Molecular analysis of the spatio-temporal distribution of sulfate-reducing bacteria (SRB) in Camargue (France) hypersaline microbial mat

Contrasting response strategies of sulfate-reducing bacteria in a microbial consortium to As3+ stress under anaerobic and aerobic environments

The sulfate-reducing efficiency of sulfate-reducing bacteria (SRB) is strongly influenced by the presence of oxygen, but little is known about the oxygen tolerance mechanism of SRB and the effect of oxygen on the metalliferous immobilization by SRB. The performance evaluation, identification of bioprecipitates, and microbial and metabolic process analyses were used here to investigate the As3+ immobilization mechanisms and survival strategies of the SRB1 consortium under different oxygen-containing environments. Results indicated that the sulfate reduction efficiency was significantly decreased under aerobic (47.37%) compared with anaerobic conditions (66.72%). SEM analysis showed that under anaerobic and aerobic conditions, the morphologies of mineral particles were different, whereas XRD and XPS analyses showed that the most of As3+ bioprecipitates under both conditions were arsenic minerals such as AsS and As4S4. The abundances of Clostridium_sensu_stricto_1, Desulfovibrio, and Thiomonas anaerobic bacteria were significantly higher under anaerobic than aerobic conditions, whereas the aerobic Pseudomonas showed an opposite trend. Network analysis revealed that Desulfovibrio was positively correlated with Pseudomonas. Metabolic process analysis confirmed that under aerobic conditions the SRB1 consortium generated additional extracellular polymeric substances (rich in functionalities such as Fe-O, SO, CO, and -OH) and the anti-oxidative enzyme superoxide dismutase to resist As3+ stress and oxygen toxicity. New insights are provided here into the oxygen tolerance and detoxification mechanism of SRB and provide a basis for the future remediation of heavy metal(loid)-contaminated environments.

Microbial mats as model to decipher climate change effect on microbial communities through a mesocosm study

Marine environments are expected to be one of the most affected ecosystems by climate change, notably with increasing ocean temperature and ocean acidification. In marine environments, microbial communities provide important ecosystem services ensuring biogeochemical cycles. They are threatened by the modification of environmental parameters induced by climate change that, in turn, affect their activities. Microbial mats, ensuring important ecosystem services in coastal areas, are well-organized communities of diverse microorganisms representing accurate microbial models. It is hypothesized that their microbial diversity and metabolic versatility will reveal various adaptation strategies in response to climate change. Thus, understanding how climate change affects microbial mats will provide valuable information on microbial behaviour and functioning in changed environment. Experimental ecology, based on mesocosm approaches, provides the opportunity to control physical-chemical parameters, as close as possible to those observed in the environment. The exposure of microbial mats to physical-chemical conditions mimicking the climate change predictions will help to decipher the modification of the microbial community structure and function in response to it. Here, we present how to expose microbial mats, following a mesocosm approach, to study the impact of climate change on microbial community.

Community Vertical Composition of the Laguna Negra Hypersaline Microbial Mat, Puna Region (Argentinean Andes)

The Altiplano-Puna region is a high-altitude plateau in South America characterized by extreme conditions, including the highest UV incidence on Earth. The Laguna Negra is a hypersaline lake located in the Catamarca Province, northwestern Argentina, where stromatolites and other microbialites are found, and where life is mostly restricted to microbial mats. In this study, a particular microbial mat that covers the shore of the lake was explored, to unravel its layer-by-layer vertical structure in response to the environmental stressors therein. Microbial community composition was assessed by high-throughput 16S rRNA gene sequencing and pigment content analyses, complemented with microscopy tools to characterize its spatial arrangement within the mat. The top layer of the mat has a remarkable UV-tolerance feature, characterized by the presence of Deinococcus-Thermus and deinoxanthin, which might reflect a shielding strategy to cope with high UV radiation. Chloroflexi and Deltaproteobacteria were abundant in the second and third underlying layers, respectively. The bottom layer harbors copious Halanaerobiaeota. Subspherical aggregates composed of calcite, extracellular polymeric substances, abundant diatoms, and other microorganisms were observed all along the mat as the main structural component. This detailed study provides insights into the strategies of microbial communities to thrive under high UV radiation and hypersalinity in high-altitude lakes in the Altiplano-Puna region.

