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Mediterranean high-mountain lakes are being increasingly affected by strong Saharan dust deposition events. However, the ecological impacts of these severe atmospheric episodes remain largely unknown. We examined the effects of a strong Saharan dust intrusion to the Iberian Peninsula in 2022 on the physicochemical parameters and prokaryotic communities in sediments of nine high-mountain lakes of Sierra Nevada (Spain) located above 2800 m.a.s.l and in different orientations (north vs. south). A previous year (2021), with lower Saharan dust deposition with respect to 2022, was used for interannual comparisons. The strong dust deposition to the high-mountain lakes resulted in a significant increase in sediment nutrient availability which was linked to changes in the composition of prokaryotic communities. Decreases in alpha diversity and changes in beta diversity of prokaryotic communities were mainly observed in lakes located in the south compared to the north orientation likely because the former was more affected by the atmospheric dust deposition episode. Dust intrusion to the high-mountain lakes resulted in significant changes in the relative abundance of specific genera involved in important nutrient cycling processes such as phosphate solubilization, nitrogen fixation, nitrification, and denitrification. Saharan dust deposition also increased predicted microbial functionality in all lakes. Our findings show that severe atmospheric dust inputs to remote high-mountain lakes of Sierra Nevada can have significant biogeochemical and biodiversity consequences through changes in nutrient availability and prokaryotic communities in sediments of these freshwater ecosystems. This information contributes to understanding how Mediterranean high-mountain lakes of Sierra Nevada face strong intrusions of Saharan dust and their ecological consequences. [ABSTRACT FROM AUTHOR]

Legumes can contribute to emissions of the potent greenhouse gas nitrous oxide (N2O) directly by some rhizobia species that are able to denitrify under free-living conditions and in symbiotic association with the plant. In this study, the capacity of Phaseolus vulgaris-Rhizobium etli symbiosis to emit N2O in response to nitrate (NO3-) has been demonstrated for the first time. We found that bacteroidal assimilatory nitrate reductase (NarB) as well as nitrite reductase (NirK) and nitric oxide reductase (cNor) denitrifying enzymes contribute to nitric oxide (NO) and N2O formation in nodules. We also show that R. etli NarK is involved in NO2- extrusion and links NO3- reduction by NarB in the cytoplasm with NirK and cNor denitrification activities in the periplasm. The knowledge generated in this work will be instrumental for exploring strategies and sustainable practices in agricultural soil management to increase legume crop yield and mitigate greenhouse gas emissions. HIGHLIGHTS Common bean nodules are able to produce N2O in response to nitrate. Nitrate reduction to nitrite in R. etli bacteroids catalyzed by assimilatory nitrate reductase NarB is involved in N2O emissions from common bean nodules. R. etli respiratory nitrite reductase NirK and nitric oxide reductase cNor support N2O production in common bean nodules. R. etli NarB and cNor are involved in NO homeostasis in common bean nodules. R. etli nitrite exporter NarK links nitrate reduction in the cytoplasm with denitrification activities in the periplasm. [ABSTRACT FROM AUTHOR]

The Moroccan endemic Vachellia gummifera grows wild under extreme desert conditions. This plant could be used as an alternative fodder for goats, and camels, in order to protect the Argan forests against overgrazing in Central and Southwestern Moroccan semiarid areas. With the aim to improve the V. gummifera population's density in semiarid areas, we proposed its inoculation with performing plant growth-promoting bacteria. Hence, 500 bacteria were isolated from the plant rhizosphere. From these, 291 isolates were retained for plant growth-promoting (PGP) activities assessment. A total of 44 isolates showed the best phosphates solubilization potential, as well as siderophore and auxin production. The combination of REP–PCR (repetitive extragenic palindromic–polymerase chain reaction) fingerprinting, PGP activities, and phenotypic properties, allowed the selection of three strains for the inoculation experiments. The three selected strains' 16S rRNA sequencing showed that they are members of the Enterobacter and Pseudomonas genera. The inoculation with three strains had diverse effects on V. gummifera growth parameters. All single and combined inoculations improved the plant shoot weight by more than 200%, and the root length by up to 139%, while some combinations further improved protein and chlorophyll content, thereby improving the plant's forage value. The three selected strains constitute an effective inoculum for use in the arid and semiarid zones of southern Morocco. [ABSTRACT FROM AUTHOR]

Purpose: Alfalfa is one of the most valuable forage crops in temperate climate zones. Ensifer meliloti, the endosymbiont of alfalfa, contains all the denitrification genes but the capacity of alfalfa root nodules to produce nitrous oxide (N2O) is not known. In this work, N2O emissions as well as the influence of bacteroidal denitrification on nodulation competitiveness and N2O release from alfalfa nodules has been investigated. Methods: Medicago sativa cv. Victoria plants were inoculated with E. meliloti 1021, a periplasmic nitrate reductase (Nap) defective mutant, a Nap overexpressing strain and a nitrous oxide reductase defective mutant. Plants were grown in the presence of different nitrate and copper treatments and subjected to flooding during one week before harvesting. N2O production by the nodules was analysed by using gas chromatography. Methyl viologen-dependent nitrate reductase (MV+-NR), nitrite reductase (MV+-NIR) and nitrous oxide reductase (N2OR) enzymatic activities were measured in isolated bacteroids. Results: Alfalfa root nodules produce N2O in response to nitrate and flooding. Overexpression of Nap improved nodulation competitiveness and induced N2O emissions from nodules. Copper is required for an effective symbiosis as well as triggered a reduction of N2O production due to the induction of the N2OR and a reduction of NIR activities in the bacteroids. Conclusion: Alfalfa root nodules emit N2O. Nap is involved in nodulation competitiveness and in N2O emissions by the nodules. Bacteroidal N2OR and NIR activities are modulated by Cu and may be considered as effective targets for the mitigation strategies of N2O emissions derived from alfalfa crops. [ABSTRACT FROM AUTHOR]

