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Submarine springs near Izola, in the Northern Adriatic Sea, appear in funnel-shaped depressions and smell strongly of sulfur. Along the Mediterranean coast there are many submarine karst springs containing brackish or fresh water, but submarine sulfur springs are not particularly common. Three submarine sulfur springs and one terrestrial sulfur spring were investigated to better understand the water properties, water–rock interaction within the aquifer, and to explore the origin of the spring water. Groundwater and seawater samples were also collected for comparison. Based on the geological setting, physicochemical parameters, hydrogeochemical data, and stable isotope data ( δ 18 O, δ 2 H, δ 13 C DIC , δ 34 S SO4 , δ 18 O SO4 ), we can affirm that (1) the large concentration of seawater in the submarine springs samples is due to sampling challenges; (2) springs recharge from precipitation where confined karst aquifers outcrop; (3) deep water circulation is indicated; (4) redox conditions can provide a suitable environment for bacterial reduction of the marine or organic sulfate to the odorous H 2 S; (5) geological data suggests that the coals beneath the alveolinic-nummulitic limestones are the source of sulfur. A multi-parameter and interdisciplinary approach has proven important in assessing submarine sulfur springs affected by seawater input.

Sulfur‐rich environments host specialized microbial communities that drive key biogeochemical processes, particularly sulfur cycling. While sulfur‐oxidizing microbiota from hydrothermal vents and volcanic systems are well studied, microbial communities in cold terrestrial sulfur springs remain less understood. In this study, we used 16S rRNA gene sequencing to examine how sulfur availability and environmental conditions shape microbial assemblages across different biofilm types in a cold sulfur spring system at Blount Springs, Alabama (33.9301° N, 86.7928° W). Sulfur‐oxidizing chemolithotrophs, including Sulfurovum and Halothiobacillus, represented the majority of the recovered reads in sulfur‐rich white biofilms, while purple phototrophic biofilms were enriched with anoxygenic sulfur‐oxidizing bacteria, such as Chromatium and Chlorobium. Nonsulfur biofilms from adjacent environments displayed greater microbial diversity, including a high abundance of photosynthetic diatoms, like, Melosira. Notably, Sulfurovum was abundant across both sulfur‐rich and phototrophic niches, suggesting ecological flexibility and a central role in sulfur metabolism. These findings highlight the influence of sulfur chemistry and light availability in structuring microbial communities and contribute to a broader understanding of microbial adaptation and sulfur cycling in cold sulfur spring ecosystems. Distinct microbial communities occur between adjacent sulfur‐rich and freshwater springs. Chemolithotrophic and phototrophic sulfur‐oxidizing taxa were enriched in cold sulfur spring biofilms, while adjacent freshwater biofilms supported more diverse microbial community composition.

Since the beginning of unicellular life, dissimilation reactions of autotrophic sulfur bacteria have been a crucial part of the biogeochemical sulfur cycle on Earth. A wide range of sulfur oxidation states is reflected in the diversity of metabolic pathways used by sulfur-oxidizing bacteria. This metabolically and phylogenetically diverse group of microorganisms inhabits a variety of environments, including extreme environments. Although they have been of interest to microbiologists for more than 150 years, meso- and psychrophilic chemolithoautotrophic sulfur-oxidizing microbiota are less studied compared to the microbiota of hot springs. Several recent studies suggested that cold sulfur waters harbor unique, yet not described, bacterial taxa.

This review integrates the biology and clinical translation of hydrogen sulfide (H2S) in balneology. It frames H2S as a gasotransmitters with dual chemical and biological actions and summarizes the H2S/HS− equilibrium as a function of pH, temperature, and oxygenation, which governs bioaccessibility in sulfurous waters. Endogenous and exogenous sources, transport, and mitochondrial catabolism are outlined, together with core cellular mechanisms: protein persulfidation; activation of Nrf2/ARE; modulation of NF-κB; regulation of ion channels; and engagement of PI3K/Akt, MAPK/ERK, and Wnt pathways, plus epigenetic interactions with HDACs and sirtuins. Preclinical and clinical evidence in dermatology, musculoskeletal disease, and respiratory care is synthesized, alongside metabolic, cardiovascular, gastrointestinal, and renal effects. Technical aspects that preserve the bioactive fraction of H2S while meeting environmental safety limits are highlighted. Routes of administration (bathing, peloids, inhalation, and drinking cures) and key operational parameters are described. Overall, the review links physicochemical and molecular foundations with clinical indications for sulfurous waters and derivatives and identifies opportunities for research and development in H2S donors and thermal cosmetics without extrapolating beyond the available data.

