Microbial communities of soda lakes and pans in the Carpathian Basin: a review

Metagenomic insights into the microbial community of the Buhera soda pans, Zimbabwe

BACKGROUND

Soda pans are unique, natural aquatic environments characterised by elevated salinity and alkalinity, creating a distinctive and often extreme geochemistry. The microbiomes of soda pans are unique, with extremophiles such as halophiles, alkaliphiles and haloalkaliphiles being important. Despite being dominated by mostly unculturable inhabitants, soda pans hold immense biotechnological potential. The application of modern "omics-based" techniques helps us better understand the ecology and true extend of the biotechnological potential of soda pan microbiomes. In this study, we used a shotgun metagenomic approach to determine the microbial diversity and functional profile of previously unexplored soda pans located in Buhera, Eastern Zimbabwe. A combination of titrimetry and inductively coupled plasma optical emission spectroscopy (ICP‒OES) was used to perform physico-chemical analysis of the soda pan water.

RESULTS

Physicochemical analysis revealed that the Buhera soda pans are highly alkaline, with a pH range of 8.74 to 11.03, moderately saline (2.94 - 7.55 g/L), and have high carbonate (3625 mg/L) and bicarbonate ion (1325 mg/L) alkalinity. High levels of sulphate, phosphate, chloride and fluoride ions were detected. Metagenomic analysis revealed that domain Bacteria dominated the soda pan microbial community, with Pseudomonadota and Bacillota being the dominant phyla. Vibrio was shown to be the predominant genus, followed by Clostridium, Candidatus Brevefilum, Acetoanaerobium, Thioalkalivibrio and Marinilactibacillus. Archaea were also detected, albeit at a low prevalence of 1%. Functional profiling revealed that the Buhera soda pan microbiome is functionally diverse, has hydrolytic-enzyme production potential and is capable of supporting a variety of geochemical cycles.

CONCLUSIONS

The results of this pioneering study showed that despite their extreme alkalinity and moderate salinity, the Buhera soda pans harbour a taxonomically and functionally diverse microbiome dominated by bacteria. Future work will aim towards establishing the full extent of the soda pan's biotechnological potential, with a particular emphasis on potential enzyme production.

Taxonomic diversity of extremophilic prokaryotes adapted to special environmental parameters in Hungary: a review

The taxonomic and metabolic diversity of prokaryotes and their adaptability to extreme environmental parameters have allowed extremophiles to find their optimal living conditions under extreme conditions for one or more environmental parameters. Natural habitats abundant in extremophilic microorganisms are relatively rare in Hungary. Nevertheless, alkaliphiles and halophiles can flourish in shallow alkaline lakes (soda pans) and saline (solonetz) soils, where extreme weather conditions favor the development of unique bacterial communities. In addition, the hot springs and thermal wells that supply spas and thermal baths and provide water for energy use are suitable colonization sites for thermophiles and hyperthermophiles. Polyextremophiles, adapted to multiple extreme circumstances, can be found in the aphotic, nutrient-poor and radioactive hypogenic caves of the Buda Thermal Karst, among others. The present article reviews the organization, taxonomic composition, and potential role of different extremophilic bacterial communities in local biogeochemical cycles, based on the most recent studies on extremophiles in Hungary.

Response of the microbial community structure to the environmental factors during the extreme flood season in Poyang Lake, the largest freshwater lake in China

Background

Poyang Lake is the largest freshwater lake in China, and there are several studies on the composition and diversity of bacteria in Poyang Lake, while few quantitative studies were carried out on the response of the bacterial community to environmental factors during the extreme flood season in Poyang Lake.

Methods

The connected-lake heterogeneity of bacterial community composition (BCC) was investigated in Poyang Lake during the flood season in 2020. Illumina high-throughput sequencing technology was used in this study.