Climate change influences chlorophylls and bacteriochlorophylls metabolism in hypersaline microbial mat

This study aimed to determine the effect of the climatic change on the phototrophic communities of hypersaline microbial mats. Ocean acidification and warming were simulated alone and together on microbial mats placed into mesocosms. As expected, the temperature in the warming treatments increased by 4 °C from the initial temperature. Surprisingly, no significance difference was observed between the water pH of the different treatments despite of a decrease of 0.4 unit pH in the water reserves of acidification treatments. The salinity increased on the warming treatments and the dissolved oxygen concentration increased and was higher on the acidification treatments. A total of 37 pigments were identified belonging to chlorophylls, carotenes and xanthophylls families. The higher abundance of unknown chlorophyll molecules called chlorophyll derivatives was observed in the acidification alone treatment with a decrease in chlorophyll a abundance. This change in pigmentary composition was accompanied by a higher production of bound extracellular carbohydrates but didn't affect the photosynthetic efficiency of the microbial mats. A careful analysis of the absorption properties of these molecules indicated that these chlorophyll derivatives were likely bacteriochlorophyll c contained in the chlorosomes of green anoxygenic phototroph bacteria. Two hypotheses can be drawn from these results: 1/ the phototrophic communities of the microbial mats were modified under acidification treatment leading to a higher relative abundance of green anoxygenic bacteria, or 2/ the highest availability of CO₂ in the environment has led to a shift in the metabolism of green anoxygenic bacteria being more competitive than other phototrophs.

New insights in bacterial and eukaryotic diversity of microbial mats inhabiting exploited and abandoned salterns at the Ré Island (France)

In order to understand the effect of human practices on microbial mats organisation, the study aimed to investigate the biodiversity within microbial mats from exploited and abandoned salterns. Despite several attempts, archaeal 16S rRNA gene fragment sequences were not obtained, indicating that microbial mats were probably dominated by Bacteria with very low abundance of Archaea (< 1%). Thus, the study compared the bacterial and meiofaunal diversity of microbial mats from abandoned and exploited salterns. The higher salinity (101 ± 3.7 psu vs. 51.1 ± 0.7 psu; Welch t-test p < 0.05) of the exploited site maintained lower bacterial diversity in comparison to the abandoned site where the salinity gradient was no longer maintained. However, the microbial mats exhibited similar bacterial class composition while the eukaryotic diversity was significantly higher in the exploited saltern. The abandoned saltern was dominated by sulfate-reducing bacteria and Nematoda, while the exploited saltern was characterized by the presence of halophilic bacteria belonging to Marinobacter, Salinivibrio and Rhodohalobacter genera, and the larger abundance of Hypotrichia (ciliates). Such bacterial and eukaryotic diversity difference might be explained by human actions for salt recovery in exploited salterns such as scraping the surface of microbial mat and increasing salinity renewing the microbial mat each year. Such action decreases the bacterial diversity changing the food web structure that favour the presence of a larger diversity of eukaryotic organisms. Our study provides new insights on microbial mat communities inhabiting salterns, especially the consequences of abandoning saltern exploitation.

