Endosymbiotic bacteria in the bivalve Loripes lacteus: Localization, characterization and aspects of symbiont regulation

Loripes lacteus inhabits reduced sediments in shallow waters and harbors chemotrophic sulfur-oxidizing bacteria in its gills. In this study, a combination of molecular (16S rRNA analysis, real time PCR and fluorescence in-situ hybridization) and microscopy approaches (confocal and transmission electron microscopy) was used to characterize this symbiotic association and evaluate factors potentially involved in endosymbiont acquisition and regulation. 16S rRNA sequencing revealed the presence of two phylotypes of endobacteria specifically associated with clam gills. The more abundant phylotype is a close relative to thiotrophic symbionts found in several symbiotic Lucinidae, while the second phylotype clustered with bacteria not known to be involved in symbiotic association. Endobacteria were not detected in other organs including gonads strongly suggesting a horizontal mode of transmission of symbionts to aposymbiotic juveniles. Clams were exposed to 3 weeks of starvation, which caused a significant decrease in the relative abundance of both phylotypes as compared to fresh animals suggesting a lytic degradation of endobacteria within clam gills. In addition, endosymbiont-depleted clams were not able to re-acquire their symbionts once moved back to “optimal” conditions. These results highlight the need for a better understanding of the molecular mechanisms that mediate acquisition, regulation and control of endosymbiotic bacteria by L. lacteus.