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Organic matter in Archean hydrothermal cherts may provide an important archive for molecular traces of the earliest life on Earth. The geobiological interpretation of this archive, however, requires a sound understanding of organic matter preservation and alteration in hydrothermal systems. Here we report on organic matter (including molecular biosignatures) enclosed in hydrothermally influenced cherts of the Pleistocene Lake Magadi (Kenya; High Magadi Beds and Green Beds). The Magadi cherts contain low organic carbon (n-alkanes, hopanes, and polycyclic aromatic hydrocarbons (PAHs) indicate that a fraction of the bitumen has been thermally altered to early or peak oil window maturity. This more mature fraction likely originated from defunctionalization of dissolved organic matter and/or hydrothermal petroleum formation at places of higher thermal flux. Like the bitumens, the kerogens also show variations in thermal maturities, which can partly be explained by admixture of thermally pre-altered macromolecules. However, findings of archaea-derived isoprenoid moieties (C20 and C25 chains) in kerogen pyrolysates indicate rapid sequestration of some archaeal lipids into kerogen while hydrothermal alteration was active. We posit that such early sequestration may enhance the resistance of molecular biosignatures against in situ hydrothermal and post-depositional alteration. Furthermore, the co-occurrence of organic matter with different thermal maturities in the Lake Magadi cherts suggests that similar findings in Archean hydrothermal deposits could partly reflect original environmental conditions and not exclusively post-depositional overprint or contamination. Our results support the view that kerogen in Archean hydrothermal cherts may contain important information on early life. Our study also highlights the suitability of Lake Magadi as an analog system for hydrothermal chert environments on the Archean Earth.

International audience; Evidence for Quaternary climate change in East Africa has been derived from outcrops on land and lake cores and from marine dust, leaf wax, and pollen records. These data have previously been used to evaluate the impact of climate change on hominin evolution, but correlations have proved to be difficult, given poor data continuity and the great distances between marine cores and terrestrial basins where fossil evidence is located. Here, we present continental coring evidence for progressive aridification since about 575 thousand years before present (ka), based on Lake Magadi (Kenya) sediments. This long-term drying trend was interrupted by many wet–dry cycles, with the greatest variability developing during times of high eccentricity-modulated precession. Intense aridification apparent in the Magadi record took place between 525 and 400 ka, with relatively persistent arid conditions after 350 ka and through to the present. Arid conditions in the Magadi Basin coincide with the Mid-Brunhes Event and overlap with mammalian extinctions in the South Kenya Rift between 500 and 400 ka. The 525 to 400 ka arid phase developed in the South Kenya Rift between the period when the last Acheulean tools are reported (at about 500 ka) and before the appearance of Middle Stone Age artifacts (by about 320 ka). Our data suggest that increasing Middle- to Late-Pleistocene aridification and environmental variability may have been drivers in the physical and cultural evolution of Homo sapiens in East Africa.

© 2020 by the authors., Mineral vesicles and chemical gardens are self-organized biomimetic structures that form via abiotic mineral precipitation. These membranous structures are known to catalyze prebiotic reactions but the extreme conditions required for their synthesis has cast doubts on their formation in nature. Apart from model solutions, these structures have been shown to form in serpentinization-driven natural silica-rich water and by fluid-rock interaction of model alkaline solutions with granites. Here, for the first time, we demonstrate that self-assembled hollow mineral vesicles and gardens can be synthesized in natural carbonate-rich soda lake water. We have synthesized these structures by a) pouring saturated metal salt solutions, and b) by immersing metal salt pellets in brines collected from Lake Magadi (Kenya). The resulting structures are analyzed by using SEM coupled with EDX analysis, Raman spectroscopy, and powder X-ray diffraction. Our results suggest that mineral self-assembly could have been a common phenomenon in soda oceans of early Earth and Earth-like planets and moons. The composition of the obtained vesicles and gardens confirms the recent observation that carbonate minerals in soda lakes sequestrate Ca, thus leaving phosphate behind in solution available for biochemical reactions. Our results strengthens the proposal that alkaline brines could be ideal sites for “one-pot” synthesis of prebiotic organic compounds and the origin of life., The authors thank the European Research Council under the European Union’s seventh Framework Program (FP7/2007-2013)/ERC grant agreement no. 340863 and the Spanish “Ministerio de Educacion y Ciencia” for the financial support to the project CGL2016-78971-P. MG. acknowledges Grant No. BES-2017-081105 of the “Ministerio de Ciencia, Innovacion y Universidades” of the Spanish government

