The five strains, designated S4, S5, S6, S9, and S10, were selected from a previously established collection of culturable bacteria sampled from Abu Dhabi’s salt flats. Whole-genome sequencing allowed the team to place each isolate on firm taxonomic ground: strains S4 and S5 belong to Bacillus paralicheniformis, S6 to Bacillus safensis, and S9 and S10 to Bacillus haynesii. Genome-wide average nucleotide identity values, ranging from 96.2 to 99.4 percent against their closest type strains, exceeded the standard threshold used to delineate bacterial species, and phylogenomic trees built from 119 conserved marker genes confirmed the assignments. The genomes themselves were high quality, with completeness values above 98.8 percent and contamination below 1.1 percent, giving the researchers a reliable blueprint of each strain’s functional potential.

That blueprint proved rich. Bioinformatic pipelines including Prokka, BlastKOALA, antiSMASH, and the plant growth-promotion predictor PGPT-Pred revealed thousands of genes tied to plant-beneficial traits: indole-3-acetic acid metabolism, nitrogen fixation, ACC deaminase activity, siderophore-mediated iron acquisition, biofilm formation, and exopolysaccharide production. The strains also carried predicted biosynthetic gene clusters for secondary metabolites such as fengycin, bacillibactin, lichenysin, bacilysin, and bacitracin, along with stress-response genes for drought, heat, cold, and heavy metal tolerance. Strain S9 stood out with the largest repertoire of functional protein clusters, harboring 5,064 clusters of orthologous groups, more than any of its four relatives.

Genomic potential, however, is only a prediction, so the team put the bacteria through a battery of laboratory stress tests. Cultures were grown in media adjusted with polyethylene glycol to mimic non-stress, moderate, and severe drought, corresponding to water potentials of 0.0, -0.2, and -1.0 megapascals. All five strains maintained growth, biofilm formation, and exopolysaccharide production under osmotic pressure, but their performance diverged sharply. Bacillus haynesii S10 was the clear laboratory champion, sustaining the highest optical density, the thickest biofilms, the most exopolysaccharide, the strongest IAA production at 95.75 micrograms per milliliter, the highest ACC deaminase activity, and the greatest siderophore output even at the harshest water potential. Strains S4 and S5 performed moderately, while S6 and S9 lagged in most in vitro assays.

The real test came in the greenhouse, where tomato seedlings of the commercial cultivar SV4224TH were inoculated with each strain by root-zone drenching and then subjected to four weeks of well-watered, moderate drought, or severe drought conditions, with soil moisture held gravimetrically at 90, 50, or 30 percent relative water content. The results revealed a striking strain-by-train patchwork. Under severe drought, S10 increased shoot length by 69 percent and shoot dry weight by 52 percent relative to uninoculated drought-stressed plants, while boosting leaf number by 85 percent and node number by 121 percent. S9, despite its weaker laboratory showing, delivered the largest root length gain of 76 percent and doubled root dry weight under severe drought. Under moderate drought, the benefits split further: S10 drove shoot elongation, S6 promoted root elongation with a 49.6 percent increase, and S9 accumulated biomass, raising root dry weight by 71.8 percent.

Physiological measurements told a coherent story about why inoculated plants fared better. Using a leaf spectrometer, the researchers tracked a suite of vegetation and pigment indices, including SPAD, the Greenness Index, the photochemical reflectance index, the water band index, NDVI, and the Zarco-Tejada and Miller Index. Drought depressed all of them, but inoculation with S9 and S10 blunted the decline. S10 raised SPAD by 46.4 percent and PRI by 77.3 percent under moderate drought, and lifted the Greenness Index by 60.7 percent and the water band index by 71.8 percent under severe drought. Its NDVI response was even more dramatic, up 70 percent under moderate and 81 percent under severe drought. Crucially, these optical gains coincided with genuine water-status improvements: S10 maintained relative water content roughly 22 percent higher than drought controls and cut electrolyte leakage, a marker of membrane damage, by more than 70 percent under both drought levels.

The biochemical layer of the response was equally coordinated. Drought-stressed tomato plants inoculated with S10 showed elevated activities of the antioxidant enzymes superoxide dismutase, peroxidase, and ascorbate peroxidase, with increases of 16.5, 56.5, and 46.5 percent respectively under moderate drought, and 21.8, 19.8, and 25.4 percent under severe drought. Proline, an osmoprotectant that helps cells retain water and stabilize proteins, rose by 48.1 percent under moderate drought and 37.6 percent under severe drought with S10 treatment. S9 again followed closely behind. Together, the data point to a mechanism in which the bacteria help plants maintain redox balance, adjust osmotically, and preserve membrane integrity, rather than exerting a single isolated effect.

