Ecological aspects and relationships of the emblematic Vachellia spp. exposed to anthropic pressures and parasitism in natural hyper-arid ecosystems: ethnobotanical elements, morphology, and biological nitrogen fixation.

MAIN CONCLUSION: Emblematic Vachellia spp. naturally exposed to hyper-arid conditions, intensive grazing, and parasitism maintain a high nitrogen content and functional mutualistic nitrogen-fixing symbioses. AlUla region in Saudi Arabia has a rich history regarding mankind, local wildlife, and fertility islands suitable for leguminous species, such as the emblematic Vachellia spp. desert trees. In this region, we investigated the characteristics of desert legumes in two nature reserves (Sharaan and Madakhil), at one archaeological site (Hegra), and in open public domains et al. Ward and Jabal Abu Oud. Biological nitrogen fixation (BNF), isotopes, and N and C contents were investigated through multiple lenses, including parasitism, plant tissues, species identification, plant maturity, health status, and plant growth. The average BNF rates of 19 Vachellia gerrardii and 21 Vachellia tortilis trees were respectively 39 and 67%, with low signs of inner N content fluctuations (2.10-2.63% N) compared to other co-occurring plants. The BNF of 23 R. raetam was just as high, with an average of 65% and steady inner N contents of 2.25 ± 0.30%. Regarding parasitism, infected Vachellia trees were unfazed compared to uninfected trees, thereby challenging the commonly accepted detrimental role of parasites. Overall, these results suggest that Vachellia trees and R. raetam shrubs exploit BNF in hyper-arid environments to maintain a high N content when exposed to parasitism and grazing. These findings underline the pivotal role of plant-bacteria mutualistic symbioses in desert environments. All ecological traits and relationships mentioned are further arguments in favor of these legumes serving as keystone species for ecological restoration and agro-silvo-pastoralism in the AlUla region.

Understanding changes in biocrust communities following phosphate mining in the Negev Desert.

Biocrusts are key ecosystem engineers that are being destroyed due to anthropogenic disturbances such as trampling, agriculture and mining. In hyper-arid regions of the Negev Desert, phosphate has been mined for over six decades, altering the natural landscape over large spatial scales. In recent years, restoration-oriented practices were mandated in mining sites, however, the impact of such practices on the ecosystem, particularly the biocrust layer, has not been tested. Here, we evaluated post-mining biocrust bacterial communities and compared them to undisturbed (reference) biocrusts. We collected samples from four mining sites (each restored at a different year) and their corresponding reference sites. We hypothesized that post-mining bacterial communities would differ significantly from reference communities, given the slow regeneration of the biocrust. We also hypothesized that bacterial communities would vary among post-mining plots based on their restoration age. To test these hypotheses, we assessed the abundance and diversity of bacterial communities by sequencing the 16S rDNA and their photosynthetic potential by quantifying the abundance of cyanobacteria and chlorophyll a. The bacterial diversity was lower, and community composition differed significantly between post-mining and reference biocrusts. In addition, cyanobacteria abundances and chlorophyll a content were lower in post-mining biocrusts, indicating lower photosynthetic potential. However, no significant changes in bacterial communities were detected, regardless of the restoration age. We suggest that the practices implemented in the Negev mines may not support the recovery of the biocrust bacterial communities, particularly the cyanobacteria. Thus, active restoration measures are needed to accelerate the regeneration time of biocrusts at the hyper-arid Negev mines.

Dark, rare and inspirational microbial matter in the extremobiosphere: 16 000 m of bioprospecting campaigns.

The rationale of our bioprospecting campaigns is that the extremobiosphere, particularly the deep sea and hyper-arid deserts, harbours undiscovered biodiversity that is likely to express novel chemistry and biocatalysts thereby providing opportunities for therapeutic drug and industrial process development. We have focused on actinobacteria because of their frequent role as keystone species in soil ecosystems and their unrivalled track record as a source of bioactive compounds. Population numbers and diversity of actinobacteria in the extremobiosphere are traditionally considered to be low, although they often comprise the dominant bacterial biota. Recent metagenomic evaluation of 'the uncultured microbial majority' has now revealed enormous taxonomic diversity among 'dark' and 'rare' actinobacteria in samples as diverse as sediments from the depths of the Mariana Trench and soils from the heights of the Central Andes. The application of innovative culture and screening options that emphasize rigorous dereplication at each stage of the analysis, and strain prioritization to identify 'gifted' organisms, have been deployed to detect and characterize bioactive hit compounds and sought-after catalysts from this hitherto untapped resource. The rewards include first-in-a-class chemical entities with novel modes of action, as well as a growing microbial seed bank that represents a potentially enormous source of biotechnological and therapeutic innovation.