Results

The bacterial community structure in the water was different from that in the sediment of Poyang Lake during extreme flood seasons. The bacterial diversity in water was much lower than that in sediment. In the water column, the dominant phyla were Actinobacteriota, while the composition of bacteria in sediment was more complex than that in water, and the dominant phyla in sediment were Proteobacteria, Chloroflexi, Acidobacteriota, and Actinobacteriota. The bacterial diversity in the water of Poyang Lake showed seasonal dynamics, while no seasonal variation of bacterial communities in sediment was observed. The bacterial community structure in the sediment from the two bays and channel areas of Poyang Lake can be distinguished from each other. The microbial diversity in sediment gradually increased from the Sancha Bay to the Zhouxi Bay and then to the channel, but the total nitrogen (TN) concentration in sediment (STN) and the total phosphorus (TP) concentration in sediment (STP) showed opposite trends. This might be due to the anthropogenic disturbances from the extreme flood. The bacterial community structure in, water column was significantly correlated with WT, NH4-N, STP, SOM, Chl a, DO, TP, and Eh, while the bacterial community structure in sediment was significantly correlated with SOM and STP.

Conclusion

The bacterial community structure in water was greatly different from that in sediment in Poyang Lake during extreme flood seasons. The bacterial community structure in the water column was not only sensitive to the geochemical characteristics of the water but also affected by some nutrient concentrations in the sediment. During the wet seasons, bacterial diversity was only affected by SOM and STP.

Response of the metabolic activity and taxonomic composition of bacterial communities to mosaically varying soil salinity and alkalinity

Soil salinity and sodicity is a worldwide problem that affects the composition and activity of bacterial communities and results from elevated salt and sodium contents. Depending on the degree of environmental pressure and the combined effect of other factors, haloalkalitolerant and haloalkaliphilic bacterial communities will be selected. These bacteria play a potential role in the maintenance and restoration of salt-affected soils; however, until recently, only a limited number of studies have simultaneously studied the bacterial diversity and activity of saline-sodic soils. Soil samples were collected to analyse and compare the taxonomic composition and metabolic activity of bacteria from four distinct natural plant communities at three soil depths corresponding to a salinity‒sodicity gradient. Bacterial diversity was detected using 16S rRNA gene Illumina MiSeq amplicon sequencing. Community-level physiological profiles (CLPPs) were analysed using the MicroResp™ method. The genus-level bacterial composition and CLPPs differed significantly in soils with different alkaline vegetation. The surface soil samples also significantly differed from the intermediate and deep soil samples. The results showed that the pH, salt content, and Na+ content of the soils were the main edaphic factors influencing both bacterial diversity and activity. With salinity and pH, the proportion of the phylum Gemmatimonadota increased, while the proportions of Actinobacteriota and Acidobacteriota decreased.

Contrasting response of microeukaryotic and bacterial communities to the interplay of seasonality and local stressors in shallow soda lakes

Seasonal environmental variation is a leading driver of microbial planktonic community assembly and interactions. However, departures from usual seasonal trends are often reported. To understand the role of local stressors in modifying seasonal succession, we sampled fortnightly, throughout three seasons, five nearby shallow soda lakes exposed to identical seasonal and meteorological changes. We characterised their microeukaryotic and bacterial communities by amplicon sequencing of the 16S and 18S rRNA gene, respectively. Biological interactions were inferred by analyses of synchronous and time-shifted interaction networks, and the keystone taxa of the communities were topologically identified. The lakes showed similar succession patterns during the study period with spring being characterised by the relevance of trophic interactions and a certain level of community stability followed by a more dynamic and variable summer-autumn period. Adaptation to general seasonal changes happened through shared core microbiome of the lakes. Stochastic events such as desiccation disrupted common network attributes and introduced shifts from the prevalent seasonal trajectory. Our results demonstrated that, despite being extreme and highly variable habitats, shallow soda lakes exhibit certain similarities in the seasonality of their planktonic communities, yet local stressors such as droughts instigate deviations from prevalent trends to a greater extent for microeukaryotic than for bacterial communities.

Habitat distribution of the genus Belliella in continental waters and the description of Belliella alkalica sp. nov., Belliella calami sp. nov. and Belliella filtrata sp. nov

