Organism motility in an oxygenated shallow-marine environment 2.1 billion years ago.

Evidence for macroscopic life in the Paleoproterozoic Era comes from 1.8 billion-year-old (Ga) compression fossils [Han TM, Runnegar B (1992) Science 257:232-235; Knoll et al. (2006) Philos Trans R Soc Lond B 361:1023-1038], Stirling biota [Bengtson S et al. (2007) Paleobiology 33:351-381], and large colonial organisms exhibiting signs of coordinated growth from the 2.1-Ga Francevillian series, Gabon. Here we report on pyritized string-shaped structures from the Francevillian Basin. Combined microscopic, microtomographic, geochemical, and sedimentologic analyses provide evidence for biogenicity, and syngenicity and suggest that the structures underwent fossilization during early diagenesis close to the sediment-water interface. The string-shaped structures are up to 6 mm across and extend up to 170 mm through the strata. Morphological and 3D tomographic reconstructions suggest that the producer may have been a multicellular or syncytial organism able to migrate laterally and vertically to reach food resources. A possible modern analog is the aggregation of amoeboid cells into a migratory slug phase in cellular slime molds at times of starvation. This unique ecologic window established in an oxygenated, shallow-marine environment represents an exceptional record of the biosphere following the crucial changes that occurred in the atmosphere and ocean in the aftermath of the great oxidation event (GOE).

Subsurface water and clay mineral formation during the early history of Mars.

Clay minerals, recently discovered to be widespread in Mars's Noachian terrains, indicate long-duration interaction between water and rock over 3.7 billion years ago. Analysis of how they formed should indicate what environmental conditions prevailed on early Mars. If clays formed near the surface by weathering, as is common on Earth, their presence would indicate past surface conditions warmer and wetter than at present. However, available data instead indicate substantial Martian clay formation by hydrothermal groundwater circulation and a Noachian rock record dominated by evidence of subsurface waters. Cold, arid conditions with only transient surface water may have characterized Mars's surface for over 4 billion years, since the early-Noachian period, and the longest-duration aqueous, potentially habitable environments may have been in the subsurface.

Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago.

The evidence for macroscopic life during the Palaeoproterozoic era (2.5-1.6 Gyr ago) is controversial. Except for the nearly 2-Gyr-old coil-shaped fossil Grypania spiralis, which may have been eukaryotic, evidence for morphological and taxonomic biodiversification of macroorganisms only occurs towards the beginning of the Mesoproterozoic era (1.6-1.0 Gyr). Here we report the discovery of centimetre-sized structures from the 2.1-Gyr-old black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, which we interpret as highly organized and spatially discrete populations of colonial organisms. The structures are up to 12 cm in size and have characteristic shapes, with a simple but distinct ground pattern of flexible sheets and, usually, a permeating radial fabric. Geochemical analyses suggest that the sediments were deposited under an oxygenated water column. Carbon and sulphur isotopic data indicate that the structures were distinct biogenic objects, fossilized by pyritization early in the formation of the rock. The growth patterns deduced from the fossil morphologies suggest that the organisms showed cell-to-cell signalling and coordinated responses, as is commonly associated with multicellular organization. The Gabon fossils, occurring after the 2.45-2.32-Gyr increase in atmospheric oxygen concentration, may be seen as ancient representatives of multicellular life, which expanded so rapidly 1.5 Gyr later, in the Cambrian explosion.

Oxygen dynamics in the aftermath of the Great Oxidation of Earth's atmosphere.

The oxygen content of Earth's atmosphere has varied greatly through time, progressing from exceptionally low levels before about 2.3 billion years ago, to much higher levels afterward. In the absence of better information, we usually view the progress in Earth's oxygenation as a series of steps followed by periods of relative stasis. In contrast to this view, and as reported here, a dynamic evolution of Earth's oxygenation is recorded in ancient sediments from the Republic of Gabon from between about 2,150 and 2,080 million years ago. The oldest sediments in this sequence were deposited in well-oxygenated deep waters whereas the youngest were deposited in euxinic waters, which were globally extensive. These fluctuations in oxygenation were likely driven by the comings and goings of the Lomagundi carbon isotope excursion, the longest-lived positive δ(13)C excursion in Earth history, generating a huge oxygen source to the atmosphere. As the Lomagundi event waned, the oxygen source became a net oxygen sink as Lomagundi organic matter became oxidized, driving oxygen to low levels; this state may have persisted for 200 million years.