Plant species shape the bacterial communities on the phyllosphere in a hyper-arid desert.

Microorganisms are an important component of global biodiversity. However, they are vulnerable to hyper-arid climates in desert regions. Xerophytes are desert vegetation with unique biodiversity. However, little is known about the identities and communities of phyllosphere epiphytic microorganisms inhabiting the xerophyte leaf surface in the hot and dry environment. The diversity and community composition of phyllosphere epiphytes on different desert plants in Gansu, China, was investigated using the next-generation sequencing technique, revealing the diversity and community composition of the phyllosphere epiphytic bacteria associated with desert xerophytes. In addition, the ecological functions of the bacterial communities were investigated by combining the sequence classification information and prokaryotic taxonomic function annotation (FAPROTAX). This study determined the phyllosphere bacterial community composition, microbial interactions, and their functions. Despite harsh environments in the arid desert, we found that there are still diverse epiphytic bacteria on the leaves of desert plants. The bacterial communities mainly included Actinobacteria (52.79%), Firmicutes (31.62%), and Proteobacteria (12.20%). Further comparisons revealed different microbial communities, including Firmicutes at the phylum and Paenibacillaceae at the family level, in the phyllosphere among different plants, suggesting that the host plants had strong filter effects on bacteria. Co-occurrence network analysis revealed positive relationships were dominant among different bacterial taxa. The abundance of Actinobacteria and Proteobacteria was positively correlated, demonstrating their mutual relationship. On the other hand, the abundance of Firmicutes was negatively correlated, which suggested that they inhibit the growth of other bacterial taxa. FAPROTAX prediction revealed that chemoheterotrophy (accounting for 39.02% of the community) and aerobic chemoheterotrophy (37.01%) were the main functions of the leaf epiphytic bacteria on desert plants. This study improves our understanding of the community composition and ecological functions of plant-associated microbial communities inhabiting scattered niches in the desert ecosystem. In addition, the study provides insight into the biodiversity assessment in the desert region.

The origin of solutes in groundwater in a hyper-arid environment: A chemical and multi-isotope approach in the Atacama Desert, Chile.

The major ion and the multi-isotopic composition (87Sr/86Sr, δ11B, δ34S(SO4) and δ18O(SO4)) of groundwater from the Central Depression in northern Chile is investigated to identify the origin of groundwater solutes in the hyper-arid core of the Atacama Desert. The study area is between the Cordillera de Domeyko and the Central Depression, at latitudes 24-25°S, and is characterized by near-zero air moisture conditions, rare precipitation and very limited runoff. Groundwater composition varies from Ca-HCO3 to Ca, Na-SO4 type below elevations of 3400 m a.s.l. The rCl/rBr ratio of meteoric waters and groundwater overlap, but significantly increase in the aquifer as salinity goes up due to evapoconcentration far from the Domeyko Cordillera. The wind-displaced dust originating in the Central Depression (87Sr/86Sr: 0.706558-0.710645; δ34S(SO4): 0 to +4‰) affects the precipitation composition in the highest parts of the Domeyko Cordillera (87Sr/86Sr: 0.706746-0.709511; δ34S(SO4): +1 to +6‰), whose δ34S(SO4) and δ11B values are greatly different from marine aerosols, discarding its contribution to dust at this distance inland. Sr and S isotopic values in groundwater indicate a strong relation with three main geological units: i) Paleozoic rocks contribute high radiogenic strontium isotope ratios to groundwater (0.707011-0.714862), while sulphate isotopic composition is probably acquired from atmospheric dust (>- 1.4‰), ii) Jurassic marine limestones contribute low-radiogenic strontium isotopic ratios to groundwater (<0.70784), while sulphate can be related to oxidized sulphides that change the isotopic signatures of sulphur (<-1.2‰), and iii) mixed salts in the Atacama Gravels contribute lower radiogenic strontium isotopic ratios and sulphate to groundwater (87Sr/86Sr: <0.707324; δ34S(SO4): +0.1 to +7.7). These three processes reflect water-rock interactions. The δ11B of groundwater generally up to +13‰, does not increase along the regional groundwater flow path, discarding fractionation by interaction with clays. These results improve the understanding of the groundwater evolution in hyper-arid systems through a new conceptual model.