Community Structure of Subsurface Biofilms in the Thermal Sulfidic Caves of Acquasanta Terme, Italy

Abstract

We performed a microbial community analysis of biofilms inhabiting thermal (35 to 50°C) waters more than 60 m below the ground surface near Acquasanta Terme, Italy. The groundwater hosting the biofilms has 400 to 830 M sulfide, <10 M O 2 , pH of 6.3 to 6.7, and specific conductivity of 8,500 to 10,500 S/cm. Based on the results of 16S rRNA gene cloning and fluorescent in situ hybridization (FISH), the biofilms have low species richness, and lithoautotrophic (or possibly mixotrophic) Gamma-and Epsilonproteobacteria are the principle biofilm architects. Deltaproteobacteria sequences retrieved from the biofilms have <90% 16S rRNA similarity to their closest relatives in public databases and may represent novel sulfate-reducing bacteria. The Acquasanta biofilms share few species in common with Frasassi cave biofilms (13°C, 80 km distant) but have a similar community structure, with representatives in the same major clades. The ecological success of Sulfurovumales-group Epsilonproteobacteria in the Acquasanta biofilms is consistent with previous observations of their dominance in sulfidic cave waters with turbulent water flow and high dissolved sulfide/oxygen ratios.

Microbial Diversity in Caves

“…Proteobacteria as the largest taxonomic group can also be found in extreme environments, such as sulfurous caves, e.g. Grotta Nuova di Rio Garrafo (Jones et al 2010), Parker (Angert et al 1998), Cesspool (Engel et al 2001), Lower Kane (Engel et al 2003), Movile (Sarbu 2000). In Grotta Sulfurea of the Frasassi cave system, Proteobacteria are involved in the sulfur cycle and represent >75% of the clone library created from the 16S rRNA genes of the water mat organisms (Macalady et al 2006).…”

“…The rich in sulfur Grotta Nuova di Rio Garrafo has neutral pH and high water temperature (50 °C). The γ-Proteobacteria class constituted the second largest group of bacterial biofilms examined (more than 10% of the population) in this environment (Jones et al 2010). Among them, sulfur-oxidizing lithothrophic bacteria were identified, e.g.…”

Fate of sulfide in the Frasassi cave system and implications for sulfuric acid speleogenesis

“…Similarly, Movile Cave (Romania) has high dissolved sulfide but low cave air H 2 S(g), slow gypsum precipitation, and scarce subaerial corrosion features (Sarbu, 2000b;Galdenzi, 2001), consistent with its largely stagnant water table. Rapid H 2 S(g) degassing and subaerial corrosion are expected for most locations within Cueva de Villa Luz, Mexico, due to high H 2 S T and turbulent water flow (Hose et al, 2000), as well as near the highly sulfidic and turbulent cave streams in Grotta Nuova di Rio Garrafo near Acquasanta Terme, Italy (Galdenzi et al, 2010;Jones et al, 2010). Water temperature and thermal air flow may also impact H 2 S(g) dynamics in these and other caves (Galdenzi, 2001;Audra et al, 2007;Audra et al, 2009), and future research will continue to explore how H 2 S T concentration, stream flow characteristics, and other factors impact subaerial sulfuric acid speleogenesis and the evolution of sulfidic karst systems.…”

Sulfur isotope values in the sulfidic Frasassi cave system, central Italy: A case study of a chemolithotrophic S-based ecosystem

“…Both rock-attached biofilms in fast-flowing waters (referred to as "streamers" hereafter) and Sediment-Water Interface Biofilms in less turbid waters (referred to as "SWIBs" hereafter) were present at most sites (Figure 1b; as described in Jones et al, 2010;Macalady et al, 2008;Macalady et al, 2006 with other S species in the system.…”