Dissimilatory nitrate reduction is a key branch of the nitrogen biogeochemical cycle. Its pathways, including nitrate reduction to nitrite, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA), are well characterized in prokaryotes (Simon and Klotz, 2013; Kuypers et al., 2018). Complete denitrification is largely restricted to bacteria particularly facultative anaerobes from the phyla Pseudomonadota and Desulfobacterota, which thrive in both terrestrial and marine ecosystems. These microorganisms play a central role in the removal of excess reactive nitrogen from the environment and wastewater, thereby mitigating eutrophication and reducing the risk of nitrogen pollution in ground and marine waters, which poses ecological and public health risks.

As in nitrogen transformations, microorganisms are also central to the degradation of aromatic compounds — structurally stable molecules that include lignin-derived monomers, quinones, flavonoids, and various petroleum-derived pollutants. Since plants and animals have limited capacity to metabolize aromatics, microbial degradation is crucial for the recycling of aromatic compounds. Many bacteria have evolved specialized enzymatic pathways to degrade chemically inert aromatic compounds under both aerobic and anaerobic conditions. (Fuchs et al., 2011). In anoxic conditions, benzoate can be decomposed by both facultative anaerobes primarily from the phylum Pseudomonadota, and strict anaerobes such as sulfate-reducing, Fe(III)-reducing, and fermentative bacteria, many of which belong to Desulfobacterota and other deep-branching clades (Boll and Fuchs, 1995; Anselmann et al., 2019; Löffler et al., 2011).

One underexplored natural environment where aromatic hydrocarbons and anaerobic conditions coexist is mud volcanoes — geological formations that periodically emit mud, gases, and oil-bearing fluids. These environments are often associated with petroleum reservoirs, tectonic activity, and deep biosphere processes. The Karabetova Gora mud volcano, located in the Taman Peninsula (Russia), is among the most active of over 50 terrestrial mud volcanoes in the region. It is characterized by continuous release of liquid mud, hydrocarbons, and gases, forming complex redox gradients on and below the surface. Such fluctuating oxygen conditions create niches favorable to facultative anaerobic microorganisms capable of both nitrate respiration and aromatic compound degradation. Previous studies have reported the presence of denitrifying bacteria in the Taman mud volcanoes (Slobodkina et al., 2020, Slobodkina et al., 2022; Ratnikova et al., 2020), and metagenomic analyses have detected genes encoding ATP-dependent benzoyl-CoA reductases in the microbial community of Karabetova Gora (Merkel et al., 2021). Moreover, an anaerobic bacterium capable of fermenting 3,4-dihydroxybenzoate was previously isolated from this site (Khomyakova et al., 2022), further suggesting the presence of aromatic-degrading, nitrate-respiring microorganisms.

Here, we report the isolation and characterization of a novel facultatively anaerobic benzoate-degrading and denitrifying bacterium, strain SMB388ᵀ, from the Karabetova Gora mud volcano. Based on its genomic, physiological, and phylogenetic properties, we propose its classification as a new genus and species, Denitraticella aggregata gen. nov., sp. nov.