Rapid volcanic ash entombment reveals the 3D anatomy of Cambrian trilobites.

Knowledge of Cambrian animal anatomy is limited by preservational processes that result in compaction, size bias, and incompleteness. We documented pristine three-dimensional (3D) anatomy of trilobites fossilized through rapid ash burial from a pyroclastic flow entering a shallow marine environment. Cambrian ellipsocephaloid trilobites from Morocco are articulated and undistorted, revealing exquisite details of the appendages and digestive system. Previously unknown anatomy includes a soft-tissue labrum attached to the hypostome, a slit-like mouth, and distinctive cephalic feeding appendages. Our findings resolve controversy over whether the trilobite hypostome is the labrum or incorporates it and establish crown-group euarthropod homologies in trilobites. This occurrence of moldic fossils with 3D soft parts highlights volcanic ash deposits in marine settings as an underexplored source for exceptionally preserved organisms.

The role of volcanic-derived clays in the preservation of Ediacaran biota from the Itajaí Basin (ca. 563 Ma, Brazil).

The early evolution of metazoans has been reconstructed by studies on exceptionally preserved molds in siliciclastic rocks from the Ediacaran Period. However, there remains considerable controversy regarding the formation mechanisms of this unusual 'Ediacaran-style' preservation. Proposed hypotheses usually include early authigenesis of minerals, but evidence for this is scarce. In a recently discovered deposit of Ediacaran biota in Brazil, we show that the classic moldic preservation is related to clay mineral authigenesis. Specifically, these clays originated from the alteration of original pyroclastic sediments, likely enhanced by microbial activity, leading to early illitization and morphological templating of the fossiliferous surfaces at a micrometric scale. Such high-fidelity preservation was made possible by rapid burial during volcanic events and the in-situ templating of tissue by clays via microbially-mediated mineralization. This newly described Lagerstätte demonstrates that a number of minerals can facilitate preservation, and that perhaps 'Ediacaran-style' preservation result from different processes leading to the same broad style of preservation.

The Fe-rich clay microsystems in basalt-komatiite lavas: importance of Fe-smectites for pre-biotic molecule catalysis during the Hadean eon.

During the Hadean to early Archean period (4.5-3.5 Ga), the surface of the Earth's crust was predominantly composed of basalt and komatiite lavas. The conditions imposed by the chemical composition of these rocks favoured the crystallization of Fe-Mg clays rather than that of Al-rich ones (montmorillonite). Fe-Mg clays were formed inside chemical microsystems through sea weathering or hydrothermal alteration, and for the most part, through post-magmatic processes. Indeed, at the end of the cooling stage, Fe-Mg clays precipitated directly from the residual liquid which concentrated in the voids remaining in the crystal framework of the mafic-ultramafic lavas. Nontronite-celadonite and chlorite-saponite covered all the solid surfaces (crystals, glass) and are associated with tiny pyroxene and apatite crystals forming the so-called "mesostasis". The mesostasis was scattered in the lava body as micro-settings tens of micrometres wide. Thus, every square metre of basalt or komatiite rocks was punctuated by myriads of clay-rich patches, each of them potentially behaving as a single chemical reactor which could concentrate the organics diluted in the ocean water. Considering the high catalytic potentiality of clays, and particularly those of the Fe-rich ones (electron exchangers), it is probable that large parts of the surface of the young Earth participated in the synthesis of prebiotic molecules during the Hadean to early Archean period through innumerable clay-rich micro-settings in the massive parts and the altered surfaces of komatiite and basaltic lavas. This leads us to suggest that Fe,Mg-clays should be preferred to Al-rich ones (montmorillonite) to conduct experiments for the synthesis and the polymerisation of prebiotic molecules.

Microbially induced potassium enrichment in Paleoproterozoic shales and implications for reverse weathering on early Earth.

Illitisation requires potassium incorporation into a smectite precursor, a process akin to reverse weathering. However, it remains unclear whether microbes facilitate K+ uptake to the sediments and whether illitisation was important in the geological past. The 2.1 billion-year-old Francevillian Series of Gabon has been shown to host mat-related structures (MRS) and, in this regard, these rocks offer a unique opportunity to test whether ancient microbes induced illitisation. Here, we show high K content confined to illite particles that are abundant in the facies bearing MRS, but not in the host sandstone and black shale. This observation suggests that microbial biofilms trapped K+ from the seawater and released it into the pore-waters during respiration, resulting in illitisation. The K-rich illite developed exclusively in the fossilized MRS thus provides a new biosignature for metasediments derived from K-feldspar-depleted rocks that were abundant crustal components on ancient Earth.