Studying recent adaptive radiations in isolated insular systems avoids complicating causal events and thus may offer clearer insight into mechanisms generating biological diversity. Here, we investigate evolutionary relationships and genomic differentiation within the recent radiation of Alcolapia cichlid fish that exhibit extensive phenotypic diversification, and which are confined to the extreme soda lakes Magadi and Natron in East Africa. We generated an extensive RAD data set of 96 individuals from multiple sampling sites and found evidence for genetic admixture between species within Lake Natron, with the highest levels of admixture between sympatric populations of the most recently diverged species. Despite considerable environmental separation, populations within Lake Natron do not exhibit isolation by distance, indicating panmixia within the lake, although individuals within lineages clustered by population in phylogenomic analysis. Our results indicate exceptionally low genetic differentiation across the radiation despite considerable phenotypic trophic variation, supporting previous findings from smaller data sets; however, with the increased power of densely sampled SNPs, we identify genomic peaks of differentiation (FST outliers) between Alcolapia species. While evidence of ongoing gene flow and interspecies hybridization in certain populations suggests that Alcolapia species are incompletely reproductively isolated, the identification of outlier SNPs under diversifying selection indicates the radiation is undergoing adaptive divergence.

Foram avaliadas alterações no metabolismo e no sistema de defesa antioxidante na larva do quironomídeo Tanytarsus minutipalpus frente a variações diárias e extremas nas condições físico-químicas no "Flamingo Pool" no Lago Magadi (Quênia). Em diferentes horas do dia (7:00 h, 13:00 h e 19:00 h do mesmo dia e 1:00 h e 7:00 h do dia seguinte), em diferentes dias de Julho/Agosto de 2010 e Julho de 2013, foram medidos a temperatura, teor de oxigênio dissolvido e pH da água. As larvas de T. minutipalpus foram coletadas com rede de zooplâncton, separadas manualmente e utilizadas para análise de parâmetros metabólicos (atividade da colinesterase e concentrações de glicose, triglicerídeos, colesterol, proteínas, uréia, hemoglobina e bilirrubina) e do estado oxidativo [espécies reativas de oxigênio (ROS), óxido nítrico (NO), glutationa reduzida (GSH), glutationa oxidada (GSSG), mucoproteínas, superóxido dismutase (SOD), catalase (CAT), glutationa peroxidase (GPx), glutationa redutase (GR), glutamato-cisteína ligase (GCL), capacidade antioxidante total (TOSC), lipoperoxidação (LPO) e dano no DNA]. Não houve variação no pH (10.0 ± 0.03) da água, porém foram observadas amplas variações diárias de temperatura (20.2-29.3oC) e oxigênio dissolvido (3.2-18.6 mg O2/L), com os valores médios mais altos sendo observados às 13:00 h. Foram observados aumentos temporários na colinesterase, glicose, colesterol, ureia, creatinina, hemoglobina SOD, GPx, GR, GSH e GSSG nas larvas coletadas às 13:00 h. Estes resultados indicam que a larva de T. minutipalpus apresenta ajustes metabólicos e no sistema de defesa antioxidante que respondem positivamente e de forma cooperativa frente às variações diárias e extremas na temperatura e oxigênio dissolvido nas águas do Lago Magadi. In the present study, we report for the first time the occurrence of the immature chironomid Tanytarsus minutipalpus in the Lake Magadi (Kenya, Africa). Adaptive responses in metabolism and antioxidant defense system to the diel variations in the extreme environmental conditions of the Lake Magadi were evaluated in chironomid larvae. In July-August 2010 and 2013, we performed measurements of water physicochemical parameters (temperature, dissolved oxygen and pH) and chironomid sampling at different times of day (7:00 am, 1:00 pm and 7:00 pm of the first day; and 1:00 am and 7:00 am of the following day). Larvae were collected using a bean trawl net (90-µm mesh), manually separated and immediately placed in liquid nitrogen or quickly transferred to laboratory for analyses of parameters indicative of metabolism (cholinesterase activity and glucose, triglycerides, cholesterol, proteins, urea, creatinine, hemoglobin and bilirubin concentration), pro-oxidant status [reactive oxygen species (ROS) and nitric oxide (NO) concentration], antioxidant status [superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione