External controls on the distribution, fabrics and mineralization of modern microbial mats in a coastal hypersaline lagoon, Cayo Coco (Cuba).

45 pagesInternational audienceActive, carbonate-mineralizing microbial mats flourish in a tropical, highly evaporative, marine-fed lagoonal network to the south of Cayo Coco Island (Cuba). Hypersaline conditions support the development of a complex sedimentary microbial ecosystem with diverse morphologies, a variable intensity of mineralization and a potential for preservation. In this study, the role of intrinsic (i.e. microbial) and extrinsic (i.e. physicochemical) controls on microbial mat development, mineralization and preservation was investigated. The network consists of lagoons, forming in the interdune depressions of a Pleistocene aeolian substratum; they developed due to a progressive increase in sea-level since the Holocene. The hydrological budget in the Cayo Coco lagoonal network changes from west to east, increasing the salinity. This change progressively excludes grazers and increases the saturation index of carbonate minerals, favouring the development and mineralization of microbial mats in the easternmost lagoons. Detailed mapping of the easternmost lagoon shows four zones with different flooding regimes. The microbial activity in the mats was recorded using light–dark shifts in conjunction with microelectrode O2 and HS− profiles. High rates of O2 production and consumption, in addition to substantial amounts of exopolymeric substances, are indicative of a potentially strong intrinsic control on mineralization. Seasonal, climate-driven water fluctuations are key for mat development, mineralization, morphology and distribution. Microbial mats show no mineralization in the permanently submersed zone, and moderate mineralization in zones with alternating immersion and exposure. It is suggested that mineralization is also driven by water-level fluctuations and evaporation. Mineralized mats are laminated and consist of alternating trapping and binding of grains and microbially induced magnesium calcite and dolomite precipitation. The macrofabrics of the mats evolve from early colonizing Flat mats to complex Cerebroid or Terrace structures. The macrofabrics are influenced by the hydrodynamic regime: wind-driven waves inducing relief terraces in windward areas and flat morphologies on the leeward side of the lagoon. Other external drivers include: (i) storm events that either promote (for example, by bioclasts covering) or prevent (for example, by causing erosion) microbial mat preservation; and (ii) subsurface degassing, through mangrove roots and desiccation cracks covered by Flat mats (i.e. forming Hemispheroids and Cerebroidal structures). These findings provide in-depth insights into understanding fossil microbialite morphologies that formed in lagoonal settings

Microbial and diagenetic steps leading to the mineralisation of Great Salt Lake microbialites.

12 pagesInternational audienceMicrobialites are widespread in modern and fossil hypersaline environments, where they provide a unique sedimentary archive. Authigenic mineral precipitation in modern microbialites results from a complex interplay between microbial metabolisms, organic matrices and environmental parameters. Here, we combined mineralogical and microscopic analyses with measurements of metabolic activity in order to characterise the mineralisation of microbial mats forming microbialites in the Great Salt Lake (Utah, USA). Our results show that the mineralisation process takes place in three steps progressing along geochemical gradients produced through microbial activity. First, a poorly crystallized Mg-Si phase precipitates on alveolar extracellular organic matrix due to a rise of the pH in the zone of active oxygenic photosynthesis. Second, aragonite patches nucleate in close proximity to sulfate reduction hotspots, as a result of the degradation of cyanobacteria and extracellular organic matrix mediated by, among others, sulfate reducing bacteria. A final step consists of partial replacement of aragonite by dolomite, possibly in neutral to slightly acidic porewater. This might occur due to dissolution-precipitation reactions when the most recalcitrant part of the organic matrix is degraded. The mineralisation pathways proposed here provide pivotal insight for the interpretation of microbial processes in past hypersaline environments

A Goal-Directed Bayesian Framework for Categorization

Categorization is a fundamental ability for efficient behavioral control. It allows organisms to remember the correct responses to categorical cues and not for every stimulus encountered (hence eluding computational cost or complexity), and to generalize appropriate responses to novel stimuli dependant on category assignment. Assuming the brain performs Bayesian inference, based on a generative model of the external world and future goals, we propose a computational model of categorization in which important properties emerge. These properties comprise the ability to infer latent causes of sensory experience, a hierarchical organization of latent causes, and an explicit inclusion of context and action representations. Crucially, these aspects derive from considering the environmental statistics that are relevant to achieve goals, and from the fundamental Bayesian principle that any generative model should be preferred over alternative models based on an accuracy-complexity trade-off. Our account is a step toward elucidating computational principles of categorization and its role within the Bayesian brain hypothesis