The genus Belliella belongs to the family Cyclobacteriaceae (order Cytophagales, phylum Bacteroidota) and harbours aerobic chemoheterotrophic bacteria. Members of this genus were isolated from various aquatic habitats, and our analysis based on global amplicon sequencing data revealed that their relative abundance can reach up to 5-10 % of the bacterioplankton in soda lakes and pans. Although a remarkable fraction of the most frequent genotypes that we identified from continental aquatic habitats is still uncultured, five new alkaliphilic Belliella strains were characterized in detail in this study, which were isolated from three different soda lakes and pans of the Carpathian Basin (Hungary). Cells of all strains were Gram-stain-negative, obligate aerobic, rod-shaped, non-motile and non-spore-forming. The isolates were oxidase- and catalase-positive, red-coloured, but did not contain flexirubin-type pigments; they formed bright red colonies that were circular, smooth and convex. Their major isoprenoid quinone was MK-7 and the predominant fatty acids were iso-C_15 : 0, iso-C_17 : 0 3-OH and summed feature 3 containing C_16 : 1  ω6c and/or C_16 : 1  ω7c. The polar lipid profiles contained phosphatidylethanolamine, an unidentified aminophospholipid, an unidentified glycolipid, and several unidentified lipids and aminolipids. Based on whole-genome sequences, the DNA G+C content was 37.0, 37.1 and 37.8 mol % for strains R4-6^T, DMA-N-10a^T and U6F3^T, respectively. The distinction of three new species was confirmed by in silico genomic comparison. Orthologous average nucleotide identity (<85.4 %) and digital DNA-DNA hybridization values (<38.9 %) supported phenotypic, chemotaxonomic and 16S rRNA gene sequence data and, therefore, the following three novel species are proposed: Belliella alkalica sp. nov. (represented by strains R4-6^T=DSM 111903^T=JCM 34281^T=UCCCB122^T and S4-10), Belliella calami sp. nov. (DMA-N-10a^T=DSM 107340^T=JCM 34280^T=UCCCB121^T) and Belliella filtrata sp. nov. (U6F3^T=DSM 111904^T=JCM 34282^T=UCCCB123^T and U6F1). Emended descriptions of species Belliella aquatica, Belliella baltica, Belliella buryatensis, Belliella kenyensis and Belliella pelovolcani are also presented.

Bacterial Communities Along Environmental Gradients in Tropical Soda Lakes

Soda lake environments are known to be variable and can have distinct differences according to geographical location. In this study, we investigated the effects of different environmental conditions of six adjacent soda lakes in the Pantanal biome (Mato Grosso do Sul state, Brazil) on bacterial communities and their functioning using a metagenomic approach combined with flow cytometry and chemical analyses. Ordination analysis using flow cytometry and water chemistry data from two sampling periods (wet and dry) clustered soda lakes into three different profiles: eutrophic turbid (ET), oligotrophic turbid (OT), and clear vegetated oligotrophic (CVO). Analysis of bacterial community composition and functioning corroborated this ordination; the exception was one ET lake, which was similar to one OT lake during the wet season, indicating drastic shifts between seasons. Microbial abundance and diversity increased during the dry period, along with a considerable number of limnological variables, all indicative of a strong effect of the precipitation-evaporation balance in these systems. Cyanobacteria were associated with high electric conductivity, pH, and nutrient availability, whereas Actinobacteria, Alphaproteobacteria, and Betaproteobacteria were correlated with landscape morphology variability (surface water, surface perimeter, and lake volume) and with lower salinity and pH levels. Stress response metabolism was enhanced in OT and ET lakes and underrepresented in CVO lakes. The microbiome dataset of this study can serve as a baseline for restoring impacted soda lakes. Altogether, the results of this study demonstrate the sensitivity of tropical soda lakes to climate change, as slight changes in hydrological regimes might produce drastic shifts in community diversity.

Habitat and Features of Development of Plankton Communities in Salt Lakes (South-Eastern Transbaikalia, Russia)

Results of studies of plankton algae and invertebrates in salt lakes of the territory of a closed runoff in the south of South-Eastern Transbaikalia (Russia) carried out in 2021–2022 are presented. Phyto- and zooplankton of sixteen saline lakes were studied during the maximum vegetation period from July to August. Lakes are different in chemical type: chloride, soda and sulfate. For chloride, sulfate and some soda lakes, data on plankton have been obtained for the first time. Fifty-four taxa of phytoplankton and twenty-seven species of zooplankton were found in soda lakes; twenty-three taxa of phytoplankton and four species of zooplankton were found in the chloride lakes; fifteen phytoplankton species and five zooplankton species were found in the sulfate lakes. For phytoplankton in soda lakes, green algae, cyanobacteria and diatoms were dominant. Green algae dominated in species composition in sulfate lakes; cryptophyte algae and cyanobacteria dominated in chloride lakes. For zooplankton, in all types of lakes, Brachionus plicatilis, Moina brachiata and Metadiaptomus asiaticus dominated. The abundance and biomass of algae and invertebrates in the surveyed lakes varied widely. Based on the results of the correlation analysis, total dissolved solids (TDS) are a key factor in the formation of planktonic communities in soda lakes; depth, transparency and temperature—in chloride lakes and pH—in sulfate lakes.

