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Arthrospira platensis was collected from Lake Bayannur and adjacent alkaline lakes located in the Mo Us Sandy Land in the Erdos Plateau in China. After subjecting the samples to temperatures of 5°C, 0°C, and -5°C for 5 days, algal morphology and ultrastructure were observed. There were minimal differences in the morphological and anatomical structures of A. platensis incubated at 5°C and 0°C; at -5°C, however, significant variations were seen, including a significant decrease in the number of intact trichomes due to breakage, thinner cell walls, disappearance of phycobilisomes, reduced clarity of thykaloid membranes, an increase in carboxysome volume, a reduction in lipid body size, and an increase in the number of polyglucan granules. Additionally, longitudinal cell walls became wave-shaped while cross walls became arc-shaped. Erdos Plateau A. platensis appears to be a typical low-temperature Arthrospira strain because of its similar response at 5°C and 0°C but divergent response at - 5 °C. These results could offer valuable insights into the management of criticality at low temperatures in industrial cultivation of the Erdos lakes' strain of A. platensis. [ABSTRACT FROM AUTHOR]

Alkaline lakes are thought to have facilitated prebiotic synthesis reactions on the early Earth because their modern analogs accumulate vital chemical feedstocks such as phosphate through the evaporation of dilute groundwaters. Yet, the conditions required for some building block synthesis reactions are distinct from others, and these conditions are generally incompatible with those permissible for nascent cellular function. However, because current scenarios for prebiotic synthesis have not taken account of the physical processes that drive the chemical evolution of alkaline lakes, the potential for the co-occurrence of both prebiotic synthesis and the origins and early evolution of life in prebiotic alkaline lake environments remains poorly constrained. Here, we investigate the dynamics of active, prebiotically relevant alkaline lakes using near-surface geophysics, aqueous geochemistry, and hydrogeologic modeling. Due to their small size, representative range of chemistry, and contrasting evaporation behavior, the investigated, neighboring Last Chance and Goodenough Lakes in British Columbia, Canada offer a uniquely tractable environment for investigating the dynamics of alkaline lake behavior. The results show that the required, extreme phosphate enrichments in alkaline lake waters demand geomorphologically-driven vulnerability to evaporation, while the resultant contrast between evaporated brines and inflowing groundwaters yields Rayleigh–Taylor instabilities and vigorous surface–subsurface cycling and mixing of lake and groundwaters. These results provide a quantitative basis to reconcile conflicting prebiotic requirements of UV light, salinity, metal concentration, and pH in alkaline lake environments. The complex physical and chemical processing inherent to prebiotic alkaline lake environments thus may have not only facilitated prebiotic reaction networks, but also provided habitable environments for the earliest evolution of life. [ABSTRACT FROM AUTHOR]

Chloroviruses exhibit a close relationship with their hosts with the phenotypic aspect of their ability to form lytic plaques having primarily guided the taxonomy. However, with the isolation of viruses that are only able to complete their replication cycle in one strain of Chlorella variabilis, systematic challenges emerged. In this study, we described the genomic features of 53 new chlorovirus isolates and used them to elucidate part of the evolutionary history and taxonomy of this clade. Our analysis revealed new chloroviruses with the largest genomes to date (>400 kbp) and indicated that four genomic features are statistically different in the viruses that only infect the Syngen 2-3 strain of C. variabilis (OSy viruses). We found large regions of dissimilarity in the genomes of viruses PBCV-1 and OSy-NE5 when compared with the other genomes. These regions contained genes related to the interaction with the host cell machinery and viral capsid proteins, which provided insights into the evolution of the replicative and structural modules in these giant viruses. Phylogenetic analysis using hallmark genes of Nucleocytoviricota revealed that OSy-viruses evolved from the NC64A-viruses, possibly emerging as a result of the strict relationship with their hosts. Merging phylogenetics and nucleotide identity analyses, we propose strategies to demarcate viral species, resulting in seven new species of chloroviruses. Collectively, our results show how genomic data can be used as lines of evidence to demarcate viral species. Using the chloroviruses as a case study, we expect that similar initiatives will emerge using the basis exhibited here. IMPORTANCE Chloroviruses are a group of giant viruses with long dsDNA genomes that infect different species of Chlorella-like green algae. They are host-specific, and some isolates can only replicate within a single strain of Chlorella variabilis. The genomics of these viruses is still poorly explored, and the characterization of new isolates provides important data on their genetic diversity and evolution. In this work, we describe 53 new chlorovirus genomes, including many isolated from alkaline lakes for the first time. Through comparative genomics and molecular phylogeny, we provide evidence of genomic gigantism in chloroviruses and show that a subset of viruses became highly specific for their hosts at a particular point in evolutionary history. We propose criteria to demarcate species of chloroviruses, paving the way for an update in the taxonomy of other groups of viruses. This study is a new and important piece in the complex puzzle of giant algal viruses. [ABSTRACT FROM AUTHOR]

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Alkaline lakes with high pH and unique ecological communities often face water-level drawdown and ecological degradation problems due to climatic and hydrologic factors. Water transfer is becoming a popular method for solving these problems. However, a high pH is often considered the key to maintaining the stability of alkaliphilic algal communities, and a lower pH induced by water transfer from a neutral-pH river may threaten ecosystems in alkaline lakes. To explore the response characteristics of phytoplankton in alkaline lakes to pH changes, we conducted cultivation experiments on one species of dominant Cyanobacteria and one species of dominant Chlorophyta from alkaline lakes under different pH conditions. Subsequently, we constructed a coupled hydrodynamic and algal mathematical model considering the effect of pH to predict the dynamic changes in phytoplankton in a typical alkaline lake under water-transfer conditions. Both species are basophilic, and pH has a "low-inhibition and high-promotion" effect on their growth. A lower pH is detrimental to cyanobacterial growth and competitiveness, which may cause Cyanobacteria to lose their dominance in weakly alkaline environments with a pH < 8.5; additionally, water transfer causes a decrease in the total biomass and proportion of Cyanobacteria in Lake Chenghai, with decreases induced by pH changes accounting for 13.4% and 70.1%, respectively. The decrease in pH is the main reason for the decrease in dominance of Cyanobacteria after water transfer. These results provide a basic summary of the effects of pH changes on the algal growth in alkaline lakes and are a useful for formulating ecological water-transfer strategies for alkaline lakes. • Experiments and simulations were combined to study the response law of phytoplankton growth in alkaline lakes to pH changes. • Constructed a coupled hydrodynamic–algal ecological model considering the impact of pH on algal growth. • Water transfer projects decrease the risk of algal blooms and the dominance of Cyanobacteria in Lake Chenghai. • A lower pH is the main reason for the decrease in dominance of Cyanobacteria after water transfer. • Cyanobacteria dominate alkaline environments due to their basophilism and strong competitive ability. [ABSTRACT FROM AUTHOR]

