International audienceAragonitic microbialites, characterized by a reticulate fabric,were discovered beneath lacustrine microbial mats on the atoll ofKiritimati, Republic of Kiribati, Central Pacific. The microbialmats, with cyanobacteria as major primary producers, grow inevaporated seawater modified by calcium carbonate and gypsumprecipitation and calcium influx via surface and/or groundwaters.Despite the high aragonite supersaturation and a high photosyntheticactivity, onlyminor aragonite precipitates are observed in thetop parts of the microbial mats. Instead, major aragonite precipitationtakes place in lower mat parts at the transition to the anoxiczone. The prokaryotic community shows a high number of phylotypesclosely related to halotolerant taxa and/or taxa with preferenceto oligotrophic habitats. Soil- and plant- inhabiting bacteriaunderline a potential surface or subsurface influx from terrestrialareas, while chitinase-producing representatives coincide with theoccurrence of insect remains in the mats. Strikingly, many of theclones have their closest relatives inmicroorganisms either involvedin methane production or consumption ofmethane or methyl compounds.Methanogens, represented by the methylotrophic genusMethanohalophilus, appear to be one of the dominant organisms inanaerobic mat parts. All this points to a significant role of methaneand methyl components in the carbon cycle of the mats. Nonetheless,thin sections and physicochemical gradients through themats,as well as the 12C-depleted carbon isotope signatures of carbonatesindicate that spherulitic components of the microbialites initiatein the photosynthesis-dominated orange mat top layer, and furthergrow in the green and purple layer below. Therefore, thesespherulites are considered as product of an extraordinary highphotosynthesis effect simultaneous to a high inhibition by pristineexopolymers. Then, successive heterotrophic bacterial activityleads to a condensation of the exopolymer framework, and finallyto the formation of crevice-like zones of partly degraded exopolymers.Here initiation of horizontal aragonite layers and verticalaragonite sheets of the microbialite occurs, which are consideredas a product of high photosynthesis at decreasing degree of inhibition.Finally, at low supersaturation and almost lack of inhibition,syntaxial growth of aragonite crystals at lamellae surfaces leadsto thin fibrous aragonite veneers. While sulfate reduction, methylotrophy,methanogenesis and ammonification play an importantrole in element cycling of the mat, there is currently no evidencefor a crucial role of them in CaCO3 precipitation. Instead, photosynthesisand exopolymer degradation sufficiently explain theobserved pattern and fabric of microbialite formation

In the euxinic waters of the NW' Black Sea shelf, tower-like carbonate build-ups up to several metres in height grow at sites of cold methane seepage. These structures are part of an unique microbial ecosystem that shows a considerable biodiversity and a remarkable degree of organization. The accretion of the build-ups is promoted by the growth of centimetre-sized, methane-filled spheres constructed by calcifying microbial mats. Progressive mineralization of these spheres involves the early precipitation of strongly luminescent high-Mg-calcite rich in iron sulphides, and closely interfingered aragonite phases that finally create the stable (mega-) thrombolithic fabric of the towers. Within the microbial mats, microorganisms occur in distinctive spatial arrangements. Major players among the microbial consortia are the archaea groups ANME-1 and ANME-2, Crenarchaeota, and sulphate-reducing bacteria (SRB) of the Desulfosarcina/Desulfobacterium group. The intracellular precipitation of iron sulphides (greigite) by some of these bacteria, growing in close association with ANME-2, suggests iron cycling as an additional biogeochemical pathway involved in the anaerobic oxidation of methane (AOM).

African trypanosomiasis is a disease caused by Trypanosoma brucei parasites with limited treatment options. Trypanosoma is unable to synthesize purines de novo and relies solely on their uptake and interconversion from the host, constituting purine nucleoside analogues a potential source of antitrypanosomal agents. Here we combine structural elements from known trypanocidal nucleoside analogues to develop a series of 3’-deoxy-7-deazaadenosine nucleosides, and investigate their effects against African trypanosomes. 3’-Deoxytubercidin is a highly potent trypanocide in vitro and displays curative activity in animal models of acute and CNS-stage disease, even at low doses and oral administration. Whole-genome RNAi screening reveals that the P2 nucleoside transporter and adenosine kinase are involved in the uptake and activation, respectively, of this analogue. This is confirmed by P1 and P2 transporter assays and nucleotide pool analysis. 3’-Deoxytubercidin is a promising lead to treat late-stage sleeping sickness.

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