Physical, chemical, and microbial feedbacks controlling brine geochemistry and lake morphology in polyextreme salar environments

Aragonite crystallization in a sulfate-rich hypersaline wetland under dry Mediterranean climate (Laguna Honda, eastern Guadalquivir basin, S Spain)

This research investigates the influence of water composition, the presence of seasonal algal mats, detrital inputs and the activity of microorganisms on the crystallization of aragonite in the sediments deposited in the hypersaline Laguna Honda wetland (S of Spain). The high alkaline and hypersaline waters (pH > 9.2 and C.E. > 70 mS/cm) of the wetland lake are rich in SO42- (>24,000 mg/l), Cl- (>21,000 mg/l), Na+ (>11,000 mg/l) Mg2+ (>8400 mg/l) and Ca2+ (>1000 mg/l), and are supersaturated for dolomite, calcite and aragonite. Sediments have lower pH values than column waters, oscillating from 8.54 in the low Eh (up to -80.9 mV) central deep sediments and 6.33 in the shallower higher Eh (around -13.6 mV) shore sediments. Erosion of the surrounding olive groves soils produced detrital silicates rich sediments with concretions of carbonate or sulfate. Aragonite (up to 19 %) and pyrite (up to 13 %) are mainly concentrated in the organic matter rich samples from the upper part of the sediment cores, whereas gypsum is preferably concentrated in low organic matter content samples. Mineral crusts containing a MgAl silicate phase, epsomite, halite and gypsum are precipitated on the floating algal mats covering the wetland waters. Floating algal mats deposit increased the organic matter content of the upper sediments which promoted the presence of fermentative microorganisms, sulfate-reducing bacteria (SRB) and sulfur-oxidizing bacteria (SOB) communities and variations of Eh that influence the authigenesis of carbonate and S-bearing minerals. Replacement of poorly crystalline MgSi phases infilling algal cells by aragonite was favored in the organic matter rich sediments with low Eh values and important SRB communities that promoted sulfate bioreduction processes to form pyrite. Aragonite precipitation was favored by the increase of carbonate and bicarbonate concentration produced by the SRB oxidation of organic matter, the CO2 degassing by high summer temperatures and the CO2 uptake by photosynthesis of the algal mats.

Identification of Playa Lakes and tracking their evolution pathways using geochemical models in the Great Indian Thar desert

Playa Lakes of the great Indian Thar desert are unique and intricate systems with pronounced scientific and ecological significance. In this study, the combined use of geospatial and field data assisted in depicting and understanding the changes within these natural systems. The purpose of this study is to provide a comprehensive dataset of Playa Lakes, which can help with an overall understanding of playas from a geochemical, ecological, and economic perspective. The 1163 surface depressions were accounted for as possible playas considering the threshold area of >5 km2. A total of 42 representative surface water samples were collected from the selected playas (Sambhar, Didwana, Pachpadra, and Pokhran) and hydrochemical analysis was carried out to identify the geochemical evolution of these playas. The major water types were Na-Cl and Na-Cl-SO4 type. Hardie and Eugster's model was used to explain the possible sequence of evolutionary pathways and brine shifts in the selected playas. Simulated evaporation modeling suggests precipitation of major evaporites (chlorides, carbonates, and sulphates) during progressive evaporation. Surface sediment analysis (X-ray diffraction) confirmed the presence of calcite, halite, and dolomite minerals in the playa sediments. The greatest economic value of these playas may be mineral production and providing food and habitat for migratory birds, though conservation and policy intervention is required for their sustainable utilization.

Compositional Changes in Sediment Microbiota Are Associated with Seasonal Variation of the Water Column in High-Altitude Hyperarid Andean Lake Systems

The lacustrine systems of La Brava and La Punta, located in the Tilopozo sector in the extreme south of Salar de Atacama, are pristine high-altitude Andean lakes found along the central Andes of South America. This shallow ecosystem suffers from permanent evaporation, leading to falling water levels, causing it to recede or disappear during the dry season. This dynamic causes physicochemical changes in lakes, such as low nutrient availability, pH change, and dissolved metals, which can influence the composition of the microbial community. In this study, we used a metataxonomic approach (16S rRNA hypervariable regions V3 to V4) to characterize the sedimentary microbiota of these lakes. To understand how the water column affects and is structured in the microbiota of these lakes, we combined the analysis of the persistence of the water column through satellite images and physicochemical characterization. Our results show a significant difference in abiotic factors and microbiota composition between La Punta and La Brava lakes. In addition, microbiota analysis revealed compositional changes in the ecological disaggregation (main and isolated bodies) and antagonistic changes in the abundance of certain taxa between lakes. These findings are an invaluable resource for understanding the microbiological diversity of high Andean lakes using a multidisciplinary approach that evaluates the microbiota behavior in response to abiotic factors. IMPORTANCE In this study, we analyzed the persistence of the water column through satellite images and physicochemical characterization to investigate the composition and diversity in High Andean Lake Systems in a hyperarid environment. In addition to the persistence of the water column, this approach can be used to analyze changes in the morphology of saline accumulations and persistence of snow or ice; for example, for establishing variable plant cover over time and evaluating the microbiota associated with soils with seasonal changes in plants. This makes it an ideal approach to search for novel extremophilic microorganisms with unique properties. In our case, it was used to study microorganisms capable of resisting desiccation and water restriction for a considerable period and adapting to survive in ecological niches, such as those with high UV irradiation, extreme drought, and high salt concentration.

Environmental and Biological Controls on Sedimentary Bottom Types in the Puquios of the Salar de Llamara, Northern Chile

The Puquios of the Salar de Llamara in the Atacama Desert, northern Chile, is a system of small lakes that is characterized by evaporitic mineral deposition and that commonly hosts microbial communities. This region is known for its extreme aridity, solar irradiance, and temperature fluctuations. The Puquios are a highly diverse ecosystem with a variety of sedimentary bottom types. Our previous results identified electrical conductivity (EC) as a first-order environmental control on bottom types. In the present paper, we extend our analysis to examine the effects of additional environmental parameters on bottom types and to consider reasons for the importance of EC as a control of sedimentology. Our results identify microbially produced extracellular polymeric substances (EPS) as a major player in the determination of bottom types. The relative amounts and properties of EPS are determined by EC. EPS, in turn, determines the consistency of bottom types, exchange of bottom substrate with the overlying water column, and mineral precipitation within the substrate. Low-EC ponds in the Puquios system have flocculent to semi-cohesive bottom types, with low-viscosity EPS that allows for high-exchange with the surrounding waters and mineral precipitation of granular gypsum, carbonate, and Mg–Si clay in close association with microbes. Ponds with elevated EC have bottom types that are laminated and highly cohesive with high-viscosity EPS that restricts the exchange between sediments and the surrounding waters; mineral precipitation in these high-EC ponds includes granular to laminated gypsum, carbonate and Mg–Si, which also form in close association with microbes. Bottom types in ponds with EC above the threshold for thriving benthic microbial communities have insufficient EPS accumulations to affect mineral precipitation, and the dominant mineral is gypsum (selenite). The variations in EPS production throughout the Puquios, associated with heterogeneity in environmental conditions, make the Puquios region an ideal location for understanding the controls of sedimentary bottom types in evaporative extreme environments that may be similar to those that existed on early Earth and beyond.