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Plastic waste is a global environmental burden and long-lasting plastic polymers, including ubiquitous and toxic polyurethanes (PUs), rapidly accumulate in the water environments. In this study, samples were collected from the three alkaline groundwater occurrences in the geotectonic regions of the Pannonian basin of northern Serbia (Torda and Slankamen Banja) and Inner Dinarides of western Serbia (Mokra Gora) with aim to isolate and identify bacteria with plastic- and lignocellulose-degrading potential, that could be applied to reduce the burden of environmental plastic pollution. The investigated occurrences belong to cold, mildly alkaline (pH: 7.6-7.9) brackish and hyperalkaline (pH: 11.5) fresh groundwaters of the SO4 - Na + K, Cl - Na + K and OH, Cl - Ca, Na + K genetic type. Full-length 16S rDNA sequencing, using Oxford Nanopore sequencing device, was performed with DNA extracted from colonies obtained by cultivation of all groundwater samples, as well as with DNA extracted directly from one groundwater sample. The most abundant genera belong to Pseudomonas, Acidovorax, Kocuria and Methylotenera. All screened isolates (100%) had the ability to grow on at least 3 of the tested plastic and lignocellulosic substrates, with 53.9% isolates degrading plastic substrate Impranil® DLN-SD (SD), a model compound for PUs degradation. Isolates degrading SD that were identified by partial 16S rDNA sequencing belong to the Stenotrophomonas, Pseudomonas, Paraburkholderia, Aeromonas, Vibrio and Acidovorax genera. Taking into account that plastics, including commonly produced PUs, are widespread in groundwater, identification of PUs-degrading bacteria may have potential applications in bioremediation of groundwater polluted with this polymer.

Plastic waste is a global environmental burden. Polyurethanes (PU), toxic and ubiquitous synthetic polymers, do not biodegrade quickly, leading to their rapid accumulation in the soil and water environments. Highly efficient PU-degrading microorganisms are rare in nature and are of fundamental importance for achieving circular plastic economy. Bacterial isolates from groundwater, originating from magmatogenic massif and Tertiary basin within metamorphic area, as well as soil isolates collected from various pristine (PS) and contaminated sites (CS), were screened using PU model compound Impranil® DLN-SD (IMP) as sole C source to identify PU-degrading isolates. Phylogenetic analysis of 16S rRNA gene sequences from IMP-degrading isolates was performed using the neighbor-joining method to observe their clustering. Thirty one of 96 isolates (32.3 %) from groundwater and 18 of 220 isolates (8.2%) from soil produced prominent IMP-clearing zones. Thirteen IMPdegrading isolates from each type of environment, belonging to 8 genera (Pseudomonas, Proteus, Enterobacter, Flavobacterium, Serratia, Pantoea, Acinetobacter and Stenotrophomonas) for groundwater and to 6 genera (Streptomyces, Pseudomonas, Rhodococcus, Achromobacter, Bacillus and Paenibacillus) for soil environment, were included in phylogenetic analysis. No clear grouping of groundwater and soil isolates was observed, indicating that isolates are too distinct. Stronger clustering was observed for groundwater compared to soil isolates. For groundwater, strongest clustering was observed for 2 isolates belonging to Proteus genus, 2 belonging to Flavobacterium and 2 to Pseudomonas. For soil samples, strongest clustering was observed for 3 isolates belonging to genus Streptomyces. There was no clear grouping within isolates from CS and PS. In the future, wider range of environmental niches should be included in screening efforts for development of biocatalytic processes for management of plastic waste. Subterranean ecosyste