Environmental changes associated with drying climate are expected to affect functional groups of pro- and microeukaryotes differently in temporary saline waters

Temporary ponds are among the most sensitive aquatic habitats to climate change. Their microbial communities have crucial roles in food webs and biogeochemical cycling, yet how their communities are assembled along environmental gradients is still understudied. This study aimed to reveal the environmental drivers of diversity (OTU-based richness, evenness, and phylogenetic diversity) and community composition from a network of saline temporary ponds, soda pans, in two consecutive spring seasons characterized by contrasting weather conditions. We used DNA-based molecular methods to investigate microbial community composition. We tested the effect of environmental variables on the diversity of prokaryotic (Bacteria, Cyanobacteria) and microeukaryotic functional groups (ciliates, heterotrophic flagellates and nanoflagellates, fungi, phytoplankton) within and across the years. Conductivity and the concentration of total suspended solids and phosphorus were the most important environmental variables affecting diversity patterns in all functional groups. Environmental conditions were harsher and they also had a stronger impact on community composition in the dry spring. Our results imply that these conditions, which are becoming more frequent with climate change, have a negative effect on microbial diversity in temporary saline ponds. This eventually might translate into community-level shifts across trophic groups with changing local conditions with implications for ecosystem functioning.

Prokaryotic and eukaryotic microbial diversity from three soda lakes in the East African Rift Valley determined by amplicon sequencing

Soda lakes are unique poly-extreme environments with high alkalinity and salinity that support diverse microbial communities despite their extreme nature. In this study, prokaryotic and eukaryotic microbial diversity in samples of the three soda lakes, Lake Abijata, Lake Chitu and Lake Shala in the East African Rift Valley, were determined using amplicon sequencing. Culture-independent analysis showed higher diversity of prokaryotic and eukaryotic microbial communities in all three soda lakes than previously reported. A total of 3,603 prokaryotic and 898 eukaryotic operational taxonomic units (OTUs) were found through culture-independent amplicon sequencing, whereas only 134 bacterial OTUs, which correspond to 3%, were obtained by enrichment cultures. This shows that only a fraction of the microorganisms from these habitats can be cultured under laboratory conditions. Of the three soda lakes, samples from Lake Chitu showed the highest prokaryotic diversity, while samples from Lake Shala showed the lowest diversity. Pseudomonadota (Halomonas), Bacillota (Bacillus, Clostridia), Bacteroidota (Bacteroides), Euryarchaeota (Thermoplasmata, Thermococci, Methanomicrobia, Halobacter), and Nanoarchaeota (Woesearchaeia) were the most common prokaryotic microbes in the three soda lakes. A high diversity of eukaryotic organisms were identified, primarily represented by Ascomycota and Basidiomycota. Compared to the other two lakes, a higher number of eukaryotic OTUs were found in Lake Abijata. The present study showed that these unique habitats harbour diverse microbial genetic resources with possible use in biotechnological applications, which should be further investigated by functional metagenomics.

Where the Little Ones Play the Main Role-Picophytoplankton Predominance in the Soda and Hypersaline Lakes of the Carpathian Basin

The extreme environmental conditions of the diverse saline inland waters (soda lakes and pans, hypersaline lakes and ponds) of the Carpathian Basin are an advantage for picophytoplankton. The abundance of picophytoplankton in these waters can be up to several orders of magnitude higher than that in freshwater shallow lakes, but differences are also found within different saline water types: higher picophytoplankton abundances were observed in hypersaline lakes compared to humic soda lakes, and their highest numbers were detected in turbid soda lakes. Moreover, their contribution to phytoplankton biomass is higher than that in shallow freshwater lakes with similar trophic states. Based on long-term data, their ratio within the phytoplankton increased with turbidity in the case of turbid soda lakes, while, in hypersaline lakes, their proportion increased with salinity. Picocyanobacteria were only detected with high abundance (>106−107 cells/mL) in turbid soda lakes, while picoeukaryotes occurred in high numbers in both turbid and hypersaline lakes. Despite the extreme conditions of the lakes, the diversity of picophytoplankton is remarkable, with the dominance of non-marine Synechococcus/Cyanobium, Choricystis, Chloroparva and uncultured trebouxiophycean green algae in the soda lakes, and marine Synechococcus and Picochlorum in the hypersaline lakes.