Much of Earth's carbon resides in the "deep" realms of our planet: sediments, crust, mantle, and core. The interaction of these deep reservoirs of carbon with the surface reservoir (atmosphere and oceans) leads to a habitable surface environment, with an equitable atmospheric composition and comfortable range in temperature that together have allowed life to proliferate. The Earth in Five Reactions project (part of the Deep Carbon Observatory program) identified the most important carbon-bearing reactions of our planet, defined as those which perhaps make our planet unique among those in our Solar System, to highlight and review how the deep and surface carbon cycles connect. Here we review the important reactions that control the concentration of carbon dioxide in our atmosphere: outgassing from magmas during volcanic eruptions and during magmatic activity; and uptake of CO2 by alkaline surface waters. We describe the state of our knowledge about these reactions and their controls, the extent to which we understand the mass budgets of carbon that are mediated by these reactions, and finally, the implications of these reactions for understanding present-day climate change that is driven by anthropogenic emission of CO2. [ABSTRACT FROM AUTHOR]

Cyanide plays a critical role in origin of life hypotheses that have received strong experimental support from cyanide-driven synthesis of amino acids, nucleotides, and lipid precursors. However, relatively high cyanide concentrations are needed. Such cyanide could have been supplied by reaction networks in which hydrogen cyanide in early Earth's atmosphere reacted with iron to form ferrocyanide salts, followed by thermal decomposition of ferrocyanide salts to cyanide. Using an aqueous model supported by new experimental data, we show that sodium ferrocyanide salts precipitate in closed-basin, alkaline lakes over a wide range of plausible early Earth conditions. Such lakes were likely common on the early Earth because of chemical weathering of mafic or ultramafic rocks and evaporative concentration. Subsequent thermal decomposition of sedimentary sodium ferrocyanide yields sodium cyanide (NaCN), which dissolves in water to form NaCN-rich solutions. Thus, geochemical considerations newly identify a particular geological setting and NaCN feedstock nucleophile for prebiotic chemistry. [ABSTRACT FROM AUTHOR]

In this study, the genetic relationships of 804 tarek (Alburnus tarichi) samples from a total of 18 populations, including the potamodromus and resident individuals from Lake Van basin in eastern Turkey, were studied by using nine microsatellite loci. A total of 93 alleles was detected, and the average number of alleles per locus was 10.3 ± 3.39. The mean estimated observed and expected heterozygosity were 0.340 ± 0.016 and 0.362 ± 0.015, respectively, which indicated a low level of polymorphism. After Bonferroni correction (P < 0.0027), the multi-locus test applied to each population revealed that 12 out of 18 populations were in Hardy-Weinberg equilibrium (HWE) (P = 0.0120–0.9981). Analysis of molecular variance (AMOVA) showed more than 76% genetic variability within individuals and 19% among populations, which was significantly higher than zero (FST = 0.19), and furthermore, a low level of genetic variation was observed among individuals within populations (4.84%: FIS = 0.06). Bayesian clustering analysis indicated that the total genetic variation grouped into 3 clusters. Additionally, the significance test results revealed that 11 of the 18 populations are threatened with extinction due to recent bottleneck events.We conclude that the tarek populations from the Lake Van basin can be classified into distinct genetic groups, based on microsatellite information. In addition, our results provide essential information for the development of a management plan that conserves the tarek's genetic diversity and achieves a sustainable fishery.

Clone libraries and morphological analysis were used to investigate cyanobacterial diversity in the cyanobacterial mat and dry crust at the bottom of the shallow, saline, alkaline Lake Khilganta (Southern Siberia, Russia). Filamentous cyanobacteria belonging to Phormidium genus and Coleofasciculus chthonoplastes were found to predominate during the dry period (2006) and the wet periods (1995 and 2012), respectively. Community composition during the dry and wet periods differed significantly. While 11 operational taxonomic units of cyanobacteria were revealed, only 3 occurred during both dry and wet periods. Occurrence of cosmopolitan C. chthonoplastes, which is common in neutral saline environments, is not typical of a continental alkaline lake and may be explained by the similarity of the dominant ions composition in lake water and in seawater. [ABSTRACT FROM AUTHOR]

