roles of noncultivable marine bacteria in marine algal-microbe interactions.