Plastic waste is a global environmental burden and long-lasting plastic polymers, including ubiquitous and toxic polyurethanes (PUs), rapidly accumulate in the water environments. In this study, samples were collected from the three alkaline groundwater occurrences in the geotectonic regions of the Pannonian basin of northern Serbia (Torda and Slankamen Banja) and Inner Dinarides of western Serbia (Mokra Gora) with aim to isolate and identify bacteria with plastic- and lignocellulose-degrading potential, that could be applied to reduce the burden of environmental plastic pollution. The investigated occurrences belong to cold, mildly alkaline (pH: 7.6–7.9) brackish and hyperalkaline (pH: 11.5) fresh groundwaters of the SO4 – Na + K, Cl – Na + K and OH, Cl – Ca, Na + K genetic type. Full-length 16S rDNA sequencing, using Oxford Nanopore sequencing device, was performed with DNA extracted from colonies obtained by cultivation of all groundwater samples, as well as with DNA extracted directly from one groundwater sample. The most abundant genera belong to Pseudomonas, Acidovorax, Kocuria and Methylotenera. All screened isolates (100%) had the ability to grow on at least 3 of the tested plastic and lignocellulosic substrates, with 53.9% isolates degrading plastic substrate Impranil® DLN-SD (SD), a model compound for PUs degradation. Isolates degrading SD that were identified by partial 16S rDNA sequencing belong to the Stenotrophomonas, Pseudomonas, Paraburkholderia, Aeromonas, Vibrio and Acidovorax genera. Taking into account that plastics, including commonly produced PUs, are widespread in groundwater, identification of PUs-degrading bacteria may have potential applications in bioremediation of groundwater polluted with this polymer.

The examined groundwaters, orginating from the fractured type of aquifer formed by Pre- Middle Jurassic ultramafic rocks, i. e. harzburgites, dunites, lherzolites and serpentinites, belong to the fresh hyperalkaline cold groundwaters of OH, Cl – Ca, Na + K genetic type. Studied occurrence was characterized by the presence of aerobic cultivable bacteria with ability to grow in a pH range of 7-12. Bacterial isolates were screened using plastic,cellulosic and hemicellulosic polymeric substrates as predominantcarbon source to assess their biotechnological potential. All 16 screened isolates (100%) demonstrated growth and/or activity on at least one tested substrate. One isolate belonging to genus Flavobacterium, produced prominent zone of clearing on Impranil® DLN-SD, a model substrate for polyurethanes (PU). Plastic-degrading potential has been previously documented for members of genus Flavobacterium. However, this is the first report of their PU-degrading activity. Accumulation of PU in the environment is of special concern, due to its toxicity, high production rates and lack of efficient recycling strategies. In the future, biotechnological potential of alkalophilic bacteria from groundwater environments should be explored more intensly, especially from the aspect of their application in the management of plastic waste and bioremediation treatments of plastic-polluted groundwater resources.

The selected brackish groundwater occurrences in the geotectonic regions of Inner Dinarides of western Serbia (Obrenovačka Banja) and Serbian crystalline core (Lomnički Kiseljak and Velika Vrbnica) were sampled for isolation and identification of plastic- and lignocellulose-degrading bacteria, as well as for the assessment of their enzymatic potential. The examined occurrences belong to the cold and warm (subthermal), weakly alkaline, neutral, and weakly acidic groundwater, and their genetic types are HCO3–Na + K and HCO3–Ca, Mg. The most abundant genera identified by next-generation 16S sequencing of cultivated groundwater samples belong to Aeromonas and Exiguobacterium. Of isolates screened on plastic and lignocellulosic substrates, 85.3% demonstrated growth and/or degrading activity on at least one tested substrate, with 27.8% isolates degrading plastic substrate Impranil® DLN-SD (SD), 1.9% plastic substrate bis(2-hydroxyethyl)terephthalate, and 5.6% carboxymethyl cellulose (CMC). Isolates degrading SD that were identified by 16S rDNA sequencing belonged to genera Stenotrophomonas, Flavobacterium, Pantoea, Enterobacter, Pseudomonas, Serratia, Acinetobacter, and Proteus, while isolates degrading CMC belonged to genera Rhizobium and Shewanella. All investigated brackish groundwaters harbor bacteria with potential in degradation of plastics or cellulose. Taking into account that microplastics contamination of groundwater resources is becoming a significant problem, the finding of plastic-degrading bacteria may have potential in bioremediation treatments of polluted groundwater. Subterranean ecosystems, which are largely untapped resources of biotechnologically relevant enzymes, are not traditionally considered the environment of choice for screening for plastic- and cellulose-degrading bacteria and therefore deserve a special attention from this aspect.