Nhecolândia is a vast sub-region of the Pantanal wetland in Brazil with great diversity in surface water chemistry evolving in a sodic alkaline pathway under the influence of evaporation. In this region, > 15,000 shallow lakes are likely to contribute an enormous quantity of greenhouse gas to the atmosphere, but the diversity of the biogeochemical scenarios and their variability in time and space is a major challenge to estimate the regional contribution. From 4 selected alkaline lakes, we compiled measurements of the physico-chemical characteristics of water and sediments, gas fluxes in floating chambers, and sedimentation rates to illustrate this diversity. Although these lakes have a similar chemical composition, the results confirm a difference between the black-water and green-water alkaline lakes, corresponding to distinct biogeochemical functioning. This difference does not appear to affect lake sedimentation rates, but is reflected in gas emissions. Black-water lakes are CO 2 and CH 4 sources, with fairly constant emissions throughout the seasons. Annual carbon dioxide and methane emissions approach 0.86 mol m − 2 y − 1 and 0.07 mol m − 2 y − 1 , respectively, and no clear trend towards N 2 O capture or emission was observed. By contrast, green-water lakes are CO 2 and N 2 O sinks but important CH 4 sources with fluxes varying significantly throughout the seasons, depending on the magnitude of the phytoplankton bloom. The results highlight important daily and seasonal variations in gas fluxes, and in particular a hot moments for methane emissions, when the O 2 -supersaturation is reached during the afternoon under extreme bloom and sunny weather conditions, provoking an abrupt O 2 purging of the lakes. Taking into account the seasonal variability, annual methane emissions are around 10.2 mol m − 2 y − 1 , i.e., much higher than reported in previous studies for alkaline lakes in Nhecolândia. Carbon dioxide and nitrous oxide consumption is estimated about 1.9 mol m − 2 y − 1 and 0.73 mmol m − 2 y − 1 , respectively. However, these balances must be better constrained with systematic and targeted measurements throughout the seasons. [ABSTRACT FROM AUTHOR]

• A series of short lanostanes are first identified in alkaline lacustrine sediments. • The carbon number of lanostanes is expanded to include C 23. • The short chain lanostanes and long chain lanostanes have common biogenic precursors. • Cyanobacteria may be a possible biological source of lanostanes. Highly mature organic-rich source rocks in the Early Permian Fengcheng Formation, Mahu Sag, Junggar Basin, northwestern China, were deposited in an alkaline lacustrine environment. Analysis of extracts of eight core samples by gas chromatography–mass spectrometry detected a series of short-chain lanostanes (C 23 –C 25) lanostanes. The relative abundance of short-chain lanostanes has a strong positive correlation with those of long chain (C 30 –C 32) homologues, indicating a possible common biological origin and genesis. Compound specific isotopic analyses of phytane, steranes, hopanes and C 30 lanostane from the core extracts reflect input from primary photosynthetic producers including cyanobacteria. Abundant lamalginite was observed in the samples from the study area. A well-defined positive relationship is also evident between the relative abundance of lanostanes and the ratios of 7-+8-methyl heptadecanes/ C max and 2-methyl hopanes/C 30 -hopanes. Given that 7-+8-methyl heptadecanes and 2-methyl hopanes are considered to be diagnostic molecular biomarkers for organic matter derived from cyanobacteria, it is possible that cyanobacteria may serve as a possible biological precursor for lanostanes in the Fengcheng Formation. The influence of maturity on the lanostanes is considered to be minor, as no significant relationship is found between lanostanes and thermal maturity parameters, although this may be due to a limited maturity range of the samples. The co-existence of high contents of phytane, β-carotanes, and gammacerane, and their correlations in abundance with lanostanes, indicates a strongly reducing sedimentary environment. [ABSTRACT FROM AUTHOR]

Soda lakes have high levels of sodium carbonates and are characterized by salinity and elevated pH. These ecosystems are found across Africa, Europe, Asia, Australia, North, Central, and South America. Particularly in Brazil, the Pantanal region has a series of hundreds of shallow soda lakes (ca. 600) potentially colonized by a diverse haloalkaliphilic microbial community. Biological information of these systems is still elusive, in particular data on the description of the main taxa involved in the biogeochemical cycling of life-important elements. Here, we used metagenomic sequencing to contrast the composition and functional patterns of the microbial communities of two distinct soda lakes from the sub-region Nhecolândia, state of Mato Grosso do Sul, Brazil. These two lakes differ by permanent cyanobacterial blooms (Salina Verde, green-water lake) and by no record of cyanobacterial blooms (Salina Preta, black-water lake). The dominant bacterial species in the Salina Verde bloom was Anabaenopsis elenkinii. This cyanobacterium altered local abiotic parameters such as pH, turbidity, and dissolved oxygen and consequently the overall structure of the microbial community. In Salina Preta, the microbial community had a more structured taxonomic profile. Therefore, the distribution of metabolic functions in Salina Preta community encompassed a large number of taxa, whereas, in Salina Verde, the functional potential was restrained across a specific set of taxa. Distinct signatures in the abundance of genes associated with the cycling of carbon, nitrogen, and sulfur were found. Interestingly, genes linked to arsenic resistance metabolism were present at higher abundance in Salina Verde and they were associated with the cyanobacterial bloom. Collectively, this study advances fundamental knowledge on the composition and genetic potential of microbial communities inhabiting tropical soda lakes.

Despite their clear economic significance, Cretaceous presalt carbonates of the South Atlantic continental margins are not well-described by published facies models. This knowledge gap arises, in part, because the chemical processes that generate distinctive sedimentary products in alkaline, non-marine environments are poorly understood. Here, we use constraints inferred from reported mineralogical and geochemical features of presalt carbonate rocks to design and perform a suite of laboratory experiments to quantify the processes of alkaline chemical sedimentation. Using real-time observations of in-situ fluid chemistry, post-experiment analysis of precipitated solids, and geochemical modeling tools, we illustrate that spherulitic carbonates and Mg-silicate clays observed in presalt carbonates were likely precipitated from elevated pH (∼10–10.5) waters with high concentrations of silica and alkali cations typical of intermediate to felsic rocks, such as Na + and K + . Charge balance constraints require that these cations were not counterbalanced to any significant degree by anions typical of seawater, such as Cl − and SO 4 −− , which implies minimal seawater involvement in presalt deposition. Experimental data suggest that, at this alkaline pH, only modest concentrations (i.e., ∼0.5–1 mmol/kg) of Ca ++ would have been required to precipitate spheroidal CaCO 3 . Given the rapid rates of CaCO 3 nucleation and growth under such conditions, it is unlikely that Ca ++ concentrations in lake waters ever exceeded these values, and sustained chemical fluxes are therefore required for extensive sediment accumulation. Moreover, our experiments indicate that the original mineralogy of presalt CaCO 3 could have been calcite or aragonite, but the differing time scales of precipitation between CaCO 3 and Mg-silicates would have tended to skew the Mg/Ca ratio in solution towards elevated values which favor aragonite. Mg-silicate nucleation and growth rates measured during our experiments suggest that elevated SiO 2 (aq) and high pH would have limited (to 1–2 mmol/kg) the Mg ++ concentrations required to precipitate poorly crystalline Mg-silicates, which, through time, crystallize to minerals such as sepiolite and stevensite. Although our results provide robust constraints on the geochemistry of Mg-silicate-carbonate interactions during alkaline lake sedimentation, they leave open the potential for biological contributions to sedimentation within the presalt basins, as well as the hydrogeochemical mechanisms that maintained a productive carbonate factory of the scale observed along the South Atlantic margins. [ABSTRACT FROM AUTHOR]

