Interconnectivity vs. isolation of prokaryotic communities in European deep-sea mud volcanoes

Abstract. During the past two decades, European cold seep ecosystems have attracted the scientific interest and to date there are several studies which have investigated the community structure and biodiversity of individual sites. In order to gain a better insight into the biology, biodiversity, and biogeography of seep-associated microbial communities along Europe's continental margins, a comparative approach was applied in the present work. By exploiting the publicly available data on 16S rRNA gene sequences retrieved from sediments of the Håkon Mosby mud volcano, Gulf of Cádiz and the eastern Mediterranean mud volcanoes/pockmarks (Anaximander area and Nile Fan), we investigated the prokaryotic biological components connecting these geographically isolated systems. The construction of interaction networks for both archaeal and bacterial shared operational taxonomic units (OTUs) among the different sites, revealed the presence of persistent OTUs, which can be considered as "key-players". One archaeal OTU (HQ588641) belonging to the ANME-3 group and one δ-Proteobacteria (HQ588562) were found in all five investigated areas. Other Archaea OTUs shared between four sites or less, belonged to the ANME-2c, -2a, MBG-D, -B and Thaumarchaeota. All other shared Bacteria belonged to the δ- and γ-Proteobacteria, with the exception of one JS1 affiliate OTU. The distribution of the majority of the shared OTUs seems to be restricted in cold seeps, mud volcanoes and other marine methane-rich environments. Although the investigated sites were connected through a small number of OTUs, these microorganisms hold central ecophysiological roles in these sediments, namely methane- and sulfur-mediated mineralization.