Proteogenomic Insights into the Physiology of Marine, Sulfate-Reducing, Filamentous Desulfonema limicola and Desulfonema magnum

The genus Desulfonema belongs to the deltaproteobacterial family Desulfobacteraceae and comprises marine, sulfate-reducing bacteria that form filaments and move by gliding. This study reports on the complete, manually annotated genomes of Dn. limicola 5ac10T (6.91 Mbp; 6,207 CDS) and Dn. magnum 4be13T (8.03 Mbp; 9,970 CDS), integrated with substrate-specific proteome profiles (8 vs. 11). The richness in mobile genetic elements is shared with other Desulfobacteraceae members, corroborating horizontal gene transfer as major driver in shaping the genomes of this family. The catabolic networks of Dn. limicola and Dn. magnum have the following general characteristics: 98 versus 145 genes assigned (having genomic shares of 1.7 vs. 2.2%), 92.5 versus 89.7% proteomic coverage, and scattered gene clusters for substrate degradation and energy metabolism. The Dn. magnum typifying capacity for aromatic compound degradation (e.g., p-cresol, 3-phenylpropionate) requires 48 genes organized in operon-like structures (87.7% proteomic coverage; no homologs in Dn. limicola). The protein complements for aliphatic compound degradation, central pathways, and energy metabolism are highly similar between both genomes and were identified to a large extent (69-96%). The differential protein profiles revealed a high degree of substrate-specificity for peripheral reaction sequences (forming central intermediates), agreeing with the high number of sensory/regulatory proteins predicted for both strains. By contrast, central pathways and modules of the energy metabolism were constitutively formed under the tested substrate conditions. In accord with their natural habitats that are subject to fluctuating changes of physicochemical parameters, both Desulfonema strains are well equipped to cope with various stress conditions. Next to superoxide dismutase and catalase also desulfoferredoxin and rubredoxin oxidoreductase are formed to counter exposure to molecular oxygen. A variety of proteases and chaperones were detected that function in maintaining cellular homeostasis upon heat or cold shock. Furthermore, glycine betaine/proline betaine transport systems can respond to hyperosmotic stress. Gliding movement probably relies on twitching motility via type-IV pili or adventurous motility. Taken together, this proteogenomic study demonstrates the adaptability of Dn. limicola and Dn. magnum to its dynamic habitats by means of flexible catabolism and extensive stress response capacities.

Bacterial community assemblages in sediments under high anthropogenic pressure at Ichkeul Lake/Bizerte Lagoon hydrological system, Tunisia

Bacterial communities inhabiting sediments in coastal areas endure the effect of strong anthropogenic pressure characterized by the presence of multiple contaminants. Understanding the effect of pollutants on the organization of bacterial communities is of paramount importance in order to unravel bacterial assemblages colonizing specific ecological niches. Here, chemical and molecular approaches were combined to investigate the bacterial communities inhabiting the sediments of the Ichkeul Lake/Bizerte Lagoon, a hydrological system under anthropogenic pressure. Although the microbial community of the Ichkeul Lake sediment was different to that of the Bizerte Lagoon, common bacterial genera were identified suggesting a lake-lagoon continuum probably due to the hydrology of the system exchanging waters according to the season. These genera represent bacterial "generalists" maintaining probably general biogeochemical functions. Linear discriminant analysis effect size (LEfSe) showed significant differential abundance distribution of bacterial genera according to the habitat, the pollution type and level. Further, correlation analyses identified specific bacterial genera which abundance was linked with pesticides concentrations in the lake, while in the lagoon the abundance of specific bacterial genera was found linked with the concentrations of PAHs (Polycyclic aromatic hydrocarbons) and organic forms of Sn. As well, bacterial genera which abundance was not correlated with the concentrations of pollutants were identified in both lake and lagoon. These findings represent valuable information, pointing out specific bacterial genera associated with pollutants, which represent assets for developing bacterial tools for the implementation, the management, and monitoring of bioremediation processes to mitigate the effect of pollutants in aquatic ecosystems.