In Europe, several gypsum karst regions occur among which the gypsum karsts located in Emilia-Romagna (UNESCO World Heritage Site since 2023) and Sicily are notable. The sediments, spring water microbial aggregates, and wall biofilms from three caves, Re Tiberio, Befana (Emilia-Romagna), and the Sicilian Santa Ninfa, have been studied from a microbiological and geochemical point of view. The samples of wall biofilms from gypsum caves were exclusively composed of Bacteria , while the sediments showed negligible abundances of Archaea . The two most abundant phyla in most sediments and biofilms were Actinomycetota and Pseudomonadota , whereas the microbial aggregates floating in the spring waters of Befana Cave showed a deviation from the typical abundance pattern as Campylobacterota replaced Actinomycetota , and the abundances of Bacteroidota and Desulfobacterota were high. The most abundant genus in the wall biofilms was Crossiella ( Actinomycetota ), but it was absent in the water aggregates collected in Befana Cave. The abundances of Crossiella in the cave sediments were very low. The dominant genera in Befana microbial aggregates showed different abundances and microbial composition when compared with the previously studied Frasassi and Fetida thermal sulfuric acid caves, located in Central and South Italy, respectively, suggesting that the community composition of the microbial aggregates is specific for each cave and related to the geochemistry of the sulfidic spring. Also, a different microbial community composition was found in Befana water aggregates with respect to the wall biofilms from Befana and Santa Ninfa caves. In the case of sediments, they significantly clustered together indicating that the microbial communities associated with sediments are similar, independently from the cave and possible other environmental parameters.

The structure and diversity of microbial communities developing in the combined gradient of temperature (44–19 °C), as well as concentration of oxygen (0–10 mg/L) and hydrogen sulfide (33–0.7 mg/L), were studied in the thermal sulfide spring on the coast of Northern Lake Baikal. The predominance of bacteria participating in sulfur and nitrogen cycles and significant changes in the composition of microbial communities were noted at changing physicochemical conditions. Thiovirga sp. (sulfur-oxidizing bacteria, up to 37%) and Azonexus sp. (nitrogen-fixing bacteria, up to 43%) were dominant at high temperatures and concentrations of hydrogen sulfide in two hydrotherms. In addition, a significant contribution of the Rhodocyclaceae family (up to 51%) which is involved in the denitrification processes, and Acetoanaerobium sp. (up to 20%) fixing carbon oxide were found in the spring water. In the stream, mainly oxygenic cyanobacteria (up to 56%) developed at a temperature of 33 °C, in the presence of hydrogen sulfide and oxygen. In addition, sulfur bacteria of the genus Thiothrix (up to 48%) found in epibiotic communities of benthic animals of Lake Baikal were present here. Thiothrix sp. formed massive fouling in the zone of mixing lake and thermal waters with a significant contribution of hydrogen-oxidizing bacteria of the genus Hydrogenophaga (up to 22.5%). As well as chemolitho- and phototrophic bacteria, chemoorganotrophs (phyla Firmicutes, Chloroflexi, Desulfobacterota, Nitrospirota, Fibrobacterota, etc.) have been identified in all communities. The chemical parameters of water in spring and coastal zones indicate a significant change in the composition of thermal waters occurring with the participation of diverse microbial communities that contribute to the assimilation of inorganic components of mineral thermal waters.