The NMDA agonist D-cycloserine facilitates fear memory consolidation in humans

Animal research suggests that the consolidation of fear and extinction memories depends on N-methyl D-aspartate (NMDA)- type glutamate receptors. Using a fear conditioning and extinction paradigm in healthy normal volunteers, we show that postlearning administration of the NMDA partial agonist D-cycloserine (DCS) facilitates fear memory consolidation, evidenced behaviorally by enhanced skin conductance responses, relative to placebo, for presentations of a conditioned stimulus (CS) at a memory test performed 72 h later. DCS also enhanced CS-evoked neural responses in a posterior hippocampus/collateral sulcus region and in the medial prefrontal cortex at test. Our data suggest a role for NMDA receptors in regulating fear memory consolidation in humans

A microRNA negative feedback loop downregulates vesicle transport and inhibits fear memory

The SNARE-mediated vesicular transport pathway plays major roles in synaptic remodeling associated with formation of long-term memories, but the mechanisms that regulate this pathway during memory acquisition are not fully understood. Here we identify miRNAs that are up-regulated in the rodent hippocampus upon contextual fear-conditioning and identify the vesicular transport and synaptogenesis pathways as the major targets of the fear-induced miRNAs. We demonstrate that miR-153, a member of this group, inhibits the expression of key components of the vesicular transport machinery, and down-regulates Glutamate receptor A1 trafficking and neurotransmitter release. MiR-153 expression is specifically induced during LTP induction in hippocampal slices and its knockdown in the hippocampus of adult mice results in enhanced fear memory. Our results suggest that miR-153, and possibly other fear-induced miRNAs, act as components of a negative feedback loop that blocks neuronal hyperactivity at least partly through the inhibition of the vesicular transport pathway.Brain & Behavior Research Foundation (Young Investigator Award)JPB Foundatio

Functional, Non-Clonal IgMa-Restricted B Cell Receptor Interactions with the HIV-1 Envelope gp41 Membrane Proximal External Region

The membrane proximal external region (MPER) of HIV-1 gp41 has several features that make it an attractive antibody-based vaccine target, but eliciting an effective gp41 MPER-specific protective antibody response remains elusive. One fundamental issue is whether the failure to make gp41 MPER-specific broadly neutralizing antibodies like 2F5 and 4E10 is due to structural constraints with the gp41 MPER, or alternatively, if gp41 MPER epitope-specific B cells are lost to immunological tolerance. An equally important question is how B cells interact with, and respond to, the gp41 MPER epitope, including whether they engage this epitope in a non-canonical manner i.e., by non-paratopic recognition via B cell receptors (BCR). To begin understanding how B cells engage the gp41 MPER, we characterized B cell-gp41 MPER interactions in BALB/c and C57BL/6 mice. Surprisingly, we found that a significant (∼7%) fraction of splenic B cells from BALB/c, but not C57BL/6 mice, bound the gp41 MPER via their BCRs. This strain-specific binding was concentrated in IgMhi subsets, including marginal zone and peritoneal B1 B cells, and correlated with enriched fractions (∼15%) of gp41 MPER-specific IgM secreted by in vitro-activated splenic B cells. Analysis of Igha (BALB/c) and Ighb (C57BL/6) congenic mice demonstrated that gp41 MPER binding was controlled by determinants of the Igha locus. Mapping of MPER gp41 interactions with IgMa identified MPER residues distinct from those to which mAb 2F5 binds and demonstrated the requirement of Fc CH regions. Importantly, gp41 MPER ligation produced detectable BCR-proximal signaling events, suggesting that interactions between gp41 MPER and IgMa determinants may elicit partial B cell activation. These data suggest that low avidity, non-paratopic interactions between the gp41 MPER and membrane Ig on naïve B cells may interfere with or divert bnAb responses

Plant-symbiotic fungi as chemical engineers: multi-genome analysis of the Clavicipitaceae reveals dynamics of alkaloid Loci