At the molecular level, the team used quantitative real-time PCR to track four drought-responsive genes that represent complementary arms of the tomato stress network. SlABF4 encodes an abscisic acid-responsive transcription factor central to stress signaling; ASR1 is a well-characterized component of osmotic and water-stress responses whose overexpression improves survival during water deficit; SlDEAD31, a DEAD-box RNA helicase, has been shown to reduce water loss and improve drought resistance when overexpressed; and SlMPK3 is a mitogen-activated protein kinase linked to drought tolerance. Inoculated plants, particularly those treated with S9 and S10, showed markedly stronger induction of all four genes under drought, with SlABF4 expression peaking in S10-treated plants under severe stress and SlMPK3 responding most strongly to the combination of S10 and severe drought. These transcriptional shifts lined up neatly with the physiological and biochemical data, suggesting the bacteria help activate the plant’s own stress-adaptation programs.

Perhaps the study’s most consequential lesson is what it says about how we screen for beneficial microbes. The two best performers in the greenhouse told opposite stories in the laboratory: S10 excelled in nearly every in vitro assay, while S9, mediocre on the lab bench, delivered some of the largest in-planta effects, including a 101 percent increase in root dry weight under severe drought. Species identity was no better a predictor, since the two B. haynesii strains diverged substantially despite overlapping genomic repertoires. The authors conclude that individual laboratory traits and genomic predictions alone cannot reliably forecast how a bacterium will perform inside a living plant, and that robust inoculant selection demands integrating bacterial phenotype, genomic potential, and plant-level response data.

The road from greenhouse to field remains long. The authors caution that their conclusions rest on controlled conditions and that field validation across heterogeneous soils and climates is essential before these strains can be deployed as commercial biofertilizers. Key open questions include how persistently S9 and S10 colonize the tomato rhizosphere, how stable they remain in formulated products, and whether their benefits extend across other tomato cultivars and crops. Still, the idea that microbes forged in one of Earth’s most hostile environments can teach a staple crop to endure its increasingly thirsty future is a compelling one, and it positions sabkha-derived Bacillus strains as promising candidates in the growing toolkit of climate-resilient agriculture.

Subject of Research: Plant growth-promoting Bacillus strains from hypersaline sabkha soils enhancing tomato drought tolerance

Article Title: Plant growth-promoting Bacillus strains enhance tomato drought tolerance through physiological, antioxidant, and stress-responsive gene expression

Article References: Ali, Q., Ali, M., Khan, T. A., Sadaiappan, B., & Mundra, S. (2026). Plant growth-promoting Bacillus strains enhance tomato drought tolerance through physiological, antioxidant, and stress-responsive gene expression. Journal of Agriculture and Food Research, Article 103341. https://doi.org/10.1016/j.jafr.2026.103341

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103341

Keywords: Bacillus, drought tolerance, tomato, plant growth-promoting bacteria, sabkha, genomics, antioxidant enzymes, gene expression, bioinoculants, sustainable agriculture, exopolysaccharides, water stress

Sabkhas form under arid to hyper-arid climates where evaporation persistently exceeds rainfall and groundwater lies close enough to the surface for capillary rise to deliver moisture into the evaporation zone. The review distinguishes two dominant types in the UAE. Coastal sabkhas, exemplified by the Abu Dhabi shoreline, sit within supratidal to marginal intertidal zones and are sustained by episodic marine flooding, tidal pumping, lagoonal exchange and saline groundwater circulation. Inland sabkhas, such as the vast Sabkha Matti that extends roughly 150 kilometres from western Abu Dhabi into Saudi Arabia, occupy continental depressions and interdune corridors where groundwater discharge and intense evaporation dominate, with little direct marine input. Both settings share the same fundamental process chain: capillary ascent, evaporative concentration, evaporite crystallization and early cementation, yet their differing hydrologic connectivity produces distinct brine chemistries and mineral assemblages.

The geological setting of the UAE gives these systems unusual scientific richness. The Abu Dhabi coastal plain is an exceptionally low-gradient carbonate ramp where offshore shoals, barrier islands and restricted lagoons dampen wave energy and allow broad tidal flats to prograde seaward. Facies belts trace an orderly progression from subtidal carbonate shoals through lagoons and tidal channels into a shoreline-parallel belt of polygonal microbial mats and finally onto supratidal sabkha plains. Because these belts are preserved as vertically stacked Holocene packages, geologists can walk laterally across modern environments and read the same succession downward in cores. This direct process-to-product linkage is why the Abu Dhabi sabkhas became the type analogue for ancient carbonate–evaporite reservoir–seal systems across the Arabian Plate, including the Upper Jurassic Arab Formation.

The Mussafah Channel, a man-made canal cut through the coastal plain, has proven especially instructive. Its walls expose a well-constrained Holocene succession in which Pleistocene aeolian deposits are overlain by lagoonal and tidal-channel carbonates, microbial mat facies and supratidal sediments overprinted by evaporite mineralization. The stacking records a post-glacial marine transgression followed by regression and progressive sabkha development, providing a high-resolution template for reconstructing facies architecture and early diagenesis in subsurface reservoirs. Quantitative comparisons between modern surfaces and shallow subsurface successions have shown that many environmentally diagnostic structures, from microbial polygons to halite crust morphologies, retain recognizable signatures during early burial, strengthening confidence in facies-based analog applications.