Within the cyanobacterial world there are many species adapted to life in hypersaline environments. Some can even grow at salt concentrations approaching NaCl saturation. Halophilic cyanobacteria often form dense mats in salt lakes, and on the bottom of solar saltern ponds, hypersaline lagoons, and saline sulfur springs, and they may be found in evaporite crusts of gypsum and halite. A wide range of species were reported to live at high salinities. These include unicellular types ( Aphanothece halophytica and similar morphotypes described as Euhalothece and Halothece ), as well as non-heterocystous filamentous species ( Coleofasciculus chthonoplastes , species of Phormidium , Halospirulina tapeticola , Halomicronema excentricum , and others). Cyanobacterial diversity in high-salt environments has been explored using both classic, morphology-based taxonomy and molecular, small subunit rRNA sequence-based techniques. This paper reviews the diversity of the cyanobacterial communities in hypersaline environments worldwide, as well as the physiological adaptations that enable these cyanobacteria to grow at high salt concentrations. To withstand the high osmotic pressure of their surrounding medium, halophilic cyanobacteria accumulate organic solutes: glycine betaine is the preferred solute in the most salt-tolerant types; Coleofasciculus produces the heteroside glucosylglycerol, and the less salt-tolerant cyanobacteria generally accumulate the disaccharides sucrose and trehalose under salt stress. Some cyanobacteria growing in benthic mats in hypersaline environments are adapted to life under anoxic conditions and they can use sulfide as an alternative electron donor in an anoxygenic type of photosynthesis through a process which involves photosystem I only.

Microbiota of sulfur-rich environments has been extensively studied due to the biotechnological potential of sulfur bacteria, or as a model of ancient life. Cold terrestrial sulfur springs are less studied compared to sulfur-oxidizing microbiota of hydrothermal vents, volcanic environments, or soda lakes. Despite that, several studies suggested that sulfur springs harbor diverse microbial communities because of the unique geochemical conditions of upwelling waters. In this study, the microbiota of five terrestrial sulfur springs was examined using a 16 S rRNA gene sequencing. The clear dominance of the Proteobacteria and Campylobacterota phyla of cold sulfur springs microbiota was observed. Contrary to that, the microbiota of the hot sulfur spring was dominated by the Aquificota and Firmicutes phylum respectively. Sulfur-oxidizing genera constituted a dominant part of the microbial populations with the Thiothrix and Sulfurovum genera identified as the core microbiota of cold sulfur terrestrial springs in Slovakia. Additionally, the study emphasizes that sulfur springs in Slovakia support unique, poorly characterized bacterial communities of sulfur-oxidizing bacteria.

During long periods with no precipitation, a sulfidic spring (Smrdljivec) appears in the dry bed of the Reka River before sinking into the karst underground. The study characterizes the area’s geological setting, development of microbial communities and an ecotone, and impact on the vulnerable karst ecosystem. Geological mapping of the area, stable isotopic analyses, field measurements, and physico-chemical and toxicity analyses were applied to elucidate the environmental conditions. The spring’s microbial diversity was assessed using cultivation methods, microscopy, and metagenomics. Sulfur compounds in the spring probably originate from coal layers in the vicinity. Metagenomic analyses revealed 175 distinct operational taxonomic units in spring water and biofilms. Proteobacteria predominated in developed biofilms, and a “core” microbiome was represented by methylotrophs, including Methylobacter, Methylomonas, and Methylotenera. Diatoms represented an important component of biofilm biomass. A combination of environmental factors and climatic conditions allows the formation and accessibility of emerging biodiversity hotspots and ecotones. Details of their dynamic nature, global impact, and distribution should be highlighted further and given more protection.

The metagenome of foulings from sulfidic spring “Serovodorodny” (Tatarstan, Russia), where members of the genus Thiothrix was observed, was sequenced. Representatives of the phyla Gammaproteobacteria, Cyanobacteria and Campilobacteriota dominated in the microbial community. The complete genome of Thiothrix sp. KT was assembled from the metagenome. It displayed 93.93–99.72% 16S rRNA gene sequence identity to other Thiothrix species. The average nucleotide identity (ANI) и digital DNA-DNA hybridization (dDDH) showed that the genome designated KT represents a new species within the genus Thiothrix, ‘Candidatus Thiothrix sulfatifontis’ sp. nov. KT. The taxonomic status has been determined of the strain Thiothrix sp. CT3, isolated about 30 years ago and not assigned to any of Thiothrix species due to high 16S rRNA gene sequence identity with related species (i.e., 98.8–99.4%). The complete genome sequence of strain CT3 was determined. The ANI between CT3 and other Thiothrix species was below 82%, and the dDDH values were less than 40%, indicating that strain CT3 belongs to a novel species, Thiothrix winogradskyi sp. nov. A genome analysis showed that both strains are chemo-organoheterotrophs, chemolithotrophs (in the presence of hydrogen sulfide and thiosulfate) and chemoautotrophs. For the first time, representatives of Thiothrix showed anaerobic growth in the presence of thiosulfate.