The selected brackish groundwater occurrences in the geotectonic regions of Inner Dinarides of western Serbia (Obrenovačka Banja) and Serbian crystalline core (Lomnički Kiseljak and Velika Vrbnica) were sampled for isolation and identification of plastic- and lignocellulose-degrading bacteria, as well as for the assessment of their enzymatic potential. The examined occurrences belong to the cold and warm (subthermal), weakly alkaline, neutral, and weakly acidic groundwater, and their genetic types are HCO3–Na + K and HCO3–Ca, Mg. The most abundant genera identified by next-generation 16S sequencing of cultivated groundwater samples belong to Aeromonas and Exiguobacterium. Of isolates screened on plastic and lignocellulosic substrates, 85.3% demonstrated growth and/or degrading activity on at least one tested substrate, with 27.8% isolates degrading plastic substrate Impranil® DLN-SD (SD), 1.9% plastic substrate bis(2-hydroxyethyl)terephthalate, and 5.6% carboxymethyl cellulose (CMC). Isolates degrading SD that were identified by 16S rDNA sequencing belonged to genera Stenotrophomonas, Flavobacterium, Pantoea, Enterobacter, Pseudomonas, Serratia, Acinetobacter, and Proteus, while isolates degrading CMC belonged to genera Rhizobium and Shewanella. All investigated brackish groundwaters harbor bacteria with potential in degradation of plastics or cellulose. Taking into account that microplastics contamination of groundwater resources is becoming a significant problem, the finding of plastic-degrading bacteria may have potential in bioremediation treatments of polluted groundwater. Subterranean ecosystems, which are largely untapped resources of biotechnologically relevant enzymes, are not traditionally considered the environment of choice for screening for plastic- and cellulose-degrading bacteria and therefore deserve a special attention from this aspect.

Three mineral water occurrences, captured by wells with a depth of 6.5-442.5 m and used for drinking and bathing purposes, were sampled and cultivated under conditions favouring the growth of anaerobic, microaerophilic or CO2 bacteria, in order to capture predominantly anaerobic portion of the bacteriome, which is dominant in water and soils. Cultivated bacteria were identified by next-generation 16S sequencing and their biotechnological potential in plastics and lignocellulose degradation was explored. Most abundant genera detected in examined samples mainly belong to facultative anaerobes that are common representatives of water and soil environments. In total, 17 genera were detected with a relative abundance over 1% in all three samples, including Aeromonas, Exiguobacterium, Comamonas and Acinetobacter. Half of the screened isolates demonstrated growth on at least one plastic or lignocellulosic polymer, with one isolate demonstrating growth on all tested substrates, one demonstrating carboxymethyl cellulose- and one arabinoxylan-degrading ability. Some of the representatives of genera identified with high relative abundance in mineral water samples, such as Aeromonas, Klebsiella, Escherichia, Salmonella, Enterobacter, Pseudomonas and Staphylococcus, have been previously documented to have pathogenic potential. Due to the use of investigated mineral waters for drinking and bathing, the health risk from such bacteria in these occurrences needs to be continuously monitored, while, on the other hand, mineral waters deserve special attention in the future from the aspect of screening for biotechnologically relevant enzymes.