Growth of an anaerobic sulfate-reducing bacterium sustained by oxygen respiratory energy conservation after O2 -driven experimental evolution

Desulfovibrio species are representatives of microorganisms at the boundary between anaerobic and aerobic lifestyles, since they contain the enzymatic systems required for both sulfate and oxygen reduction. However, the latter has been shown to be solely a protective mechanism. By implementing the oxygen-driven experimental evolution of Desulfovibrio vulgaris Hildenborough, we have obtained strains that have evolved to grow with energy derived from oxidative phosphorylation linked to oxygen reduction. We show that a few mutations are sufficient for the emergence of this phenotype and reveal two routes of evolution primarily involving either inactivation or overexpression of the gene encoding heterodisulfide reductase. We propose that the oxygen respiration for energy conservation that sustains the growth of the O₂ -evolved strains is associated with a rearrangement of metabolite fluxes, especially NAD⁺ /NADH, leading to an optimized O₂ reduction. These evolved strains are the first sulfate-reducing bacteria that exhibit a demonstrated oxygen respiratory process that enables growth.

Variation of bacterial biodiversity from saline soils and estuary sediments present near the Mediterranean Sea coast of Camargue (France)

Salinity is an important environmental factor influencing microbial community composition. To better understand this influence, we determined the bacterial communities present in 17 different sites of brackish sediment (underwater) and soil (surface) samples from the Camargue region (Rhône river delta) in southern France during the fall of 2013 and 2014 using pyrosequencing of the V3-V4 regions of the 16S rRNA genes amplified by PCR. This region is known for abundant flora and fauna and, though saline, 30% of rice consumed in France is grown here. We found that bacterial abundance in 1 g of soil or sediment, calculated by qPCR, was higher in sediments than in surface soil samples. Members belonging to the Proteobacteria, Bacteroidetes, Chloroflexi and Firmicutes phyla dominated the bacterial communities of sediment samples, while members belonging to the Proteobacteria, Bacteroidetes, Gemmatimonadetes, Actinobacteria, Firmicutes and Acidobacteria phyla dominated the bacterial communities of the soil samples. The most abundant bacterial genera present in the saline sediments and soils from the Camargue belonged mostly to halophilic and sulphate reducing bacteria, suggesting that the Camargue may be a valuable system to investigate saline, yet agriculturally productive, sediment and soil microbial ecosystem.

Use of specific PCR primers for the study of sulfate-reducing bacteria diversity in microbial mats of Ebro Delta, Spain

Microbial mats are prokaryotic communities that provide model systems to analyze microbial diversity and ecophysiological interactions. Sulfate-reducing bacteria (SRB) play a key role in sulfur and nutrient recycling in these ecosystems. In this work, specific primers for 16S rRNA encoding gene, previously described, were used to study the diversity of SRB in microbial mats of the Ebro Delta. We confirm that this method is reliable to identify the diversity of SRB in these ecosystems. However, some mismatches in obtained sequences had been observed in our system and must be taken under consideration. Various genera of SRB in Ebro Delta microbial mats were identified, such as Desulfonema, Desulfatitalea, Desulfosalsimonas, Desulfoccocus, and Desulfovibrio. The diversity observed in our samples is very similar to previously reported in other microbial mats communities.

Functional Stability and Community Dynamics during Spring and Autumn Seasons Over 3 Years in Camargue Microbial Mats