reductase (GR) and glutamate-cysteine ligase (GCL) activity, as well as total glutathione (GSH), oxidized glutathione (GSSG) and mucoproteins concentration], and oxidative status [total antioxidant capacity (TOSC), lipid peroxidation (LPO) and DNA damage]. Wide variations in water temperature (20.2-29.3oC) and dissolved oxygen (3.2-18.6 mg O2/L) were observed at different times of day, without any significant change in water pH (10.0 ± 0.03). Higher mean values of water temperature and dissolved oxygen were observed at 1:00 pm (29.3 ± 0.8oC and 18.6 ± 1.0 mg O2/L, respectively) and 7:00 pm (29.3 ± 1.4oC and 16.2 ± 1.6 mg O2/L, respectively). Significant increases in parameters related to metabolism (cholinesterase, glucose, cholesterol, urea, creatinine and hemoglobin) paralleled by significant increases in the major components of the 19 antioxidant system (SOD, GPx, GR, GSH and GSSG) were observed in chironomid larvae collected at 1:00 pm. On the other hand, no significant changes were observed in pro-oxidants (ROS and NO), TOSC and oxidative damage parameters (LPO and DNA damage). These findings clearly show that the antioxidant system of T. minutipalpus larvae acts positively and cooperatively to face an increased metabolism in response to increasing temperature and O2 content in Lake Magadi waters. They also indicate that biochemical and physiological adjustments performed by the chironomid larvae were efficient in keeping body homeostasis, as well as protecting biomolecules against oxidative damage. GSH-GSSG and GPx-GR systems were shown to play an essential role in the adaptive response displayed by the chironomid larvae when facing the diel changes in the extreme conditions of the Lake Magadi.

During recent humid episodes, stromatolites were built along a paleolake margin, some 60 m above the modern water level of lakes Natron and Magadi (southern Gregory Rift Valley). Three generations of stromatolites are observed, the more recent ones frequently coating pebbles and boulders eroded from the older ones. Therefore, three distinct lake level rises are distinguished, ranging in age from middle Pleistocene to Holocene. The stromatolitic incrustations indiscriminately cover the whole hard substrates, including the bedrock surfaces as well as some vegetal remains. They show a variety of growth forms from oncolites, tabular and cylindrical encrustations to various type bioherms. Through detailed morphologic and pétrographie examination of the stromatolites it was possible to establish a precise littoral zonation. Several parameters of the physical environment, such as depth, hydrodynamism and light intensity have a strong influence on the stromatolite gross morphologies. Both blue-green algae and bacteria, growing in mats or colonies, precipitate calcite which is the main component of the stromatolites. The most common stromatolitic microstructure appears to be the repetition of a doublet composed of a dark-colored micritic lamina and a light-colored lamina of radial fibrous calcite. This doublet represents an ecological cycle of the microbial mat growth and is interpreted in terms of seasonal contrast., Au cours d'épisodes humides récents, des stromatolites se sont formés en bordure d'un paléolac, dont le rivage se situe 60 m au-dessus du niveau actuel des lacs Natron et Magadi. Trois générations de stromatolites sont décrites, les plus récentes encroûtant généralement les formes érodées plus anciennes. Elles correspondent à trois hauts niveaux lacustres différents, dont les âges se distribuent du Pléistocène moyen à l'Holocène. Les stromatolites recouvrent l'ensemble des substrats durs, parmi lesquels figurent de nombreux supports végétaux. Oncolites, encroûtements plans et cylindriques, ainsi que diverses formes de bioherms sont les principales morphologies rencontrées. Une analyse pétrographique et morphologique détaillée a permis d'établir une zonation littorale précise. Les principaux facteurs de l'environnement, tels que la profondeur, l'hydrodynamisme ou l'intensité lumineuse, influencent directement la morphologie des stromatolites. La phase carbonatée est de la calcite précipitée conjointement par des cyanophycées et des bactéries sous forme de tapis ou de colonies. La microstructure la plus fréquente résulte de la répétition d'un doublet composé d'une lamine sombre micritique et d'une lamine claire sparitique. Ce doublet représente un cycle écologique de la croisance du tapis microbien ; il est interprété en termes de contraste saisonnier., Casanova Joel. Stromatolites et hauts niveaux lacustres pléistocènes du bassin Natron-Magadi (Tanzanie-Kenya). Pleistocene stromatolites and high lake levels of the Natron-Magadi basin (Tanzania-Kenya). In: Sciences Géologiques. Bulletin, tome 40, n°1-2, 1987. Lac Natron. Géologie, géochimie et paléontologie. D'un bassin évaporatique du rift est-africain. pp. 135-153.