Targeted physiological groups of bacteria were cultivated and identified in the brackish groundwaters of Obrenovačka Banja (OB), Lomnički Kiseljak (LK) and Velika Vrbnica (VV) using biological activity reaction tests (BARTs) to assess the biocorrosion, biofouling and health risks. The highest density of iron-related, sulfate-reducing, slime-forming, facultatively anaerobic heterotrophic, denitrifying bacteria and representatives of Pseudomonas spp. was recorded in the OB sample, while the lowest density of the same physiological groups of bacteria was recorded in the LK sample. Facultatively anaerobic heterotrophic bacteria were the most abundant in the OB and LK samples, while, in contrast, heterotrophic aerobic bacteria were the most abundant in the VV sample. All tested samples were characterized by a high degree of biochemical activity associated with iron-related, sulfate-reducing, slime-forming, heterotrophic aerobic and facultatively anaerobic bacteria. Also, high biochemical activity of denitrifying bacteria was recorded in the OB sample, and the same activity of Pseudomonas species was recorded in the OB and VV samples. For OB and LK groundwaters, the highest degree of risk was estimated for biocorrosion process, while for the OB and VV occurrences, the highest degree of risk was estimated for biofouling process. The health risk was present for all examined groundwaters. Caution is warranted in further use of all investigated occurrences due to the established public health risk and an immediate revitalization of the OB, LK and VV wells is necessary.

Sa namerom definisanja hidrogeološke uloge i značaja mikrobiološkog diverziteta odabranih pojava mineralnih voda Srbije, primenom sistema biodetektora, dokazano je prisustvo gvožđevitih, sulfato-redukujućih, sluz-produkujućih, heterotrofnih aerobnih i denitrifikacionih bakterija, na osnovu čega je proračunat rizik od razvoja procesa biokorozije i biohemijske inkrustacije, te zdravstveni rizik. Uz procenu rizika, izvedene su analize skenirajuće elektronske mikroskopije biofilmova i hidrogeohemijsko modeliranje, te makroskopska, otpička i difrakciona ispitivanja naslaga inkrustacije, dok je fizičko-hemijskim analizama utvrđena vrednost ukupne mineralizacije iznad 1 g/L, kao i povišeni sadržaji odgovarajućih metala i nemetala. Metabarkoding analizama mikrobioma mineralnih voda dokazano je prisustvo razdela Proteobacteria, Bacteroidetes, Patescibacteria, Chlorflexi, Firmicutes, Spirochaetes i Acidobacteria u većem procentu. Pseudomonas, Limnobacter i Rheinheimera su bili najzastupljeniji rodovi u uzorku mineralnih voda Torde, Acinetobacter i Massilia u uzorku iz Obrenovačke Banje, Maritimimonas, Phreatobacter, Planktosalinus, Acinetobacter, Geminocystis i Sulfuritalea u uzorku iz Lomnice, Pseudorhodobacter i Limnohabitans u mineralnim vodama Velike Vrbnice, dok su u uzorku iz Slankamen Banje dominirale zelene sumporne bakterije, Methylotenera, Flavobacterium, Melioribacter i Dechloromonas. Realizovanim istraživanjima zaključeno je da odgovarajući predstavnici mikroorganizama utiču na starenje bunara i definišu ekološki status vodnih resursa, dok s druge strane pojedini rodovi poseduju biotehnološki potencijal u tretmanu remedijacije kontaminiranih voda, zbog čega postoji neophodnost implementacije biohidrogeologije u okviru standardne metodike hidrogeoloških istraživanja., The presence of iron related, sulphate reducing, slime producing, heterotrophic aerobic and denitrifying bacteria was proved applying biodetectors, in order to define the hydrogeological role and importance of the microbiological diversity of select occurrences of mineral waters in Serbia. Then the biocorrosion, biochemical encrustation and health risks were assessed. The research included scanning electron microscopy of biofilms, hydrogeochemical modelling and macroscopic, optical and diffraction examination of the encrustations. Physical and chemical analyses indicated total dissolved salts of more than 1 g/L and elevated concentrations of certain metals and non-metals. Metabarcoding of the microbiomes of the mineral waters revealed the presence of a high percentage of the Proteobacteria, Bacteroidetes, Patescibacteria, Chlorflexi, Firmicutes, Spirochaetes and Acidobacteria divisions. Pseudomonas, Limnobacter and Rheinheimera were the most representative genera in the mineral water sample from Torda, Acinetobacter and Massilia in the sample collected in Obrenovačka Banja, Maritimimonas, Phreatobacter, Planktosalinus, Acinetobacter, Geminocystis and Sulfuritalea in the sample from Lomnica, and Pseudorhodobacter and Limnohabitans in the sample from Velika Vrbnica, while in the sample from Slankamen Banja was dominated by green sulphur bacteria, Methylotenera, Flavobacterium, Melioribacter and Dechloromonas. The research concludes that the respective microorganisms cause water well aging and define the ecological status of the water resources, while, on the other hand, some of the genera possess the biotechnological potential for treating and remediating contaminated waters, which is a compelling reason for implementing biohydrogeology in the standard hydrogeological research methodology