Author Correction: Microbially induced potassium enrichment in Paleoproterozoic shales and implications for reverse weathering on early Earth.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Unusual microbial mat-related structural diversity 2.1 billion years ago and implications for the Francevillian biota.

The 2.1-billion-year-old (Ga) Francevillian series in Gabon hosts some of the oldest reported macroscopic fossils of various sizes and shapes, stimulating new debates on the origin, evolution and organization of early complex life. Here, we document ten representative types of exceptionally well-preserved mat-related structures, comprising "elephant-skin" textures, putative macro-tufted microbial mats, domal buildups, flat pyritized structures, discoidal microbial colonies, horizontal mat growth patterns, wrinkle structures, "kinneyia" structures, linear patterns and nodule-like structures. A combination of petrographic analyses, scanning electron microscopy, Raman spectroscopy and organic elemental analyses of carbon-rich laminae and microtexture, indicate a biological origin for these structures. The observed microtextures encompass oriented grains, floating silt-sized quartz grains, concentrated heavy minerals, randomly oriented clays, wavy-crinkly laminae and pyritized structures. Based on comparisons with modern analogues, as well as an average δ13 C organic matter (Corg ) composition of -32.94 ± 1.17‰ (1 standard deviation, SD) with an outlier of -41.26‰, we argue that the mat-related structures contain relicts of multiple carbon pathways including heterotrophic recycling of photosynthetically derived Corg . Moreover, the relatively close association of the macroscopic fossil assemblages to the microbial mats may imply that microbial communities acted as potential benthic O2 oases linked to oxyphototrophic cyanobacterial mats and grazing grounds. In addition, the mat's presence likely improved the preservation of the oldest large colonial organisms, as they are known to strongly biostabilize sediments. Our findings highlight the oldest community assemblage of microscopic and macroscopic biota in the aftermath of the "Great Oxidation Event," widening our understanding of biological organization during Earth's middle age.

Prolonged hypoxia concomitant with serum deprivation induces massive human mesenchymal stem cell death.

Mesenchymal stem cells (MSCs) have been proposed for the repair of damaged tissue including bone, cartilage, and heart tissue. Upon in vivo transplantation, the MSCs encounter an ischemic microenvironment characterized by reduced oxygen (O2) tension and nutrient deprivation that may jeopardize viability of the tissue construct. The aim of this study was to assess the effects of serum deprivation and hypoxia on the MSC survival rates in vitro. As expanded MSCs are transferred from plastic to a scaffold in most tissue engineering approaches, possibly inducing loss of survival signals from matrix attachments, the effects of a scaffold shift on the MSC survival rates were also assessed. Human MSCs were exposed for 48 hours to (i) a scaffold substrate shift, (ii) serum deprivation, and (iii) O2 deprivation. MSCs were also exposed to prolonged (up to 120 hours) hypoxia associated with serum deprivation. Cell death was assessed by Live/Dead staining and image analysis. The MSC death rates were not affected by the shift to scaffold or 48-hour hypoxia, but increased with fetal bovine serum (FBS) starvation, suggesting that between the two components of ischemia, nutrient deprivation is the stronger factor. Long-term hypoxia combined with serum deprivation resulted in the complete death of MSCs (99 +/- 1%), but this rate was reduced by half when MSCs were exposed to hypoxia in the presence of 10% FBS (51 +/- 31%). These results show that MSCs are sensitive to the concurrent serum and O2 deprivation to which they are exposed when transplanted in vivo, and call for the development of new transplantation methods.

Bovine BMP osteoinductive potential enhanced by functionalized dextran-derived hydrogels.

This study evaluated functionalized dextran-derived hydrogels as BMP carriers using both in vitro and in vivo models. In vitro release kinetics indicated that dextran-derived hydrogels could retain rhBMP-2 growth factor in a variable manner depending on their functionalization ratio. The potential of these hydrogels when combined with extracted bovine BMP to enhance the bone formation was evaluated in a rat ectopic model. The largest osteoinduction was found when using hydrogels exhibiting the highest growth factor retention capacity. In addition, some implanted hydrogels demonstrated a capacity to induce an in-vivo calcification certainly related to their chemical composition. These properties make these materials interesting osteoconductive BMP carriers, allowing to decrease the amount of implanted factor required for bone regeneration.