(δ²H et δ¹⁸O) values of hydrothermal springs from the Lake Magadi -Lake Natron area show a linear relationship with a 4.7 slope indicative of evaporated waters. The springs are considered to be mixtures of three components : (1) deep saline groundwaters of the rift floor (¹⁸O~-7‰ ; δ²H ~-50‰); (2) shallow dilute groundwaters from the basaltic and Precambrian plateaus (δ¹⁸O~-4‰ ; δ²H~-20‰) ; and (3) interstitial brines from the lake sediments (δ¹⁸O~8‰δ²H ~-24‰) . No apparent traces of deep rock-water isotopic exchange are observed. 13C and 14C contents of the inorganic dissolved carbon are considered to represent deep carbon inputs (δ¹³C, Les sources hydrothermales du bassin Magadi-Natron présentent des compositions isotopiques (δ²H et δ¹⁸O) groupées autour d'une droite de pente 4,7 indiquant une évaporation assez importante. Elles sont interprétées comme le produit d'un mélange de trois composantes : la nappe profonde du rift (¹⁸O~-7‰ ; δ²H ~-50‰), la nappe superficielle des plateaux basaltique et précambrien (δ¹⁸O~-4‰ ; δ²H~-20‰) et les saumures interstitielles des sédiments lacustres (δ¹⁸O~8‰ ; δ²H ~-24‰). Le circuit géothermal de l'eau n'a laissé aucune trace isotopique apparente. Les teneurs en I3C et en l4C du carbone inorganique dissous reflètent des apports de carbone profond (δ¹³C, Hillaire-Marcel Claude. Hydrologie isotopique des lacs Magadi (Kenya) et Natron (Tanzanie). Isotopic hydrology of the lakes Magadi (Kenya ) and Natron (Tanzania ). In: Sciences Géologiques. Bulletin, tome 40, n°1-2, 1987. Lac Natron. Géologie, géochimie et paléontologie. D'un bassin évaporatique du rift est-africain. pp. 111-120.

The Lake Natron-Lake Magadi basin has been occupied by extensive lakes on several occasions during the late Pliocene and Pleistocene times. During the most recent lacustrine episodes, stromatolites were built at the lake margins, some 50 m above the modern water level. Three distinct phases of encrustation were identified by detailed morphologic and pétrographie examination of these algal limestones. The more recent phases frequently cover pebbles and boulders eroded from the older phases. More than 30¹⁴C measurements on the third generation yielded ages ranging from approximately 12,000 to 10,000 yrs B.P. Their ¹³C values (≥ 2.6 ‰) suggest isotopic equilibrium between the paleolake total inorganic dissolved carbon and the atmospheric CO₂, thereby lending credence to the reliability of the ¹⁴C ages. I80 and 13C measurements on these algal limestones when compared to values on modern waters and carbonates, give some indication about the paleohydrology. Long residence time of the paleolake waters and less seasonally contrasted regimes have been inferred for the corresponding humid episodes. The ²³⁰Th/²³²Th ratio of the detrital component was determined by Th/U measurements on the recent stromatolites. Using this values we then calculated a ²³⁰Th/²³⁴U chronology for the older stromatolites. Age ≥240,000 yrs and 135,000 ± 10,000 yrs were obtained respectively for the first and second generations. A humid episode seems therefore to characterize eastern Africa during each glacial-interglacial transition period., Trois générations de stromatolites marquent l'extension récente de paléolacs dans le bassin Natron-Magadi, à la frontière tanzano-kenyenne. Leurs teneurs en ¹⁸O et en ¹³C, relativement élevées, reflètent un long temps de résidence de l'eau des paléolacs favorable à l'établissement d'un équilibre entre le CO₂ atmosphérique et le carbone inorganique dissous malgré les forts apports de carbone profond lié au volcanisme carbonatitique régional. De ce fait, les âges ¹⁴C obtenus sur les stromatolites les plus récents (12 00 à 10 000 B.P.) sont considérés comme représentatifs. Les stromatolites plus anciens