The fungal family Clavicipitaceae includes plant symbionts and parasites that produce several psychoactive and bioprotective alkaloids. The family includes grass symbionts in the epichloae clade (Epichloë and Neotyphodium species), which are extraordinarily diverse both in their host interactions and in their alkaloid profiles. Epichloae produce alkaloids of four distinct classes, all of which deter insects, and some—including the infamous ergot alkaloids—have potent effects on mammals. The exceptional chemotypic diversity of the epichloae may relate to their broad range of host interactions, whereby some are pathogenic and contagious, others are mutualistic and vertically transmitted (seed-borne), and still others vary in pathogenic or mutualistic behavior. We profiled the alkaloids and sequenced the genomes of 10 epichloae, three ergot fungi (Claviceps species), a morning-glory symbiont (Periglandula ipomoeae), and a bamboo pathogen (Aciculosporium take), and compared the gene clusters for four classes of alkaloids. Results indicated a strong tendency for alkaloid loci to have conserved cores that specify the skeleton structures and peripheral genes that determine chemical variations that are known to affect their pharmacological specificities. Generally, gene locations in cluster peripheries positioned them near to transposon-derived, AT-rich repeat blocks, which were probably involved in gene losses, duplications, and neofunctionalizations. The alkaloid loci in the epichloae had unusual structures riddled with large, complex, and dynamic repeat blocks. This feature was not reflective of overall differences in repeat contents in the genomes, nor was it characteristic of most other specialized metabolism loci. The organization and dynamics of alkaloid loci and abundant repeat blocks in the epichloae suggested that these fungi are under selection for alkaloid diversification. We suggest that such selection is related to the variable life histories of the epichloae, their protective roles as symbionts, and their associations with the highly speciose and ecologically diverse cool-season grasses

Microbial Origin of the Organic Matter Preserved in the Cayo Coco Lagoonal Network, Cuba

The southern part of the tropical Cayo Coco Island (Cuba) hosts a complex, highly evaporative and marine-fed lagoonal network. In the easternmost lagoon of this network, hypersaline conditions favour the development of complex sedimentary microbial ecosystems within the water column at the bottom water-sediment interface and on the shore. Some of these ecosystems are producing microbial mats and biofilms with variable mineralisation rates, depending on their location. Since the mineralisation of these microbial deposits is rare, the sedimentary record does not provide a direct window on the evolution of these ecosystems or their distribution through space and time. However, microbial deposits also produce copious amounts of organic matter, which may be used to decipher any microbial-related origin within the sedimentary record. Microbial mats and biofilms were identified as the potential source of organic material in addition to the surrounding mangrove, soils and suspended particulate matter (SPM). The origin and evolution of the sedimentary organic matter preserved within the lagoonal sediments has been analysed using geochemical parameters such as elemental (TOC, TN and [C/N]atomic ratio) and isotopic (δ13Corg and δ15NTN) signals on four sedimentary cores retrieved from different locations in the lagoon and compared with the geochemical signatures of the potential sources. Despite the high potential for organic matter accumulation in the studied lagoon, the TOC and TN downcore values in sediments that were analysed (i.e., micritic muds and bioclastic sands) remain very low compared to the sediment-water interface. The relative contributions of the different potential sources of organic matter were estimated using [C/N]atomic ratios and δ13Corg values. The δ15NTN signature was discarded as a source signature as it records synsedimentary, early diagenetic, secondary evolution of the nitrogen signal associated with OM remineralisation (i.e., denitrification). Finally, among the microbial deposits, the slime recognised in the permanently submersed zone of the waterbody appears to be the main contributor to the organic matter preserved within the sediments of the lagoon. SPM, mainly composed of microbial-rich particles, also contribute and cannot be ruled out as a source

External controlling factors of modern microbial deposits at the transition between continental and marine environments

