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 eubacte rium (Proteobacteria [alpha subclass], Rhodobacter grouping), which, typica l of bacterial symbionts, has not yet been cultivated or isolated in pure c ulture. This investigation tested the hypothesis that P. laceolata gall for mation was caused by the associated eubacterium using a species-specific rD NA probe (S-S-P.l.sym-0949-a-A-25) to identify and assay for symbiont prese nce during consecutive laboratory induction trials. Gall induction was quan tified and whole-cell in situ hybridization used to determine the relative percentage of symbiotic eubacteria in inoculation homogenates. In situ hybr idization of symbionts in sections allowed localization and monitoring of t his microbe during gall development. Induction trial results indicate a sig nificant correlation between bacterial symbiont presence and gall initiatio n (P = 0.00005). The gall bacterium comprised the majority of the eubacteri a hybridized in laboratory induction homogenates (85-97%?), in galls induce d in the laboratory and in three algal populations in nature. The evidence presented here demonstrates the causative role of the identified eubacteriu m in gall induction and formation. This investigation is significant in the application of molecular methods towards understanding the roles of noncul tivable marine bacteria in marine algal-microbe interactions.