Soda lakes have high levels of sodium carbonates and are characterized by salinity and elevated pH. These ecosystems are found across Africa, Europe, Asia, Australia, North, Central, and South America. Particularly in Brazil, the Pantanal region has a series of hundreds of shallow soda lakes (ca. 600) potentially colonized by a diverse haloalkaliphilic microbial community. Biological information of these systems is still elusive, in particular data on the description of the main taxa involved in the biogeochemical cycling of life-important elements. Here, we used metagenomic sequencing to contrast the composition and functional patterns of the microbial communities of two distinct soda lakes from the sub-region Nhecolândia, state of Mato Grosso do Sul, Brazil. These two lakes differ by permanent cyanobacterial blooms (Salina Verde, green-water lake) and by no record of cyanobacterial blooms (Salina Preta, black-water lake). The dominant bacterial species in the Salina Verde bloom was Anabaenopsis elenkinii. This cyanobacterium altered local abiotic parameters such as pH, turbidity, and dissolved oxygen and consequently the overall structure of the microbial community. In Salina Preta, the microbial community had a more structured taxonomic profile. Therefore, the distribution of metabolic functions in Salina Preta community encompassed a large number of taxa, whereas, in Salina Verde, the functional potential was restrained across a specific set of taxa. Distinct signatures in the abundance of genes associated with the cycling of carbon, nitrogen, and sulfur were found. Interestingly, genes linked to arsenic resistance metabolism were present at higher abundance in Salina Verde and they were associated with the cyanobacterial bloom. Collectively, this study advances fundamental knowledge on the composition and genetic potential of microbial communities inhabiting tropical soda lakes. [ABSTRACT FROM AUTHOR]

Thick Oligocene-Miocene lacustrine limestones of the Lake Mead region in southern Nevada have few adequate modern analogues that can shed light on their origin. We contend that the Lower Pahranagat Lake (LPAH), a spring-fed, alkaline lake in east-central Nevada, provides such an analogue. Through sediment cores, sampling of the lake margin, and characterization of sediment physical and chemical properties, we show that the LPAH shares a number of lithofacies in common with the Late Oligocene to Early Miocene Rainbow Gardens (RGF) and Miocene Horse Spring Formations (HSF) in the Lake Mead area. These include: stratiform stromatolites, domal stromatolites, reed beds, and intermittently laminated and massive limestones. Oxygen and carbon isotopes from authigenic carbonates support our interpretation in that the LPAH values strongly overlap those of the Oligocene-Miocene units. As a result, we interpret the Oligocene-Miocene RGF and HSF lacustrine carbonates as having been deposited in restricted, alkaline lake basins with high salinity. These basins were not conducive to the ecological success of animals and other, large multicellular organisms and, instead, fostered the growth of significant microbial communities. Lake margins, particularly during RGF deposition, were inhabited by reedy plants and salt tolerant shore grass much as is encountered in the LPAH today. These lakes were likely sourced by springs, but were nonetheless highly evaporative. Some unique aspects of both the LPAH and the Lake Mead region Oligocene-Miocene strata may shed light on the rarity of thick lacustrine carbonates in the rock record. Both of these systems are underlain by thick, Paleozoic carbonate sequences that host the principal aquifers that feed the lakes. [ABSTRACT FROM AUTHOR]

This study provides the first characterization of the variability of bromine and chlorine stable isotopic composition in evaporites, associated residual brines, and shoreline gases in terrestrial evaporative saline lakes. The lakes investigated here are groundwater discharge locations, and include both potash-rich alkaline lakes and sodic-dominated neutral pH lakes at a variety of salinities and evaporative stages. The chlorine and bromine isotope systems behave consistently different during evaporative salt precipitation, with37Cl more enriched in the salt than in the fluid, but81Br more enriched in the fluid compared with the precipitated salt. The81Br concentration of shore off-gassing was even smaller than mineral precipitate compositions. The trends observed for bromine isotopes are surprising compared with published laboratory studies, indicating that a process besides inorganic mineral precipitation is affectingδ81Br. Additional processes explored include diffusion, salt deflation, microbial and photolytic conversion to the gas phase, and oxidative bromination of organic matter. Dedicated to Professor Peter Fritz on the occasion of his 80th birthday [ABSTRACT FROM PUBLISHER]

Cyanotoxins are secondary metabolites produced by cyanobacteria, of varied chemical nature and toxic effects. Although cyanobacteria thrive in all kinds of ecosystems on Earth even under very harsh conditions, current knowledge on cyanotoxin distribution is almost restricted to freshwaters from temperate latitudes. In this review, we bring to the forefront the presence of cyanotoxins in extreme environments. Cyanotoxins have been reported especially in polar deserts (both from the Arctic and Antarctica) and alkaline lakes, but also in hot deserts, hypersaline environments, and hot springs. Cyanotoxins detected in these ecosystems include neurotoxins-anatoxin-a, anatoxin-a (S), paralytic shellfish toxins, β-methylaminopropionic acid, N-(2-aminoethyl) glycine and 2,4-diaminobutyric acid- and hepatotoxins -cylindrospermopsins, microcystins and nodularins-with microcystins being the most frequently reported. Toxin production there has been linked to at least eleven cyanobacterial genera yet only three of these (Arthrospira, Synechococcus and Oscillatoria) have been confirmed as producers in culture. Beyond a comprehensive analysis of cyanotoxin presence in each of the extreme environments, this review also identifies the main knowledge gaps to overcome (e.g., scarcity of isolates and -omics data, among others) toward an initial assessment of ecological and human health risks in these amazing ecosystems developing at the very edge of life. [ABSTRACT FROM AUTHOR]

