Investigation of Capitella spp. symbionts in the context of varying anthropic pressures: First occurrence of a transient advantageous epibiosis with the giant bacteria Thiomargarita sp. to survive seasonal increases of sulfides in sediments

Capitella spp. is considered as an important ecological indicator of eutrophication due to its high densities in organic-rich, reduced, and sometimes polluted coastal ecosystems. We investigated whether such ability cope with adverse contexts might be a response the microorganisms these worms are associated with. In populations from French Atlantic coast (Roscoff, Brittany), we observed epibiotic association covering tegument 20–30% specimens anthropized site while individuals reference, non-anthropized were devoid any visible epibionts. Using RNAseq, molecular microscopic analyses, described compared microbial communities versus non-epibiotic at both locations. Interestingly, data showed that epibiosis characterized by sulfur-oxidizing bacteria among which giant bacterium Thiomargarita sp., date only deep sea habitats. Survey combined geochemical analysis their sediment revealed always found muds highest concentration sulfides, mostly during summer. Concomitantly, tolerance tests demonstrated acquisition epibionts increased survival against toxic level sulfides. Overall, present highlight for first time peculiar plastic adaptation seasonal variations habitat based on transcient allowing species survive temporary harsher conditions.