(1) Background: Microbial communities in terrestrial, calcifying high-alkaline springs are not well understood. In this study, we investigate the structure and composition of microbial mats in ultrabasic (pH 10–12) serpentinite springs of the Voltri Massif (Italy). (2) Methods: Along with analysis of chemical and mineralogical parameters, environmental DNA was extracted and subjected to analysis of microbial communities based upon next-generation sequencing. (3) Results: Mineral precipitation and microbialite formation occurred, along with mat formation. Analysis of the serpentinite spring microbial community, based on Illumina sequencing of 16S rRNA amplicons, point to the relevance of alkaliphilic cyanobacteria, colonizing carbonate buildups. Cyanobacterial groups accounted for up to 45% of all retrieved sequences; 3–4 taxa were dominant, belonging to the filamentous groups of Leptolyngbyaceae, Oscillatoriales, and Pseudanabaenaceae. The cyanobacterial community found at these sites is clearly distinct from creek water sediment, highlighting their specific adaptation to these environments.

Siboglinids 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 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 supported by 16S rRNA sequence analysis. Furthermore, elemental analyses with electron microscopy and energy-dispersive X-ray spectroscopy point to the mineralization and silicification of the tube, most likely induced by the microbial metabolisms. Bacterial and archaeal 16S rRNA sequence libraries revealed abundant microorganisms related to these siboglinid specimens and certain variations in microbial communities among samples. Thus, the tube remarkably increases the microbial biomass related to the worms and provides an additional microbial niche in deep-sea ecosystems.

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.

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.

Suppl. Figure 1. δ 13 C and δ 18 O values of samples taken from weak and heavy calcified areas of the stromatolite framework as well as from cyanobacterial sheaths as indicated.