Historical Change and Ecological Risk of Potentially Toxic Elements in the Lake Sediments from North Aral Sea, Central Asia

Viral Community Structure and Potential Functions in the Dried-Out Aral Sea Basin Change along a Desiccation Gradient

The dried-out Aral Sea basin represents an extreme environment due to a man-made ecological disaster. Studies conducted in this unique environment revealed high levels of pollution and a specifically adapted microbiota; however, viral populations remained entirely unexplored. By employing an in-depth analysis based on the sequencing of metagenomic DNA recovered from rhizosphere samples of Suaeda acuminata (C. A. Mey.) Moq. along a desiccation gradient of 5, 10, and 40 years, we detected a diverse viral community comprising 674 viral populations (viral operational taxonomic units [vOTUs]) dominated by Caudovirales. Targeted analyses highlighted that viral populations in this habitat are subjected to certain dynamics that are driven mainly by the gradient of desiccation, the corresponding salinity, and the rhizosphere bacterial populations. In silico predictions linked the viruses to dominant prokaryotic taxa in the Aral Sea basin, such as Gammaproteobacteria, Actinomycetia, and Bacilli. The lysogenic lifestyle was predicted to be predominant in areas that dried out 5 years ago, representing the early revegetation phase. Metabolic prediction of viral auxiliary metabolic genes (AMGs) suggests that viruses may play a role in the biogeochemical cycles, stress resilience, and competitiveness of their hosts due to the presence of genes that are involved in biofilm formation. Overall, our study provides important insights into viral ecology in an extreme environment and expands our knowledge related to virus occurrence in terrestrial systems. IMPORTANCE Environmental viruses have added a wealth of knowledge to ecological studies with the emergence of metagenomic technology and approaches. They are also becoming recognized as important genetic repositories that underpin the functioning of terrestrial ecosystems but have remain moslty unexplored. Using shotgun metagenome sequencing and bioinformatic tools, we found that the viral community structure was affected during natural revegetation in the dried-up Aral Sea area, a model habitat for investigating natural ecological restoration but still understudied. In this study, we highlight the importance of viruses, elements that are overlooked, for their potential contribution to terrestrial ecosystems, i.e., nutrient cycles, stress resilience, and host competitiveness, during natural revegetation.

Function-Based Rhizosphere Assembly along a Gradient of Desiccation in the Former Aral Sea

The desiccation of the Aral Sea represents one of the largest human-made environmental regional disasters. The salt- and toxin-enriched dried-out basin provides a natural laboratory for studying ecosystem functioning and rhizosphere assembly under extreme anthropogenic conditions. Here, we investigated the prokaryotic rhizosphere communities of the native pioneer plant Suaeda acuminata (C.A.Mey.) Moq. in comparison to bulk soil across a gradient of desiccation (5, 10, and 40 years) by metagenome and amplicon sequencing combined with quantitative PCR (qPCR) analyses. The rhizosphere effect was evident due to significantly higher bacterial abundances but less diversity in the rhizosphere compared to bulk soil. Interestingly, in the highest salinity (5 years of desiccation), rhizosphere functions were mainly provided by archaeal communities. Along the desiccation gradient, we observed a significant change in the rhizosphere microbiota, which was reflected by (i) a decreasing archaeon-bacterium ratio, (ii) replacement of halophilic archaea by specific plant-associated bacteria, i.e., Alphaproteobacteria and Actinobacteria, and (iii) an adaptation of specific, potentially plant-beneficial biosynthetic pathways. In general, both bacteria and archaea were found to be involved in carbon cycling and fixation, as well as methane and nitrogen metabolism. Analysis of metagenome-assembled genomes (MAGs) showed specific signatures for production of osmoprotectants, assimilatory nitrate reduction, and transport system induction. Our results provide evidence that rhizosphere assembly by cofiltering specific taxa with distinct traits is a mechanism which allows plants to thrive under extreme conditions. Overall, our findings highlight a function-based rhizosphere assembly, the importance of plant-microbe interactions in salinated soils, and their exploitation potential for ecosystem restoration approaches. IMPORTANCE The desertification of the Aral Sea basin in Uzbekistan and Kazakhstan represents one of the most serious anthropogenic environmental disasters of the last century. Since the 1960s, the world's fourth-largest inland body of water has been constantly shrinking, which has resulted in an extreme increase of salinity accompanied by accumulation of many hazardous and carcinogenic substances, as well as heavy metals, in the dried-out basin. Here, we investigated bacterial and archaeal communities in the rhizosphere of pioneer plants by combining classic molecular methods with amplicon sequencing as well as metagenomics for functional insights. By implementing a desiccation gradient, we observed (i) remarkable differences in the archaeon-bacterium ratio of plant rhizosphere samples, (ii) replacement of archaeal indicator taxa during succession, and (iii) the presence of specific, potentially plant-beneficial biosynthetic pathways in archaea present during the early stages. In addition, our results provide hitherto-undescribed insights into the functional redundancy between plant-associated archaea and bacteria.

