Stable microbial communities in the sponge Crambe crambe from inside and outside a polluted Mediterranean harbor

Marine sponges have been shown to harbor diverse microbial symbiont communities that play key roles in host functioning, yet little is known about how anthropogenic disturbances impact sponge-microbe interactions. The Mediterranean sponge Crambe crambe is known to accumulate heavy metals in polluted harbors. In this study, we investigated whether the microbiome of C. crambe differed between sponges inhabiting a polluted harbor in Blanes (Spain) and a nearby (<1 km) natural environment. Triplicate sponge and ambient seawater samples were collected from each site and the microbial composition of each sample was determined by 16S rRNA gene sequence analysis (Illumina Hi-Seq platform). No significant differences in the diversity or structure of microbial communities in C. crambe were detected between habitats, while a significant difference in community structure was observed in ambient seawater inside and outside of the polluted harbor. The microbiome of C. crambe was clearly differentiated from free-living seawater microbes and dominated by Proteobacteria, specifically a single betaproteobacterium that accounted for >86% of all sequence reads. These results indicate that sponge microbiomes exhibit greater stability and pollution tolerance than their free-living microbial counterparts, potentially mitigating the effects of pollutants on coastal marine communities.