Anion-type modulates the effect of salt stress on saline lake bacteria

Beside sodium chloride, inland saline aquatic systems often contain other anions than chloride such as hydrogen carbonate and sulfate. Our understanding of the biological effects of salt composition diversity is limited; therefore, the aim of this study was to examine the effect of different anions on the growth of halophilic bacteria. Accordingly, the salt composition and concentration preference of 172 strains isolated from saline and soda lakes that differed in ionic composition was tested using media containing either carbonate, chloride or sulfate as anion in concentration values ranging from 0 to 0.40 mol/L. Differences in salt-type preference among bacterial strains were observed in relationship to the salt composition of the natural habitat they were isolated from indicating specific salt-type adaptation. Sodium carbonate represented the strongest selective force, while majority of strains was well-adapted to growth even at high concentrations of sodium sulfate. Salt preference was to some extent associated with taxonomy, although variations even within the same bacterial species were also identified. Our results suggest that the extent of the effect of dissolved salts in saline lakes is not limited to their concentration but the type of anion also substantially impacts the growth and survival of individual microorganisms.

Grazing pressure-induced shift in planktonic bacterial communities with the dominance of acIII-A1 actinobacterial lineage in soda pans

Astatic soda pans of the Pannonian Steppe are unique environments with respect to their multiple extreme physical and chemical characteristics (high daily water temperature fluctuation, high turbidity, alkaline pH, salinity, polyhumic organic carbon concentration, hypertrophic state and special ionic composition). However, little is known about the seasonal dynamics of the bacterial communities inhabiting these lakes and the role of environmental factors that have the main impact on their structure. Therefore, two soda pans were sampled monthly between April 2013 and July 2014 to reveal changes in the planktonic community. By late spring in both years, a sudden shift in the community structure was observed, the previous algae-associated bacterial communities had collapsed, resulting the highest ratio of Actinobacteria within the bacterioplankton (89%, with the dominance of acIII-A1 lineage) ever reported in the literature. Before these peaks, an extremely high abundance (> 10,000 individuum l-1) of microcrustaceans (Moina brachiata and Arctodiaptomus spinosus) was observed. OTU-based statistical approaches showed that in addition to algal blooms and water-level fluctuations, zooplankton densities had the strongest effect on the composition of bacterial communities. In these extreme environments, this implies a surprisingly strong, community-shaping top-down role of microcrustacean grazers.

A Large-Scale Survey of the Bacterial Communities in Lakes of Western Mongolia with Varying Salinity Regimes

In recent years, climate change coupled with anthropogenic activities has led to monumental changes in saline lakes which are rapidly drying up across the globe and particularly in Central Asia. The landlocked country of Mongolia is rich in lakes which have remained primarily undisturbed by human impact, and many of these lakes have varying salinity regimes and are located across various geographical landscapes. In this study, we sampled 18 lakes with varying salinity regimes (hyperhaline, mesohaline, oligohaline, and polyhaline) covering 7000 km of western Mongolia and its various geographical landscapes (Gobi Desert, forests, and steppe). We identified that the bacterial communities that dominate these lakes are significantly influenced by salinity (p < 0.001) and geographical landscape (p < 0.001). Further, only five zOTUs were shared in all the lakes across the salinity regimes, providing evidence that both local and regional factors govern the community assembly and composition. Furthermore, the bacterial communities of hyperhaline lakes were significantly positively correlated with salinity (ANOVA, p < 0.001) and arsenic concentrations (ANOVA, p < 0.001), whereas bacterial communities of mesohaline and polyhaline lakes situated in forest and steppe landscapes were positively correlated with temperature (ANOVA, p < 0.001) and altitude (ANOVA, p < 0.001), respectively. Functional predictions based on the 16S rRNA gene indicated enrichment of KEGG Ontology terms related to transporters for osmoprotection and -regulation. Overall, our study provides a comprehensive view of the bacterial diversity and community composition present in these lakes, which might be lost in the future.