Terrestrial mud volcanoes (TMVs) occur where high fluid pressure in the deep subsurface results in the transport of mud, water and gas to the surface (Feyzullayev and Movsumova, 2010; Niemann and Boetius, 2010). This process creates diverse morphological features rich in methane and other hydrocarbons (Dimitrov, 2002), and hosting a suite of electron acceptors (Alain et al., 2006; Chang et al., 2012; Mazzini et al., 2009; Planke et al., 2003). Mud volcanic settings promote a range of chemosynthetic microbial metabolisms, where electron donors (e.g., hydrocarbons, hydrogen sulfide) meet electron acceptors (e.g., oxygen, nitrate, sulfate), driving microbial processes crucial for the cycling of carbon, sulfur, and other key elements. Methane oxidation in mud volcanoes is performed by aerobic methanotrophic bacteria and anaerobic methanotrophic archaea (ANME) while dominant bacterial component may differ depending on physicochemical input (Chang et al., 2012; Chen et al., 2024; Mardanov et al., 2020; Merkel et al., 2021; Slobodkin et al., 2024).

More than 40 terrestrial mud volcanoes are located at Taman Peninsula (Russia) (Lavrushin et al., 2003). The microbial diversity of Taman Peninsula TMVs has only recently begun to be studied (Mardanov et al., 2020; Merkel et al., 2021; Slobodkin et al., 2024). Gnilaya Gora is one of the most alkaline TMVs in Taman Peninsula with waters, characterized by pH 8.7–9.1, moderate salinity (12–20 g l−1), high Cl− concentration (up to 412 mM) and low amounts of sulfate and nitrate (Slobodkin et al., 2024). The bacterial members of Gnilaya Gora microbial community accounted for 65–99% of prokaryotic diversity with the most abundant phyla Pseudomonadota, Desulfobacterota, and Halobacterota (Slobodkin et al., 2024). Using high-throughput sequencing of 16S rRNA gene amplicons, sulfur-reducing and sulfur-disproportionating representatives of the phylum Desulfobacterota such as Desulfuromusa, Desulfurivibrio, Desulfocapsa, etc. were detected in the samples from Gnilaya Gora (Slobodkin et al., 2024). Additionally, several alkaliphilic isolates taking part in sulfur cycle have been previously isolated from this TMV (Frolova et al., 2021, Frolova et al., 2023a, Frolova et al., 2023b; Slobodkin et al., 2024).

Azerbaijan represents one of the most densely populated region of mud volcanoes and mud volcanism is one of the major factors controlling oil and gas fields in the region. Analyses of oil source rocks ejected from mud volcanoes of Azerbaijan revealed low organic sulfur contents with the majority of sulfur occurring as pyrite (Isaksen et al., 2007). In particular, the expelled waters of Bakhar satellite mud volcano contained as dominant ions Na+ and Cl− (concentrations 22 and 33 g l−1 respectively), while concentrations of sulfate, iron and nitrate were low or near zero (Planke et al., 2003). At the same time, TMVs of this region are characterized by unusually high concentrations of bicarbonates, most probably originated from oil biodegradation (Feyzullayev and Movsumova, 2010). As a result, pH values of all investigated TMVs were approximately 8.0 (Green-Saxena et al., 2012). However, very little is known about the microbial communities of Azerbaijan TMVs with no strains being isolated. To date a single work with the combination of geochemical analyses, biological rate measurements and molecular diversity surveys shed light on the presence of active sulfur-oxidizing and sulfate-reducing representatives in four TMVs of Azerbaijan, including Bakhar (Green-Saxena et al., 2012). Thereby methane and sulfur cycles are recognized as the most important biogeochemical cycles in such environments. TMVs have the potential for supporting active sulfur cycling with typical sulfate concentrations of approximately 2 mM (Alain et al., 2006, 2006; Mazzini et al., 2009; Nakada et al., 2011; Planke et al., 2003; Yakimov et al., 2002), and these natural venting structures may also serve as a window into sulfur-cycling processes in the deep biosphere. At the same time the ecology of organisms involved in sulfur cycling in TMVs remains largely unexplored.

One of the largest and oldest taxonomic groups of sulfur-metabolizing microorganisms is the order Desulfobacterales (Kuever et al., 2005), which was reclassified and divided into several orders, in particular, the order Desulfobulbales was separated from it including newly proposed family Desulfurivibrionaceae (Waite et al., 2020). Genus Desulfurivibrio up to date included only two alkaliphilic species (D. alkaliphilus and D. dismutans), isolated from soda lakes. (Sorokin et al., 2008; Sorokin et al., 2025). Both species are able to disproportionate elemental sulfur in combination with autotrophy, which is rare peculiarity among bacteria (Poser et al., 2013). Most microorganisms capable of disproportionation of sulfur compounds use thiosulfate and sulfite, but the ability to disproportionate elemental sulfur is less common (Slobodkin and Slobodkina, 2019). To date, only three strains capable of S0-disproportionation have been isolated from terrestrial environments, which are directly connected with subsurface fluids: our two strains and Thiovibrio frasassiensis. Our novel isolates from TMVs are the only S0-disproportionating strains (with the exception of Thiovibrio frasassiensis) (Aronson et al., 2023) isolated from a terrestrial subsurface.

In this paper we describe two new species of Desulfurivibrio, isolated from TMVs of Taman Peninsula (Russia) and Azerbaijan, which are able to disproportionate sulfur compounds. It is the first report on the isolation of sulfur-disproportionating microorganisms from terrestrial mud volcanoes.