Probabilistic risk assessment of soil contamination related to agricultural and industrial activities

Soil contamination related to industrial and agricultural activities were associated with many adverse health effects and climate change could exacerbate this effect. However, the evidence on this topic in low and middle-income countries (LMICs) is still scarce. We, therefore, aimed to investigate the heavy metals (HMs) concentrations in topsoil around Lake Urmia (which dried about 50 % of it in recent years). Accordingly, 96 surface soil samples were collected from an area of about 4000 km² in 2019. An inductively coupled plasma-atomic emission spectrometry (ICP-AES) was used to measure the HMs concentrations, including cadmium (Cd), arsenic (As), chromium (Cr), cobalt (Co), nickel (Ni), copper (Cu), lead (Pb) and zinc (Zn). Health risk assessment of exposure to HMs was performed using Monte Carlo simulations technique. The mean concentrations of Zn, Cu, Ni, Co, Pb, Cr, As and Cd were 68.66, 35, 31.66, 15, 14.4, 11.2, 7.04, and 0.26 mg/kg, respectively, which was in the allowable range of USEPA guideline. The mean enrichment factor (EF) values for Zn, Cd, Cu, Cr, Ni, Pb, Co and As were1.3, 1.9, 1, 1.2, 1.7, 2.8, 1.8 and 2, respectively. Carcinogenic risks of exposure to HMs for inhalation, dermal and ingestion exposure pathways were at safe level. Similarly, the hazard index (HI) was at safe level (HI < 1). The sensitivity analysis indicated that the exposure duration (ED) for As, exposure frequency (EF) for Cd and Ni, concentration (C_soi) for Cr and Pb had the highest impact on ELCR values. Our finding confirmed that HMs concentrations around Lake Urmia had no health risk for inhabitants.

A review of water pollution arising from agriculture and mining activities in Central Asia: Facts, causes and effects

Central Asia is one of many regions worldwide that face severe water shortages; nevertheless, water pollution in this region exacerbates the existing water stress and increases the risk of regional water conflicts. In this study, we perform an extensive literature review, and the data show that water pollution in Central Asia is closely linked to human activities. Within the Asian Gold Belt, water pollution is influenced mainly by mining, and the predominant pollutants are heavy metals and radionuclides. However, in the irrigated areas along the middle and lower reaches of inland rivers (e.g., the Amu Darya and Syr Darya), water pollution is strongly associated with agriculture. Hence, irrigated areas are characterized by high concentrations of ammonia, nitrogen, and phosphorus. In addition, the salinities of rivers and groundwater in the middle and lower reaches of inland rivers generally increase along the flow path due to high rates of evaporation. Soil salinization and frequent salt dust storms in the Aral Sea basin further increase the pollution of surface water bodies. Ultimately, the pollution of surface water and groundwater poses risks to human health and deteriorates the ecological environment. To prevent further water pollution, joint monitoring of the surface water and groundwater quantity and quality throughout Central Asia must be implemented immediately.