Bacterial fingerprints of marine sponges with different bacterial transmission strategies

Many marine sponges host diverse communities of microorganisms that are often vertically transmitted from mother to oocyte or embryo. However, not all sponges obtain all consistently associated bacteria via vertical transmission. Horizontal transmission (acquisition from the environment) has often been proposed to occur in marine sponges, but the mechanism is poorly understood. To assess the contribution of the mode of transmission to the bacterial composition in the sponge tissue, we analyzed the bacterial fingerprints of three sympatric sponges that have previously been reported to acquire bacteria either via vertical or horizontal transmission. Detailed studies have shown that Petrosia ficiformis does not transmit its associated bacteria vertically. Thus, this species was taken as a model for horizontal transmission. The bacterial fingerprint of Corticium candelabrum adults closely resembles that of C. candelabrum embryos, and hence this sponge was included as a model for vertical transmission. Crambe crambe is in contrast to the other species a low-bacterial-abundance sponge and was included to detect unspecific environmental acquisition of bacteria. The comparative study was performed by pyrosequencing of barcoded PCR-amplified 16S rRNA gene fragments. We found that P. ficiformis and C. candelabrum harbour bacteria similar to other high-microbial-abundance sponges, whereas C. crambe shows some overlap with local seawater bacteria. In addition, the bacteria associated with P. ficiformis were closely related to bacteria that were associated with other sponges for which vertical transmission was proven. Therefore, our results suggest that the mode of transmission has no major impact on the bacterial profile of marine sponges