Sulfur cycling and methanogenesis primarily drive microbial colonization of the highly sulfidic Urania deep hypersaline basin

Abstract

Urania basin in the deep Mediterranean Sea houses a lake that is >100 m deep, devoid of oxygen, 6 times more saline than seawater, and has very high levels of methane and particularly sulfide (up to 16 mM), making it among the most sulfidic water bodies on Earth. Along the depth profile there are 2 chemoclines, a steep one with the overlying oxic seawater, and another between anoxic brines of different density, where gradients of salinity, electron donors and acceptors occur. To identify and differentiate the microbes and processes contributing to the turnover of organic matter and sulfide along the water column, these chemoclines were sampled at a high resolution. Bacterial cell numbers increased up to a hundredfold in the chemoclines as a consequence of elevated nutrient availability, with higher numbers in the upper interface where redox gradient was steeper. Bacterial and archaeal communities, analyzed by DNA fingerprinting, 16S rRNA gene libraries, activity measurements, and cultivation, were highly stratified and metabolically more active along the chemoclines compared with seawater or the uniformly hypersaline brines. Detailed analysis of 16S rRNA gene sequences revealed that in both chemoclines delta- and epsilon-Proteobacteria, predominantly sulfate reducers and sulfur oxidizers, respectively, were the dominant bacteria. In the deepest layers of the basin MSBL1, putatively responsible for methanogenesis, dominated among archaea. The data suggest that the complex microbial community is adapted to the basin's extreme chemistry, and the elevated biomass is driven largely by sulfur cycling and methanogenesis.

Fig. 1.

Microbiological and geochemical profiling of the Urania water column. SW, seawater; I1,2, Interface 1,2; B1,2, brine 1, 2. (A) Filled circles, salinity profile measured by the CTD probe mounted on the SciPack system during deployment in the Urania water column; open diamonds, redox potential (Eh). (B) DAPI microbial counts; circles, data obtained in 2003 cruise; diamonds, data obtained in 2002 cruise. (C) 16S rRNA gene abundance; filled circles, prokaryote (prok); open diamonds, archaea (arch). (D) Filled circles, ATP; open diamonds, Shannon–Weaver index calculated from ARISA fingerprinting. (E) Open diamonds, sulfate reduction rates (SRR); filled triangles, methane production rates (MPR); filled circles, methane concentrations (upper x axis). (F) Open diamonds, sulfate concentrations; filled circles, dissolved Mn²⁺ concentrations. (G) Open diamonds, nitrates; filled circles ammonium. Error bars are within 4–48% of the reported values for DAPI counts; 3–5% for geochemical measurements; 1–55% for real-time 16S rRNA quantification; 7–32% for ATP quantification; 2–38% for SRR; and 2–59% for MPR.

Fig. 2.

δ- and ϵ-Proteobacteria are stratified along t he water column of Urania basin. (A and C) Relative distribution of sequences belonging to the ε- (A) and δ- (C) Proteobacteria. (B) Relative distribution of sequences at subfamily level, belonging to the Campylobacteraceae. CT, new candidate taxa.