Les microbialites, dépôts lithifiés d’origine microbienne, sont des structures organosédimentaires documentées depuis plus de 3,5 milliards d’années. La compréhension de ces structures, des conditions et des modalités de leur mise en place, nous apporte des informations précieuses sur l’origine de la vie sur Terre. Cette thématique de recherche est également en plein essor du fait de ses implications économiques majeures concernant les hydrocarbures. La reconstitution des environnements associés aux structures microbiennes fossiles reste incertaine, notamment entre milieux de dépôts marins ou continentaux. Ces deux environnements contrastés ont été étudiés à travers deux exemples modernes, Cayo Coco (Cuba) en domaine marin lagunaire et le Grand Lac Salé (Utah, USA) en domaine lacustre continental dans le but d’identifier des critères diagnostiques pour discriminer ces environnements dans le registre fossile.La formation des microbialites résulte de la minéralisation et de la lithification d’un tapis microbien sous l’influence d’une composante métabolique (intrinsèque) liée à l’activité du tapis microbien, et d’une composante environnementale (extrinsèque). L’objectif de ce travail était de replacer à différentes échelles les dépôts microbiens dans leur contexte environnemental et de comprendre les influences directes des paramètres extrinsèques sur : (1) les processus de minéralisation des tapis microbiens et la formation de microbialites, (2) la morphologie des structures microbiennes, (3) la distribution spatio-temporelle des microbialites et des sédiments associés, et enfin (4) les relations entre les tapis microbiens et microbialites et leur environnement.Lithified microbial deposits, considered as microbialites, are organosedimentary structures observed since 3.5 billion years. Understanding the mechanisms and environmental conditions leading to their formation may provide valuable information about the origin of life on Earth. Our interest on this research topic has increased owing to the economic implications of these deposits, especially concerning the hydrocarbons. The reconstruction of the environments associated with microbial structures remains uncertain in the fossil record, especially between marine and continental domains. These two contrasting environments were studied through two modern examples: (1) a marine-fed lagoon area in Cayo Coco (Cuba), and the continental lake of the Great Salt Lake (Utah, USA) in order to identify diagnostic criteria allowing to distinguish both environments in the fossil record.Microbialite result from mineralization and lithification of microbial mats under the influence of metabolic chemical reactions (intrinsic) related to the activity of the microbial biomass and environmental (extrinsic) conditions. The objective of this PhD is to replace microbial deposits, at different scales, in their context of formation to highlight the significance of extrinsic parameters on: (1) the mineralization processes and the formation of microbialites, (2) the morphologies of observed microbial structures, (3) their spatial distribution, and (4) the relationship between microbial mats and microbialite and their environment

Microbial origin of the organic matter preserved in the Cayo Coco lagoonal network, Cuba.

34 pagesInternational audienceThe southern part of the tropical Cayo Coco Island (Cuba) hosts a complex, highly evaporative and marine-fed lagoonal network. In the easternmost lagoon of this network, hypersaline conditions favour the development of complex sedimentary microbial ecosystems within the water column at the bottom water-sediment interface and on the shore. Some of these ecosystems are producing microbial mats and biofilms with variable mineralisation rates, depending on their location. Since the mineralisation of these microbial deposits is rare, the sedimentary record does not provide a direct window on the evolution of these ecosystems or their distribution through space and time. However, microbial deposits also produce copious amounts of organic matter, which may be used to decipher any microbial-related origin within the sedimentary record. Microbial mats and biofilms were identified as the potential source of organic material in addition to the surrounding mangrove, soils and suspended particulate matter (SPM). The origin and evolution of the sedimentary organic matter preserved within the lagoonal sediments has been analysed using geochemical parameters such as elemental (TOC, TN and [C/N]atomic ratio) and isotopic (δ13Corg and δ15NTN) signals on four sedimentary cores retrieved from different locations in the lagoon and compared with the geochemical signatures of the potential sources. Despite the high potential for organic matter accumulation in the studied lagoon, the TOC and TN downcore values in sediments that were analysed (i.e., micritic muds and bioclastic sands) remain very low compared to the sediment-water interface. The relative contributions of the different potential sources of organic matter were estimated using [C/N]atomic ratios and δ13Corg values. The δ15NTN signature was discarded as a source signature as it records synsedimentary, early diagenetic, secondary evolution of the nitrogen signal associated with OM remineralisation (i.e., denitrification). Finally, among the microbial deposits, the slime recognised in the permanently submersed zone of the waterbody appears to be the main contributor to the organic matter preserved within the sediments of the lagoon. SPM, mainly composed of microbial-rich particles, also contribute and cannot be ruled out as a source