The Pantanal of Nhecolândia, the world's largest and most diversified field of tropical lakes, comprises approximately 10,000 lakes, which cover an area of 24,000 km² and vary greatly in salinity, pH, alkalinity, colour, physiography and biological activity. The hyposaline lakes have variable pHs, low alkalinity, macrophytes and low phytoplankton densities. The saline lakes have pHs above 9 or 10, high alkalinity, a high density of phytoplankton and sand beaches. The cause of the diversity of these lakes has been an open question, which we have addressed in our research. Here we propose a hybrid process, both geochemical and biological, as the main cause, including (1) a climate with an important water deficit and poverty in Ca2+ in both superficial and phreatic waters; and (2) an elevation of pH during cyanobacteria blooms. These two aspects destabilise the general tendency of Earth's surface waters towards a neutral pH. This imbalance results in an increase in the pH and dissolution of previously precipitated amorphous silica and quartzose sand. During extreme droughts, amorphous silica precipitates in the inter-granular spaces of the lake bottom sediment, increasing the isolation of the lake from the phreatic level. This paper discusses this biogeochemical problem in the light of physicochemical, chemical, altimetric and phytoplankton data.O Pantanal da Nhecolândia é o maior e mais diversificado campo de lagos da região tropical do planeta, com cerca de 10.000 lagos de variadas salinidade, pH, alcalinidade, cor, fisiografia e atividade biológica dispostos em uma área de 24.000 km². Os lagos hipossalinos têm pH variável, baixa alcalinidade, macrófitas e baixa densidade de fitoplâncton. Os lagos salinos tem pH acima de 9 ou 10, elevada alcalinidade, alta densidade de fitoplâncton e praias de areia. A causa da diversidade desses lagos é uma questão ainda em aberto que é abordada nesta pesquisa. Propõe-se como principal causa um processo híbrido, geoquímico e biológico, baseado em (1) clima com um déficit hídrico importante e pobreza em Ca2+na água superficial e do freático e (2) na elevação do pH durante florações de cianobactérias. Estes dois aspectos desestabilizam a tendência geral de pH neutro para as águas superficiais da Terra. Este desequilíbrio resulta em aumento do pH e dissolução da areia quartzosa do fundo dos lagos salino-alcalinos. Durante secas extremas há precipitação de sílica amorfa nos espaços inter-granulares dos sedimentos de fundo destes lagos, aumentando seu isolamento do freático. O artigo discute este processo biogeoquímico, à luz de dados físico-químicos, químicos, fitoplânctonicos e de altimetria de precisão.

A haloalkaline anaerobic microbial community obtained from soda lake sediments was used to inoculate anaerobic reactors for the production of methane rich biogas. The microalga Spirulina was successfully digested by the haloalkaline microbial consortium at alkaline conditions (pH 10, 2.0 M Na+). Continuous biogas production was observed and the obtained biogas was rich in methane, up to 96 %. Alkaline medium acted as a CO2 scrubber which resulted in low amounts of CO2 and no traces of H2S in the produced biogas. A hydraulic retention time of 15 days and 0.25 g Spirulina L-1 day-1 organic loading rate were identified as the optimal operational parameters. Metagenomics and metatranscriptomics analysis showed that the hydrolysis of the supplied substrate was mainly carried out by Bacteroidetes of the ML635J-40 aquatic group while the hydrogenotrophic pathway was the main producer of methane in a methanogenic community dominated by Methanocalculus.

Cyanotoxins are secondary metabolites produced by cyanobacteria, of varied chemical nature and toxic effects. Although cyanobacteria thrive in all kinds of ecosystems on Earth even under very harsh conditions, current knowledge on cyanotoxin distribution is almost restricted to freshwaters from temperate latitudes. In this review, we bring to the forefront the presence of cyanotoxins in extreme environments. Cyanotoxins have been reported especially in polar deserts (both from the Arctic and Antarctica) and alkaline lakes, but also in hot deserts, hypersaline environments, and hot springs. Cyanotoxins detected in these ecosystems include neurotoxins—anatoxin-a, anatoxin-a (S), paralytic shellfish toxins, β-methylaminopropionic acid, N-(2-aminoethyl) glycine and 2,4-diaminobutyric acid- and hepatotoxins –cylindrospermopsins, microcystins and nodularins—with microcystins being the most frequently reported. Toxin production there has been linked to at least eleven cyanobacterial genera yet only three of these (Arthrospira, Synechococcus and Oscillatoria) have been confirmed as producers in culture. Beyond a comprehensive analysis of cyanotoxin presence in each of the extreme environments, this review also identifies the main knowledge gaps to overcome (e.g., scarcity of isolates and –omics data, among others) toward an initial assessment of ecological and human health risks in these amazing ecosystems developing at the very edge of life.