Microbial mats are complex biofilms in which the major element cycles are represented at a millimeter scale. In this study, community variability within microbial mats from the Camargue wetlands (Rhone Delta, southern France) were analyzed over 3 years during two different seasons (spring and autumn) and at different layers of the mat (0–2, 2–4, and 4–6 mm). To assess bacterial diversity in the mats, amplicons of the V1–V2 region of the 16S rRNA gene were sequenced. The community’s functionality was characterized using two approaches: (i) inferred functionality through 16S rRNA amplicons genes according to PICRUSt, and (ii) a shotgun metagenomic analysis. Based on the reads distinguished, microbial communities were dominated by Bacteria (∼94%), followed by Archaea (∼4%) and Eukarya (∼1%). The major phyla of Bacteria were Proteobacteria, Bacteroidetes, Spirochaetes, Actinobacteria, Firmicutes, and Cyanobacteria, which together represented 70–80% of the total population detected. The phylum Euryarchaeota represented ∼80% of the Archaea identified. These results showed that the total bacterial diversity from the Camargue microbial mats was not significantly affected by seasonal changes at the studied location; however, there were differences among layers, especially between the 0–2 mm layer and the other two layers. PICRUSt and shotgun metagenomic analyses revealed similar general biological processes in all samples analyzed, by season and depth, indicating that different layers were functionally stable, although some taxa changed during the spring and autumn seasons over the 3 years. Several gene families and pathways were tracked with the oxic-anoxic gradient of the layers. Genes directly involved in photosynthesis (KO, KEGG Orthology) were significantly more abundant in the top layer (0–2 mm) than in the lower layers (2–4 and 4–6 mm). In the anoxic layers, the presence of ferredoxins likely reflected the variation of redox reactions required for anaerobic respiration. Sulfatase genes had the highest relative abundance below 2 mm. Finally, chemotaxis signature genes peaked sharply at the oxic/photic and transitional oxic-anoxic boundary. This functional differentiation reflected the taxonomic diversity of the different layers of the mat.

Variation of Oxygenation Conditions on a Hydrocarbonoclastic Microbial Community Reveals Alcanivorax and Cycloclasticus Ecotypes

Deciphering the ecology of marine obligate hydrocarbonoclastic bacteria (MOHCB) is of crucial importance for understanding their success in occupying distinct niches in hydrocarbon-contaminated marine environments after oil spills. In marine coastal sediments, MOHCB are particularly subjected to extreme fluctuating conditions due to redox oscillations several times a day as a result of mechanical (tide, waves and currents) and biological (bioturbation) reworking of the sediment. The adaptation of MOHCB to the redox oscillations was investigated by an experimental ecology approach, subjecting a hydrocarbon-degrading microbial community to contrasting oxygenation regimes including permanent anoxic conditions, anoxic/oxic oscillations and permanent oxic conditions. The most ubiquitous MOHCB, Alcanivorax and Cycloclasticus, showed different behaviors, especially under anoxic/oxic oscillation conditions, which were more favorable for Alcanivorax than for Cycloclasticus. The micro-diversity of 16S rRNA gene transcripts from these genera revealed specific ecotypes for different oxygenation conditions and their dynamics. It is likely that such ecotypes allow the colonization of distinct ecological niches that may explain the success of Alcanivorax and Cycloclasticus in hydrocarbon-contaminated coastal sediments during oil-spills.

Distribution of Sulfate-Reducing Communities from Estuarine to Marine Bay Waters

Estuaries are highly dynamic ecosystems in which freshwater and seawater mix together. Depending on tide and river inflows, particles originating from rivers or from the remobilization of sediments accumulate in the water column. Due to the salinity gradient and the high heterotrophic activity in the estuarine plume, hypoxic and anoxic microniches may form in oxygenated waters, sustaining favorable conditions for resuspended anaerobic microorganisms. In this context, we tested the hypothesis that anaerobic sulfate-reducing prokaryotes may occur in the water column of the Adour River. Using 16S ribosomal RNA (rRNA) and dsrAB-based terminal restriction fragment length polymorphism (T-RFLP) techniques, we characterized total prokaryotic and sulfate-reducing communities along a gradient from estuarine to marine bay waters. Sulfate-reducing prokaryotes were further characterized by the description of dsrB genes and the cultivation of sulfidogenic anaerobic microorganisms. As a result, physical-chemical parameters had a significant effect on water bacterial diversity and community structure along the studied gradient. The concentration of cultured sulfidogenic microorganisms ranged from 1 to 60 × 10³ cells l^-1 in the water column. Sulfate-reducing prokaryotes occurring in estuarine waters were closely related to microorganisms previously detected in freshwater sediments, suggesting an estuarine origin, mainly by the remobilization of the sediments. In the marine bay station, sediment-derived sulfate-reducing prokaryotes were not cultured anymore, probably due to freshwater dilution, increasing salinity and extended oxic stress. Nevertheless, isolates related to the type strain Desulfovibrio oceani were cultured from the diluted plume and deep marine waters, indicating the occurrence of autochthonous sulfate-reducing bacteria offshore.