One of the most celebrated contributions of the UAE sabkhas lies in addressing the long-standing Dolomite Problem: the puzzle of how abundant dolomite forms at low temperatures when laboratory experiments struggle to precipitate it. Cryo-scanning electron microscopy has revealed authigenic dolomite crystals nucleating within the extracellular polymeric substances of Abu Dhabi microbial mats, and laboratory studies with sulfate-reducing bacteria isolated from the sabkha have reproduced poorly ordered Ca-dolomite and very high-Mg calcite under simulated pore-water conditions. These findings establish microbial mats as geochemical micro-reactors that locally modify pH, alkalinity and magnesium-to-calcium ratios, demonstrating that low-temperature dolomitization in evaporitic settings is fundamentally microbially mediated rather than a simple consequence of evaporation.

The hydrogeochemistry of the coastal sabkha is equally counterintuitive. Water-budget and mass-balance analyses show that most water entering the shallow sabkha aquifer is meteoric, derived from infrequent but efficiently infiltrating rainfall, whereas more than 95 percent of the dissolved solutes ascend from continental brines in underlying Tertiary formations. This decoupling between water sources and solute sources explains why marine-like evaporite assemblages develop even where modern marine flooding is sporadic. The characteristic mineral sequence of carbonate, gypsum and anhydrite, then halite, reflects recurrent concentration–reset cycles driven by evaporation, flooding and brine–sediment interaction rather than a single linear evaporation path. Density contrasts between concentrated near-surface brines and dilute underlying groundwater even drive free convection, first documented at field scale in the Abu Dhabi aquifer, redistributing solutes and shaping cementation patterns.

The review also brings radiogeochemistry into the framework. Natural radioelement distributions prove strongly facies-dependent: potassium-40 and thorium-232 track detrital inputs of K-bearing silicates, clays and heavy minerals inherited from desert dunes and the ophiolite-dominated Hajar Mountains, while uranium behaves far more dynamically, migrating as uranyl–carbonate complexes in oxidizing alkaline brines and immobilizing in reducing microbial mat microenvironments. Radium, by contrast, can be trapped through substitution into the abundant gypsum and anhydrite that pervade sabkha soils, making sulfate horizons potential radiogeochemical sinks. The authors argue that robust environmental baselines therefore require sampling stratified by facies and depth rather than treating sabkhas as uniform salt flats.

On the question of critical elements, particularly lithium, the synthesis is deliberately cautious. Lithium remains in solution through early evaporation and concentrates mainly in late-stage, bittern-like residual brines enriched in magnesium, potassium, bromine and boron. Whether UAE sabkhas reach that stage depends on hydrologic restriction, residence time and flushing frequency, and the reviewers note that facies-resolved lithium datasets for UAE sabkhas are effectively absent from the open literature. Regional benchmarks, including lithium concentrations of roughly 0.2 to 0.3 milligrams per litre measured in Arabian Gulf desalination brines, and the persistent technical obstacle posed by high magnesium-to-lithium ratios in direct extraction technologies, all counsel against assuming inherent high-grade enrichment. The authors instead propose a staged screening strategy: vertical porewater profiling across representative facies, quantification of lithium alongside its chemical companions, seasonal repetition to capture evaporation and flooding cycles, and integration with techno-economic and environmental assessments before any resource claims are made.

Finally, the review elevates the sabkhas to geoheritage priorities. Abu Dhabi Sabkha is listed on UNESCO’s World Heritage Tentative List, recognized for the completeness of its roughly 7,000-year-old system, where subtidal lagoon muds, intertidal microbial mats and supratidal gypsum and anhydrite nodules co-occur within a single locality. Yet the scientific archive is disappearing fast: studies estimate that of about 150 kilometres of coastal sabkha present in the 1960s, only around 36 percent remained by the early 2010s, lost to industrial and urban encroachment. The reviewers call for protected reference zones, controlled access and integration of research with public interpretation, framing the sabkhas not as wasteland but as irreplaceable outdoor laboratories. From reservoir calibration and lithium screening to radon baselines and geotechnical risk in sulfate-cemented soils, they conclude, the fate of these reactive evaporitic systems now matters far beyond the shorelines of the southern Gulf.

Subject of Research: Integrated sedimentology, hydrogeochemistry, geomicrobiology and resource potential of UAE sabkha carbonate–evaporite systems

Article Title: Sabkha systems of the United Arab Emirates as integrated carbonate–evaporite laboratories: facies architecture, brine evolution, critical-element potential, and geoheritage significance

Article References: Abukhadra, M. R., Al-zharani, M., Allam, A. A., Hamdan, M. A., Szűcs, P., & Eid, M. H. (2026). Sabkha systems of the United Arab Emirates as integrated carbonate–evaporite laboratories: facies architecture, brine evolution, critical-element potential, and geoheritage significance. Environmental Earth Sciences, 85(16), Article 411. https://doi.org/10.1007/s12665-026-13146-2

Image Credits: AI Generated

DOI: 10.1007/s12665-026-13146-2

Keywords: sabkha, United Arab Emirates, evaporites, carbonate sedimentology, brine geochemistry, dolomite, microbial mats, lithium, radioactivity, Abu Dhabi, geoheritage, reservoir analogue