While water transfer from rivers to alkaline lakes has been proposed to solve lake water level drawdown and ecological degradation problems, its effectiveness for achieving ecological goals is often questionable. A sudden pH decline in alkaline lakes due to water transfer is considered likely to harm the lake ecology. However, it remains unclear to what extent water transfer affects alkaline lake pH. Thus, a three-dimensional numerical model coupling a pH calculation method considering the carbonate balance with the MIKE3 hydrodynamic model was developed to predict pH changes in an alkaline lake. Laboratory and field measurements verified the model reliability. The model accurately simulated the mixed-water pH during water transfer, with a root mean square error of 0.03–0.07 and a coefficient of determination of 0.894–0.998. The model was then applied to predict the pH response to water transfer in Lake Chenghai. The results showed that the pH response to water transfer demonstrated spatial and temporal variability, and a low-pH diffusion zone (pH ≤ 9) formed in the northern parts of the lake during annual water transfer; the effects of water transfer on the pH in the lake were cumulative over time, and the average pH in Lake Chenghai after five years decreased by 0.2 units; strong wind and low inflow could effectively reduce the low-pH diffusion area; and daily thermal stratification of the plateau region threatened the low-pH diffusion area control in Lake Chenghai. Our results provide a new reference for formulating ecological water transfer strategies for alkaline lakes and similar water bodies. [Display omitted] • A three-dimensional pH model based on the carbonate equilibrium system was developed. • pH simulation in an alkaline lake with water transfer from a neutral river was satisfactory. • The pH response in an alkaline lake to water transfer demonstrated spatial and temporal variability. • The daily thermal stratification of the plateau region threatened the low-pH diffusion area control in an alkaline lake. • Strong wind and the water transfer mode with low inflow in a relatively long time are beneficial to reduce the eco-environment risks for alkaline lakes. [ABSTRACT FROM AUTHOR]

Specific responses of cells from different cell-cycle phases to environmental variations strongly impact their ecology and productivity. We quantified responses of both the motile, calcified growing cell phase (G1 phase) and the calcite shell forming, non-motile mitotic phase (S, G2 and M phase) of the freshwater phytoflagellate Phacotus lenticularis to changes in depth-specific physicochemical parameters and meteorological parameters over two growing seasons. Growing (GCs) and mitotic cells (MCs) segregated vertically during peak development. GCs accumulated within the thermocline, whereas MCs aggregated in the epilimnion down to 4 m depth, which corresponded to the maximum depth of mixing. GCs and MCs differed in their preference with respect to pH and water temperature, but not to water density. Interestingly, pH and temperature variations in the epilimnion, where MCs aggregate and calcify, explained up to 83% of the variance of GC density at a time lag of one and more days in a daily sampling campaign. Among the meteorological variables we tested, rainfall and air temperature had an additional positive effect on GC density. The results suggest that Phacotus cells migrate upwards prior to loss of motility to layers that reduce sinking but likely enhance calcification rates, which translates into a response of GC density. [ABSTRACT FROM AUTHOR]

A striking feature of the Mongolian plateau is the wide range of air temperatures during a year, -30 to 30°C. High summer temperatures, atmospheric weathering and the arid climate lead to formation of numerous alkaline soda lakes that are covered by ice during 6-7 months per year. During the study period, the lakes had pH values between 8.1 to 10.4 and salinity between 1.8 and 360 g/L. According to chemical composition, the lakes belong to sodium carbonate, sodium chloride-carbonate and sodium sulfate-carbonate types. This paper presents the data on the water chemical composition, results of the determination of the rates of microbial processes in microbial mats and sediments in the lakes studied, and the results of a Principal Component Analysis of environmental variables and microbial activity data. Temperature was the most important factor that influenced both chemical composition and microbial activity. pH and salinity are also important factors for the microbial processes. Dark CO fixation is impacted mostly by salinity and the chemical composition of the lake water. Total photosynthesis and sulfate-reduction are impacted mostly by pH. Photosynthesis is the dominant process of primary production, but the highest rate (386 mg C/(L∙d)) determined in the lakes studied were 2-3 times lower than in microbial mats of lakes located in tropical zones. This can be explained by the relatively short warm period that lasts only 3-4 months per year. The highest measured rate of dark CO assimilation (59.8 mg C/(L∙d)) was much lower than photosynthesis. The highest rate of sulfate reduction was 60 mg S/(L∙d), while that of methanogenesis was 75.6 μL СН/(L∙d) in the alkaline lakes of Mongolian plateau. The rate of organic matter consumption during sulfate reduction was 3-4 orders of magnitude higher than that associated with methanogenesis. [ABSTRACT FROM AUTHOR]

Nitrogen isotope ratios in ancient sedimentary rocks are generally interpreted as a proxy for metabolic nitrogen pathways and the redox state of the water column. Fractionation processes occurring under anoxic, alkaline conditions during the dissociation of NH 4 + to H + and volatile NH 3 are frequently overlooked, although this mechanism imparts large isotopic fractionations. Here we propose that NH 3 volatilization is largely responsible for δ 15 N values of up to + 50 ‰ at high C/N ratios in the late Archean Tumbiana Formation. This sequence of sedimentary rocks represents a system of lakes that formed on subaerial flood basalts and were partly filled by basaltic volcanic ash. Aqueous alteration of volcanic glass followed by evaporative concentration of ions should have led to the development of high alkalinity with a pH of 9 or higher, as in modern analogues. In this sedimentologically unusual setting, nitrogen isotope ratios thus provide indirect evidence for the oldest alkaline lake system in the rock record. These very heavy lacustrine δ 15 N values contrast markedly with those of Archean marine sedimentary rocks, making a Precambrian “soda ocean” unlikely. Today, alkaline lakes are among the most productive ecosystems on Earth. Some nutrients, in particular molybdenum, are more soluble at high pH, and certain prebiotic reactions would likely have been favored under alkaline conditions in similar settings earlier in Earth's history. Hence alkaline lakes in the Archean could have been significant for the origin and early evolution of life. [ABSTRACT FROM AUTHOR]