Sa namerom definisanja hidrogeološke uloge i značaja mikrobiološkog diverziteta odabranih pojava mineralnih voda Srbije, primenom sistema biodetektora, dokazano je prisustvo gvožđevitih, sulfato-redukujućih, sluz-produkujućih, heterotrofnih aerobnih i denitrifikacionih bakterija, na osnovu čega je proračunat rizik od razvoja procesa biokorozije i biohemijske inkrustacije, te zdravstveni rizik. Uz procenu rizika, izvedene su analize skenirajuće elektronske mikroskopije biofilmova i hidrogeohemijsko modeliranje, te makroskopska, otpička i difrakciona ispitivanja naslaga inkrustacije, dok je fizičko-hemijskim analizama utvrđena vrednost ukupne mineralizacije iznad 1 g/L, kao i povišeni sadržaji odgovarajućih metala i nemetala. Metabarkoding analizama mikrobioma mineralnih voda dokazano je prisustvo razdela Proteobacteria, Bacteroidetes, Patescibacteria, Chlorflexi, Firmicutes, Spirochaetes i Acidobacteria u većem procentu. Pseudomonas, Limnobacter i Rheinheimera su bili najzastupljeniji rodovi u uzorku mineralnih voda Torde, Acinetobacter i Massilia u uzorku iz Obrenovačke Banje, Maritimimonas, Phreatobacter, Planktosalinus, Acinetobacter, Geminocystis i Sulfuritalea u uzorku iz Lomnice, Pseudorhodobacter i Limnohabitans u mineralnim vodama Velike Vrbnice, dok su u uzorku iz Slankamen Banje dominirale zelene sumporne bakterije, Methylotenera, Flavobacterium, Melioribacter i Dechloromonas. Realizovanim istraživanjima zaključeno je da odgovarajući predstavnici mikroorganizama utiču na starenje bunara i definišu ekološki status vodnih resursa, dok s druge strane pojedini rodovi poseduju biotehnološki potencijal u tretmanu remedijacije kontaminiranih voda, zbog čega postoji neophodnost implementacije biohidrogeologije u okviru standardne metodike hidrogeoloških istraživanja., The presence of iron related, sulphate reducing, slime producing, heterotrophic aerobic and denitrifying bacteria was proved applying biodetectors, in order to define the hydrogeological role and importance of the microbiological diversity of select occurrences of mineral waters in Serbia. Then the biocorrosion, biochemical encrustation and health risks were assessed. The research included scanning electron microscopy of biofilms, hydrogeochemical modelling and macroscopic, optical and diffraction examination of the encrustations. Physical and chemical analyses indicated total dissolved salts of more than 1 g/L and elevated concentrations of certain metals and non-metals. Metabarcoding of the microbiomes of the mineral waters revealed the presence of a high percentage of the Proteobacteria, Bacteroidetes, Patescibacteria, Chlorflexi, Firmicutes, Spirochaetes and Acidobacteria divisions. Pseudomonas, Limnobacter and Rheinheimera were the most representative genera in the mineral water sample from Torda, Acinetobacter and Massilia in the sample collected in Obrenovačka Banja, Maritimimonas, Phreatobacter, Planktosalinus, Acinetobacter, Geminocystis and Sulfuritalea in the sample from Lomnica, and Pseudorhodobacter and Limnohabitans in the sample from Velika Vrbnica, while in the sample from Slankamen Banja was dominated by green sulphur bacteria, Methylotenera, Flavobacterium, Melioribacter and Dechloromonas. The research concludes that the respective microorganisms cause water well aging and define the ecological status of the water resources, while, on the other hand, some of the genera possess the biotechnological potential for treating and remediating contaminated waters, which is a compelling reason for implementing biohydrogeology in the standard hydrogeological research methodology