Exploring Actinobacteria assemblages in coastal marine sediments under contrasted Human influences in the West Istria Sea, Croatia

The exploration of marine Actinobacteria has as major challenge to answer basic questions of microbial ecology that, in turn, will provide useful information to exploit Actinobacteria metabolisms in biotechnological processes. The ecological functions performed by Actinobacteria in marine sediments are still unclear and belongs to the most burning basic questions. The comparison of Actinobacteria communities inhabiting marine sediments that are under the influence of different contamination types will provide valuable information in the adaptation capacities of Actinobacteria to colonize specific ecological niche. In the present study, the characterization of different Actinobacteria assemblages according to contamination type revealed the ecological importance of Actinobacteria for maintaining both general biogeochemical functions through a "core" Actinobacteria community and specific roles associated with the presence of contaminants. Indeed, the results allowed to distinguish Actinobacteria genera and species operational taxonomic units (OTUs) able to cope with the presence of either (i) As, (ii) metals Ni, Fe, V, Cr, and Mn, or (iii) polycyclic aromatic hydrocarbons (PAHs) and toxic metals (Hg, Cd, Cu, Pb, and Zn). Such observations highlighted the metabolic capacities of Actinobacteria and their potential that should be taken into consideration and advantage during the implementation of bioremediation processes in marine ecosystems.

Application of denaturing high-performance liquid chromatography for monitoring sulfate-reducing bacteria in oil fields

Sulfate-reducing bacteria (SRB) participate in microbially induced corrosion (MIC) of equipment and H2S-driven reservoir souring in oil field sites. Successful management of industrial processes requires methods that allow robust monitoring of microbial communities. This study investigated the applicability of denaturing high-performance liquid chromatography (DHPLC) targeting the dissimilatory sulfite reductase ß-subunit (dsrB) gene for monitoring SRB communities in oil field samples from the North Sea, the United States, and Brazil. Fifteen of the 28 screened samples gave a positive result in real-time PCR assays, containing 9 × 10(1) to 6 × 10(5) dsrB gene copies ml(-1). DHPLC and denaturing gradient gel electrophoresis (DGGE) community profiles of the PCR-positive samples shared an overall similarity; both methods revealed the same samples to have the lowest and highest diversity. The SRB communities were diverse, and different dsrB compositions were detected at different geographical locations. The identified dsrB gene sequences belonged to several phylogenetic groups, such as Desulfovibrio, Desulfococcus, Desulfomicrobium, Desulfobulbus, Desulfotignum, Desulfonatronovibrio, and Desulfonauticus. DHPLC showed an advantage over DGGE in that the community profiles were very reproducible from run to run, and the resolved gene fragments could be collected using an automated fraction collector and sequenced without a further purification step. DGGE, on the other hand, included casting of gradient gels, and several rounds of rerunning, excising, and reamplification of bands were needed for successful sequencing. In summary, DHPLC proved to be a suitable tool for routine monitoring of the diversity of SRB communities in oil field samples.

