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.