Investigations focusing on wellhead encrustations within select zones of emergence of mineral water, in different hydrogeological provinces, were conducted from 2014 to 2017 in Serbia. They included: well BB-1 in Bogatić (Inner Dinaric Alps of western Serbia), wells LB-4 and LB-5 in Lukovska Banja (Šumadija-Kopaonik-Kosovo Province), and wells VG-2 and VG-3 in Vranjska Banja (Serbian Crystalline Core). The studied occurrences belong to the group of hot mineral waters, with temperatures ranging from 62°C to 93.9°C, and total dissolved solids (TDS) from 600 to 1267 mg/L. They are mildly acidic to mildly alkaline (pH 6.6 – 8.0) and their genetic types are HCO3 --Na++K+ (Bogatić and Lukovska Banja) and SO4 2-, HCO3 --Na++K+ (Vranjska Banja). Macroscopic, optical and diffraction assessment revealed that the encrustations comprised calcite and aragonite minerals, along with some local quartz, muscovite, albite and clinochlore. The conclusion was that there was a predominant presence of calcium carbonate in all the occurrences, represented by crystalline calcite grains, characteristically birefractive, and dark, semitranslucent grains in the form of microcrystalline calcite highly stained by iron hydroxides (Bogatić), as well as crystalline calcite with radially developed carbonate - aragonite (Lukovska Banja) and rod-like and filamentous aggregates of aragonite and calcite, highly translucent in places (Vranjska Banja).The texture of the samples exhibits alternating horizontal or wavy carbonate laminae. In Vranjska Banja, they include thin micritic to medium crystalline laminae and laminae composed of aggregates of aragonite needles in the form of bunch. In Bogatić and Lukovska Banja, there are thick micritic and highly porous laminae, 0.025 to 1 mm thick, or interlayers of a coarser crystalline structure. Apart from encrustations, all the occurrences exhibit phototrophic microbial mats, which were analyzed by scanning electron microscopy. The onset and progress of encrus

Investigations focusing on wellhead encrustations within select zones of emergence of mineral water, in different hydrogeological provinces, were conducted from 2014 to 2017 in Serbia. They included: well BB-1 in Bogatic (Inner Dinaric Alps of western Serbia), wells LB-4 and LB-5 in Lukovska Banja (Sumadija-Kopaonik-Kosovo Province), and wells VG-2 and VG-3 in Vranjska Banja (Serbian Crystalline Core). The studied occurrences belong to the group of hot mineral waters, with temperatures ranging from 62 degrees C to 93.9 degrees C, and total dissolved solids (TDS) from 600 to 1267 mg/L. They are mildly acidic to mildly alkaline (pH 6.6 -8.0) and their genetic types are HCO3--Na++K+ (Bogatic and Lukovska Banja) and SO42-, HCO3--Na++K+ (Vranjska Banja). Macroscopic, optical and diffraction assessment revealed that the encrustations comprised calcite and aragonite minerals, along with some local quartz, muscovite, albite and clinochlore. The conclusion was that there was a predominant presence of calcium carbonate in all the occurrences, represented by crystalline calcite grains, characteristically birefractive, and dark, semi-translucent grains in the form of microcrystalline calcite highly stained by iron hydroxides (Bogatic), as well as crystalline calcite with radially developed carbonate - aragonite (Lukovska Banja) and rod-like and filamentous aggregates of aragonite and calcite, highly translucent in places (Vranjska Banja). The texture of the samples exhibits alternating horizontal or wavy carbonate laminae. In Vranjska Banja, they include thin micritic to medium crystalline laminae and laminae composed of aggregates of aragonite needles in the form of bunch. In Bogatic and Lukovska Banja, there are thick micritic and highly porous laminae, 0.025 to 1 mm thick, or interlayers of a coarser crystalline structure. Apart from encrustations, all the occurrences exhibit phototrophic microbial mats, which were analyzed by scanning electron microscopy. The onset and prog

