New insights into Siboglinidae microbiota - external tube contributes to an increment of the total microbial biomass

Siboglinid worms were sampled from four mud volcanoes in the Gulf of Cádiz (El Cid MV, Bonjardim MV, Al Gacel MV and Anastasya MV). These invertebrates are characteristic to cold seeps and are known to host chemosynthetic endosymbionts in a dedicated trophosome organ. However, little is known about their tube as a potential niche for other chemosynthetic and non-chemosynthetic microorganisms. Analyses by scanning and transmission electron microscopy showed dense biofilms on the tube in Al Gacel MV and Anastasya MV specimens by prokaryotic cells. Methanotrophic bacteria were the most abundant forming these biofilms as further confirmed by 16S rRNA sequence analysis. Furthermore, elemental analyses with electron microscopy and EDX point to the progressive mineralization of the biofilm and the tube in absence of nutrients. Environmental bacterial and archaeal 16S rRNA sequence libraries revealed abundant microorganisms related to these siboglinid worms and variation in microbial communities among samples. We argue that these differences must be related to variance in seepage activity, as it is the main source of nutrients. Thus, the tube remarkably increases the microbial biomass related to the worms and needs to be incorporated as an important part of the worm’s microbiota. Furthermore, empty tubes may still influence the composition of the active microbial community at those sites.