ont donné des âges U/Th de 135 000 ans et de ca. 240 000 ans. Un épisode humide semble donc caractériser l'Afrique de l'Est au cours de chaque transition entre les périodes glaciaires et interglaciaires., Casanova Joel, Hillaire-Marcel Claude. Chronologie et paléohydrologie des hauts niveaux quaternaires du bassin Natron-Magadi (Tanzanie-Kenya) d'après la composition isotopique (18O, 13C, 14C, U/Th) des stromatolites littoraux. Chronology and paleohydrology of former lake levels in the Natron-Magadi basin (Tanzania-Kenya) from isotopic composition (18O, 13C, 14C, U/Th) of shoreline stromatolites. In: Sciences Géologiques. Bulletin, tome 40, n°1-2, 1987. Lac Natron. Géologie, géochimie et paléontologie. D'un bassin évaporatique du rift est-africain. pp. 121-134.

Background Lake Magadi and little Magadi are hypersaline, alkaline lakes situated in the southern part of Kenyan Rift Valley. Solutes are supplied mainly by a series of alkaline hot springs with temperatures as high as 86 °C. Previous culture-dependent and culture-independent studies have revealed diverse groups of microorganisms thriving under these conditions. Previous culture independent studies were based on the analysis of 16S rDNA but were done on less saline lakes. For the first time, this study combined illumina sequencing and analysis of amplicons of both total community rDNA and 16S rRNA cDNA to determine the diversity and community structure of bacteria and archaea within 3 hot springs of L. Magadi and little Magadi. Methods Water, wet sediments and microbial mats were collected from springs in the main lake at a temperature of 45.1 °C and from Little Magadi “Nasikie eng’ida” (temperature of 81 °C and 83.6 °C). Total community DNA and RNA were extracted from samples using phenol-chloroform and Trizol RNA extraction protocols respectively. The 16S rRNA gene variable region (V4 – V7) of the extracted DNA and RNA were amplified and library construction performed following Illumina sequencing protocol. Sequences were analyzed done using QIIME while calculation of Bray-Curtis dissimilarities between datasets, hierarchical clustering, Non Metric Dimensional Scaling (NMDS) redundancy analysis (RDA) and diversity indices were carried out using the R programming language and the Vegan package. Results Three thousand four hundred twenty-six and one thousand nine hundred thirteen OTUs were recovered from 16S rDNA and 16S rRNA cDNA respectively. Uncultured diversity accounted for 89.35 % 16S rDNA and 87.61 % 16S rRNA cDNA reads. The most abundant phyla in both the 16S rDNA and 16S rRNA cDNA datasets included: Proteobacteria (8.33–50 %), Firmicutes 3.52–28.92 %, Bacteroidetes (3.45–26.44 %), Actinobacteria (0.98–28.57 %) and Euryarchaeota (3.55–34.48 %) in all samples. NMDS analyses of taxonomic composition clustered the taxa into three groups according to sample types (i.e. wet sediments, mats and water samples) with evident overlap of clusters between wet sediments and microbial mats from the three sample types in both DNA and cDNA datasets. The hot spring (45.1 °C) contained less diverse populations compared to those in Little Magadi (81–83 °C). Conclusion There were significant differences in microbial community structure at 95 % level of confidence for both total diversity (P value, 0.009) based on 16S rDNA analysis and active microbial diversity (P value, 0.01) based on 16S rRNA cDNA analysis, within the three hot springs. Differences in microbial composition and structure were observed as a function of sample type and temperature, with wet sediments harboring the highest diversity. Electronic supplementary material The online version of this article (doi:10.1186/s12866-016-0748-x) contains supplementary material, which is available to authorized users.