Investigations focusing on wellhead encrustations within select zones of emergence of mineral water, in different hydrogeological provinces, were conducted from 2014 to 2017 in Serbia. They included: well BB-1 in Bogatić (Inner Dinaric Alps of western Serbia), wells LB-4 and LB-5 in Lukovska Banja (Šumadija-Kopaonik-Kosovo Province), and wells VG-2 and VG-3 in Vranjska Banja (Serbian Crystalline Core). The studied occurrences belong to the group of hot mineral waters, with temperatures ranging from 62°C to 93.9°C, and total dissolved solids (TDS) from 600 to 1267 mg/L. They are mildly acidic to mildly alkaline (pH 6.6 – 8.0) and their genetic types are HCO3 --Na++K+ (Bogatić and Lukovska Banja) and SO4 2-, HCO3 --Na++K+ (Vranjska Banja). Macroscopic, optical and diffraction assessment revealed that the encrustations comprised calcite and aragonite minerals, along with some local quartz, muscovite, albite and clinochlore. The conclusion was that there was a predominant presence of calcium carbonate in all the occurrences, represented by crystalline calcite grains, characteristically birefractive, and dark, semitranslucent grains in the form of microcrystalline calcite highly stained by iron hydroxides (Bogatić), as well as crystalline calcite with radially developed carbonate - aragonite (Lukovska Banja) and rod-like and filamentous aggregates of aragonite and calcite, highly translucent in places (Vranjska Banja).The texture of the samples exhibits alternating horizontal or wavy carbonate laminae. In Vranjska Banja, they include thin micritic to medium crystalline laminae and laminae composed of aggregates of aragonite needles in the form of bunch. In Bogatić and Lukovska Banja, there are thick micritic and highly porous laminae, 0.025 to 1 mm thick, or interlayers of a coarser crystalline structure. Apart from encrustations, all the occurrences exhibit phototrophic microbial mats, which were analyzed by scanning electron microscopy. The onset and progress of encrus

Phototropic microorganisms in thermal mats had not attracted enough attention in Serbia in the past. The research presented in this paper is the first to address biofilms at selected occurrences of mineral water in Serbia (Bogatic, Metkovic and Belotic in the Macva District as well as Radaljska Banja, Lukovska Banja and Vranjska Banja) (in Serbian, banja denotes a spa town). The study period is from 2014 to 2016. The temperatures of the studied occurrences are in the 30-93.9 degrees C range and the chemical composition corresponds to the HCO3- -Na++ K+ and SO42-, HCO3--Na++ K+ types of mineral water. One of the objectives was to examine the condition of wellheads (incrustation, biofouling and corrosion), focusing on the presence of phototrophic microorganisms whose metabolites may lead to changes in mineral water quality, yield and chemistry. Analysis by light and scanning electron microscopy provides insight into the diversity of phototrophic microorganisms that populate biofilms near the points of emergence and on wellhead. The research reveals the presence of representatives of three groups of phototrophic microorganisms: Cyanobacteria, Chlorophyta and Bacillariophyta. Cyanobacteria were the most numerous, dominated by the order Oscillatoriales with ten recorded genera, of which Leptolynbgya and Phormidium feature the largest numbers of species. The order Chroococcales are the second most numerous, with seven recorded genera, of which Gloeocapsa and Synechocystisare the most noteworthy. The largest numbers of representative taxa were noted at Radaljska Banja and Vranjska Banja, where mineral waters originate from lithostratigraphic units of igneous origin, followed by mineral water occurrences in karst aquifers (Bogatic and Belotic) and an intergranular aquifer (Metkovic). The smallest number of identified phototrophic microorganisms is noted at Lukovska Banja, where the origin is associated with a fractured aquifer formed in sedimentary and metamorphic rocks, wi