The pelagic phytoflagellate Phacotus lenticularis creates a shell of highly symmetrical calcite crystals at its outer extracellular layer and thereby constitutes a significant source of calcite in temperate alkaline lakes worldwide. Responses of P. lenticularis to a changing lake environment therefore have potential implications for the calcium carbonate flux in lakes. How meteorological variability in interaction with the CaCO3 saturation state may influence the abundance and phenology of P. lenticularis has not been investigated so far. We measured, biweekly to daily, algal abundance, CaCO3 saturation, pH, water temperature and nutrient status in three hard-water lakes that varied in lake morphology and wind exposure. Our data provide evidence that increasing supersaturation of CaCO3 mediates the onset of P. lenticularis exponential growth and stimulates seasonal peak development. However, total cell concentration in the lake water appears to be independent of the CaCO3 saturation state and was the highest in the most wind exposed lake. Phacotus lenticularis abundance and epilimnetic CaCO3 supersaturation were significantly positively correlated at a time lag of about 1 day. Wind caused an immediate decline in epilimnetic pH followed by a decline in CaCO3 saturation about 2 days later. Solar irradiance positively affected both pH and water temperature, but water temperature changes were insufficient to produce a significant CaCO3 saturation response. We conclude that the CaCO3 saturation state is a determinant factor driving the timing of P. lenticularis peaks in alkaline lakes. [ABSTRACT FROM AUTHOR]

This study assesses the potential use of Mg isotopes to trace Mg carbonate precipitation in natural waters. Salda Lake (SW Turkey) was chosen for this study because it is one of the few modern environments where hydrous Mg carbonates are the dominant precipitating minerals. Stromatolites, consisting mainly of hydromagnesite, are abundant in this lake. The Mg isotope composition of incoming streams, groundwaters, lake waters, stromatolites, and hydromagnesite-rich sediments were measured. Because Salda Lake is located in a closed basin, mass balance requires that the Mg isotopic offset between Lake Salda water and precipitated hydromagnesite be comparable to the corresponding offset between Salda Lake and its water inputs. This is consistent with observations; a δMg offset of 0.8-1.4 ‰ is observed between Salda Lake water and it is the incoming streams and groundwaters, and precipitated hydromagnesite has a δMg 0.9-1.1 ‰ more negative than its corresponding fluid phase. This isotopic offset also matches closely that measured in the laboratory during both biotic and abiotic hydrous Mg carbonate precipitation by cyanobacteria (Mavromatis, V., Pearce, C., Shirokova, L. S., Bundeleva, I. A., Pokrovsky, O. S., Benezeth, P. and Oelkers, E.H.: Magnesium isotope fractionation during inorganic and cyanobacteria-induced hydrous magnesium carbonate precipitation, Geochim. Cosmochim. Acta, . 76, 161-174). Batch reactor experiments performed in the presence of Salda Lake cyanobacteria and stromatolites resulted in the precipitation of dypingite (Mg(CO)(OH)·5(HO)) and hydromagnesite (Mg(CO)(OH)·4HO) with morphological features similar to those of natural samples. Concurrent abiotic control experiments did not exhibit carbonate precipitation demonstrating the critical role of cyanobacteria in the precipitation process. [ABSTRACT FROM AUTHOR]

Lake Kitagata, Uganda, is a hypersaline crater lake with Na-SO-Cl-HCO-CO chemistry, high pH and relatively small amounts of SiO. EQL/EVP, a brine evaporation equilibrium model (Risacher and Clement ), was used to model the major ion chemistry of the evolving brine and the order and masses of chemically precipitated sediments. Chemical sediments in a 1.6-m-long sediment core from Lake Kitagata occur as primary chemical mud (calcite, magadiite [NaSiO(OH)·3HO], burkeite [Na(CO)(SO)]) and as diagenetic intrasediment growths (mirabilite (NaSO·10HO)). Predicted mineral assemblages formed by evaporative concentration were compared with those observed in salt crusts along the shoreline and in the core from the lake center. Most simulations match closely with observed natural assemblages. The dominant inflow water, groundwater, plays a significant role in driving the chemical evolution of Lake Kitagata water and mineral precipitation sequences. Simulated evaporation of Lake Kitagata waters cannot, however, explain the large masses of magadiite found in cores and the formation of burkeite earlier in the evaporation sequence than predicted. The masses and timing of formation of magadiite and burkeite may be explained by past groundwater inflow with higher alkalinity and SiO concentrations than exist today. [ABSTRACT FROM AUTHOR]

Abstract: The Palaeogene Maíz Gordo Formation is one of the main lacustrine events recorded in northwestern Argentina. It consists of sandstone, mudstone, and limestone beds 200 m thick, deposited in a brackish–alkaline lake and braided alluvial systems. The Maíz Gordo Lake evolved mainly as a closed system, with brief periods as an open one. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study samples from seven sites, corresponding respectively to proximal, intermediate, and transitional positions of the fluvial environment and marginal and inner-lake environment, focusing on the clay mineralogy and analcime formation. The basinward zonation of diagenetic minerals identified in the Maíz Gordo Lake was: mordenite → analcime → K-feldspar. Although not a typical zonation of saline–alkaline lakes, it does indicate an increase in salinity and alkalinity towards the centre. In proximal fluvial settings, smectite predominates at the base of the sequence, with scarce kaolinite. Towards the top, a striking increase in kaolinite content suggests a change from a relatively arid climate with alternating humid and dry seasons, towards a warm and humid climate. Kaolinite content clearly decreases in a basinward direction. Such a variation is attributable to changes in hydro-geochemistry, denoting the progressive influence of the brackish and alkaline lake water on interstitial pores. SEM images of intermediate fluvial samples reveal authigenesis of illite at the expense of kaolinite booklets. In littoral and inner-lake settings the clay fraction is composed of muscovite, sometimes with subordinate smectite. Analcime occurs in variable amounts in all sedimentary facies, in rock pores or filling veins. It forms subhedral square to hexagonal, or anhedral rounded crystals, denoting that they coarsened at low to moderate degrees of supersaturation. Although the mordenite identified in a fluvial level would have been the precursor of analcime in the Maíz Gordo Basin, no textural evidence of analcime formation through replacement of mordenite or other precursor zeolite was found. Hence it is more probable that analcime formation took place by direct authigenic precipitation or through the reaction between interstitial brines and clay minerals or plagioclase. [Copyright &y& Elsevier]