Lagoon systems are often confined, and their waters are poorly renewed, which makes them vulnerable to pollutants' accumulation. Here, the impact of different sources of anthropogenic contamination (domestic, urban, industrial, and agricultural) on the nitrate (NO3−) content, emission of the greenhouse gas nitrous oxide (N2O), abundance of total bacterial archaeal, nitrifying, and denitrifying communities, and diversity and composition of bacterial communities in the sediments of the RAMSAR-protected Marchica lagoon (Nador, Morocco) was investigated. Six lake sites differing in NO3− concentration were selected. Wastewater coming from industrial activities results in the greatest concentration of NO3− in sediments and emissions of N2O. Increased carbon to nitrogen content in sites near domestic activities resulted in an increase in the abundance of total bacterial and archaeal communities, as well as nitrification and denitrification genes, but low N2O emissions due to a greater presence of microorganisms involved in N2O production over those able to reduce N2O. Significant differences in bacterial community composition between sites were observed, with the NO3− content being the main driver of these changes. Increased NO3− content in the sampling sites significantly reduced bacterial diversity. Bacterial genera involved in the degradation of organic and inorganic pollutants and nitrous oxide reduction, such as Robiginitalea, Symbiobacterium, Bacillus, Fusibacter, Neptunomonas, Colwellia, and Alteromonas, were the most abundant in the lagoon. The results suggest that the type of anthropogenic contamination directly impacts the nitrate content in the sediments of the Marchica lagoon, which determines variations in nitrous oxide emissions, nitrogen-cycling gene abundances, and bacterial diversity. [ABSTRACT FROM AUTHOR]

The soybean endosymbiont Bradyrhizobium diazoefficiens harbours the complete denitrification pathway that is catalysed by a periplasmic nitrate reductase (Nap), a copper (Cu)-containing nitrite reductase (NirK), a c -type nitric oxide reductase (cNor), and a nitrous oxide reductase (Nos), encoded by the napEDABC, nirK, norCBQD , and nosRZDFYLX genes, respectively. Induction of denitrification genes requires low oxygen and nitric oxide, both signals integrated into a complex regulatory network comprised by two interconnected cascades, FixLJ–FixK2–NnrR and RegSR–NifA. Copper is a cofactor of NirK and Nos, but it has also a role in denitrification gene expression and protein synthesis. In fact, Cu limitation triggers a substantial down-regulation of nirK, norCBQD , and nosRZDFYLX gene expression under denitrifying conditions. Bradyrhizobium diazoefficiens genome possesses a gene predicted to encode a Cu-responsive repressor of the CsoR family, which is located adjacent to copA , a gene encoding a putative Cu+-ATPase transporter. To investigate the role of CsoR in the control of denitrification gene expression in response to Cu, a csoR deletion mutant was constructed in this work. Mutation of csoR did not affect the capacity of B. diazoefficiens to grow under denitrifying conditions. However, by using qRT-PCR analyses, we showed that nirK and norCBQD expression was much lower in the csoR mutant compared to wild-type levels under Cu-limiting denitrifying conditions. On the contrary, copA expression was significantly increased in the csoR mutant. The results obtained suggest that CsoR acts as a repressor of copA. Under Cu limitation, CsoR has also an indirect role in the expression of nirK and norCBQD genes. [ABSTRACT FROM AUTHOR]

FixK2 is a CRP/FNR-type transcription factor that plays a central role in a sophisticated regulatory network for the anoxic, microoxic and symbiotic lifestyles of the soybean endosymbiont Bradyrhizobium diazoefficiens. Aside from the balanced expression of the fixK2 gene under microoxic conditions (induced by the two-component regulatory system FixLJ and negatively auto-repressed), FixK2 activity is posttranslationally controlled by proteolysis, and by the oxidation of a singular cysteine residue (C183) near its DNA-binding domain. To simulate the permanent oxidation of FixK2, we replaced C183 for aspartic acid. Purified C183D FixK2 protein showed both low DNA binding and in vitro transcriptional activation from the promoter of the fixNOQP operon, required for respiration under symbiosis. However, in a B. diazoefficiens strain coding for C183D FixK2, expression of a fixNOQP'-'lacZ fusion was similar to that in the wild type, when both strains were grown microoxically. The C183D FixK2 encoding strain also showed a wild-type phenotype in symbiosis with soybeans, and increased fixK2 gene expression levels and FixK2 protein abundance in cells. These two latter observations, together with the global transcriptional profile of the microoxically cultured C183D FixK2 encoding strain, suggest the existence of a finely tuned regulatory strategy to counterbalance the oxidation-mediated inactivation of FixK2 in vivo. [ABSTRACT FROM AUTHOR]