An oxygen reduction chain in the hyperthermophilic anaerobe Thermotoga maritima highlights horizontal gene transfer between Thermococcales and Thermotogales

The hyperthermophile Thermotoga maritima, although strictly anaerobic, is able to grow in the presence of low amounts of O(2). Here, we show that this bacterium consumes O(2) via a three-partner chain involving an NADH oxidoreductase (NRO), a rubredoxin (Rd) and a flavo-diiron protein (FprA) (locus tags: TM_0754, TM_0659 and TM_0755, respectively). In vitro experiments showed that the NADH-dependent O(2) consumption rate was 881.9 (± 106.7) mol O(2) consumed min(-1) per mol of FprA at 37°C and that water was the main end-product of the reaction. We propose that this O(2) reduction chain plays a central role in the O(2) tolerance of T. maritima. Phylogenetic analyses suggest that the genes coding for these three components were acquired by an ancestor of Thermotogales from an ancestor of Thermococcales via a single gene transfer. This event likely also involved two ROS scavenging enzymes (neelaredoxin and rubrerythrin) that are encoded by genes clustered with those coding for FprA, NRO and Rd in the ancestor of Thermococcales. Such genomic organization would have provided the ancestor of Thermotogales with a complete set of enzymes dedicated to O(2)-toxicity defence. Beside Thermotogales and Thermococcales, horizontal gene transfers have played a major role in disseminating these enzymes within the hyperthermophilic anaerobic prokaryotic communities, allowing them to cope with fluctuating oxidative conditions that exist in situ.

Intact phospholipid and quinone biomarkers to assess microbial diversity and redox state in microbial mats

Microbial mats are stratified microbial communities composed by highly inter-related populations and therefore are frequently chosen as model systems to study diversity and ecophysiological strategies. The present study describes an integrated approach to analyze microbial quinones and intact polar lipids (IPLs) in microbial mats within layers as thin as 500 microm by liquid chromatography-tandem mass spectrometry. Quinone profiles revealed important depth-related differences in community composition in two mat systems. The higher abundance of ubiquinones, compared to menaquinones, reflected the clear predominance of microorganisms belonging to aerobic alpha-, beta-, and gamma-Proteobacteria in Ebro delta estuarine mats. Hypersaline photosynthetic Camargue mats (France) showed a predominance of menaquinone-9 at the top of the mat, which is consistent with an important contribution of facultative aerobic or anaerobic bacteria in its photic zone. Quinone indices also indicated a higher diversity of non-phototrophs and a more anaerobic character in the hypersaline mats. Besides, the dissimilarity index suggested that the samples were greatly influenced by a depth-related redox state gradient. In the analysis of IPLs, there was a predominance of phosphatidylglycerols and sulfoquinovosyldiacylglycerols, the latter being an abundant biomarker of Cyanobacteria. This combined approach based on quinone and IPL analysis has proven to be a useful method to establish differences in the microbial diversity and redox state of highly structure microbial mat systems at a fine-scale level.

Extremely halophilic microbial communities in anaerobic sediments from a solar saltern

The prokaryotic communities inhabiting hypersaline sediments underlying a crystallizer pond of a Mediterranean solar saltern have been studied in a polyphasic approach including 16S rRNA and dsrAB gene libraries analysis [the last encoding for dissimilatory (bi)sulfite reductase], most probable number of cultivable counts, and metabolic measurements of sulfate reduction. The samples studied here represent one of the most hypersaline anoxic environments sampled worldwide that harbour a highly diverse microbial community different from those previously reported in other hypersaline sediments. Both bacterial and archaeal types are present but, contrarily to the overlying brine system, the former dominates. Molecular analyses indicated that the bacterial fraction is highly diverse and mostly composed by groups related to sulfate-reducing bacteria (SRB). In good agreement with this, sulfate-reducing activity was detected in the sediment, as well as the metabolic diversity within SRB (as indicated by the use of different electron donors in enrichments). On the other hand, the archaeal fraction was phylogenetically homogeneous and, surprisingly, strongly affiliated with the MBSl-1 candidate division, an euryarchaeotal group only reported in deep-sea hypersaline anoxic basins of the Western Mediterranean, for which a methanogenic metabolism was hypothesized. The hypersaline studied samples constitute a valuable source of new prokaryotic types with metabolisms adapted to the prevalent in situ extreme conditions.