Significant concentrations of dissolved inorganic arsenic can be found in the waters of a number of lakes located in the western USA and in other water bodies around the world. These lakes are often situated in arid, volcanic terrain. The highest concentrations of arsenic occur in hypersaline, closed basin soda lakes and their remnant brines. Although arsenic is a well-known toxicant to eukaryotes and prokaryotes alike, some prokaryotes have evolved biochemical mechanisms to exploit arsenic oxyanions (i.e., arsenate and arsenite); they can use them either as an electron acceptor for anaerobic respiration (arsenate), or as an electron donor (arsenite) to support chemoautotrophic fixation of CO2 into cell carbon. Unlike in freshwater or marine ecosystems, these processes may assume quantitative significance with respect to the carbon cycle in arsenic-rich soda lakes. For the past several years our research has focused on the occurrence and biogeochemical manifestations of these processes in Mono Lake, a particularly arsenic-rich environment. Herein we review some of our findings concerning the biogeochemical arsenic cycle in this lake, with the hope that it may broaden the understanding of the influence of microorganisms upon the speciation of arsenic in more common, less “extreme” environments, such as drinking water aquifers. [Copyright &y& Elsevier]

The fluoride content of Tanzanian and Kenyan magadi has been estimated to be in the range 0.1-17.9 mg F[sup -] g[sup -1], which is comparable with that reported elsewhere, but indicating a considerable variation in levels. The median fluoride content of crystalline magadi harvested from the alkaline lakes was 2.1 mg g[sup -1], which was higher than the median of 1.4 mg g[sup -1] for scooped magadi harvested from the surface soil. The highest median fluoride contents of 3.2 and 2.9 mg g[sup -1] were found in magadi originating from Lake Magadi, Kenya, and Lake Natron, Tanzania, respectively. It was found that the fluoride content varied significantly even for magadi originating from the individual lake, e.g. the fluoride content in magadi from Lake Magadi was between 0.1 and 8.7 mg g[sup -1]. In a lump of magadi originating from Lake Magadi, it was found that the fluoride content in 20 smaller part samples was subject to considerable variation indicating that the fluoride-bearing minerals were unevenly distributed in the lump. Results show that the fluoride is mainly present in grains <1.0 mm that made up 25% of the magadi sample. When daily eating the popular meal makande as in Tanzania, the exposure to fluoride through magadi in 70% of cases was estimated to be <4 mg per adult day[sup -1], as recommended by the WHO. Thus, the health hazard from magadi-fluoride is estimated to be significant in cases where the magadi is heavily contaminated. [ABSTRACT FROM AUTHOR]

Kenya's Baringo-Tugen Hills-Barsemoi drill site is one of six localities across Kenya and Ethiopia from which the Hominin Sites and Paleolakes Drilling Project has obtained sediment cores in an effort to investigate the role of environmental forcing in shaping human evolution. The Baringo Basin site features extensive exposures of the Chemeron Formation, which contains >100 fossil vertebrate localities including five hominin sites. The 228-m drill core, dating from ~3.29 to 2.56 Ma, is characterized by fluvio-lacustrine sediments, including multiple diatomites, with evidence of variable degrees of later pedogenic modification. In the lower part of the core (~3.29–3.04 Ma), diatoms were preserved only in very low abundance, consistent with predominantly fluvial or lake marginal environments. In contrast, five diatomites and two additional diatom-rich intervals were deposited after ~3.04 Ma, reflecting a major shift in the basin hydrology. Planktonic freshwater species dominated these diatom-rich intervals, whereas periphytic taxa were present in proportions less than 2%, suggesting that these intervals represent open-water deposition during lake highstands. Littoral or saline assemblages are largely absent throughout the core. Instead, we observed a pattern of increasing diatom frustule dissolution at the tops and bottoms of diatomite units, indicating increased alkalinity during the transgressive/regressive phases. A Na-bearing zeolite (analcime) indicative of saline waters precipitated in clastic-dominated intervals between diatomites, suggesting extreme environmental variability between lake highstands and lowstands. Diatom assemblages were consistently dominated by a few species belonging to the genera Aulacoseira and Stephanodiscus , which were at times co-dominant. We infer that assemblages dominated by Aulacoseira represent a well-mixed lake with abundant supply of silica. When Stephanodiscus was dominant, which occurred more frequently in the later freshwater phases, we infer incomplete mixing and reduced silica flux to the epilimnion (upper water layer). • Diatom evidence of deep, productive, fresh lakes from 3.04–2.99 Ma and 2.66–2.58 Ma. • Diatom dissolution and zeolite precipitation reflect alkaline and saline waters. • Shifts in planktonic species' dominance indicate changes in intensity of lake mixing. [ABSTRACT FROM AUTHOR]

Mg clay minerals are usually associated with carbonates in alkaline-saline environments, precipitated from solution and/or transformation from other minerals. The aim of this research is to identify the mineralogy and geochemistry of clay minerals in different alkaline lakes in the Nhecolândia region, the southernmost region of the Pantanal wetland (Brazil). Sediment samples were analyzed by X-ray diffraction, X-ray fluorescence, scanning electron microscopy and transmission electron microscopy. Water samples were analyzed, determining their main cations and anions, in order to understand their relationship with the clays. The analyses allowed classifying the water bodies as saline, oligosaline and freshwater lakes. The sediments are composed mainly of quartz and a fine-clay fraction, dominated by illite, kaolinite and smectite. The XRD results showed illite and smectite mixed-layered in the saline lakes at Barranco Alto farm, whereas at Nhumirim farm, trioctahedral smectite was only observed in one lake. The smectite minerals were normally identified coupled with calcite at the top of the sequences, associated with exopolymeric substances (EPS) in the lakes, suggesting that these minerals are precipitating due to the physical-chemical and biological conditions of the water bodies. [ABSTRACT FROM AUTHOR]