In vitro acoustic waves propagation in human and bovine cancellous bone.

UNLABELLED: The acoustic behavior of cancellous bone with regard to its complex poroelastic nature has been investigated. The existence of two longitudinal modes of propagation is demonstrated in both bovine and human cancellous bone. Failure to take into account the presence of these two waves may result in inaccurate material characterization. INTRODUCTION: Acoustic wave propagation is now a commonly used nondestructive method for cancellous bone characterization. However, wave propagation in this material may be affected by fluid-solid interactions inherent to its poroelastic nature, resulting in two different longitudinal waves. This phenomenon has been demonstrated in previous studies and is in agreement with Biot's theory. The purpose of this paper is to extend these findings to human trabecular bone and to thoroughly investigate these two waves. MATERIALS AND METHODS: Sixty human and 14 bovine cancellous bone cubic specimens were tested in vitro in three different directions using an immersion acoustic transmission method. Original procedures were developed to quantify both velocity and attenuation characteristics of each wave. In term of attenuation, a modified broadband ultrasound attenuation (BUA), describing the rate of change of the frequency-dependent attenuation, was defined for each wave (FDUA). RESULTS: Both waves were identified in most of the specimens. The fast wave velocities demonstrated a negative linear correlation with porosity (1500-2300 m/s, R2 = 0.44, p < 10(-3)), whereas the slow wave velocities exhibited two different behaviors: (1) a first set of data clearly dependent on porosity showing a positive linear correlation (1150-1450 m/s, R2 = 0.26, p < 10(-3)) and (2) a second group independent on porosity. The fast wave FDUA (20-140 dB/cmMHz) showed a parabolic behavior and reached a maximum for 75% porosity (second degree relationship R2 = 0.41,p < 10(-3)), whereas a positive linear behavior was observed for the slow wave FDUA (15-40 dB/cmMHz; R2 = 0.15, p < 10(-2)). CONCLUSIONS: Existence of two wave propagation modes were demonstrated in human cancellous bone. Our data suggest that, in some cases, the amplitude of the slow wave is much larger than the amplitude of the fast wave. For this reason, care should be taken when using measurement systems that incorporate simple threshold detection because the fast wave could remain undetected. Moreover, failure to consider the presence of these two waves could result in an inaccurate quantification of cancellous bone physical properties.

Interaction of specifically chemically modified dextrans with transforming growth factor beta1: potentiation of its biological activity.

Transforming growth factor beta (TGFbeta), a potent multifunctional cytokine, is well known to demonstrate heparin binding ability. This study investigated the binding capacity of heparin-like family of chemically modified dextrans to TGFbeta1. Dextran derivatives with various substitution contents in carboxymethyl, benzylamide and sulfate groups were evaluated using a gel mobility shift assay. This structure-function study indicated that a synergistic role of benzylamide and sulfate substituents resulted in an optimal interaction with the growth factor. The effect of these polymers on the biological response of TGFbeta1 was assessed using mink lung epithelial cells transfected with a plasminogen activator inhibitor-1 promoter-luciferase construct (PAI/Luc). When the growth factor was mixed with 250 microg/mL of carboxymethyl-benzylamide-dextran (DCMB) or carboxymethyl-benzylamide-sulfate-dextran (DCMBSu), the luciferase gene expression was enhanced. Only polymers exhibiting TGFbeta1 binding demonstrated a biological potentiating effect. However, this effect was strongly amplified as the cell plating time increased (35-fold increase with a 2 days plating time versus 1.1-fold increase with a 4 hr plating time at a 0.25 ng/mL concentration of TGFbeta1). TGFbeta1 induced the PAI/Luc construct in a dose-dependent fashion but its effect diminished when added to cells previously cultured for 24 and 48 hr. The results indicated that the potentiating effect required a complex formation between TGFbeta1 and polymers, the action of which seeming to locally maintain TGFbeta1 in an active form. TGFbeta isoforms playing a key role in the process of bone repair, specifically designed functionalized dextrans could potentiate the in vivo TGFbeta1 biological effect and be used in the field of wound healing applications.

A technique for creating critical-size defects in the metatarsus of sheep for use in investigation of healing of long-bone defects.

OBJECTIVE: To develop a technique for use in investigation of healing of long-bone defects by creation of a critical-size defect in the left metarsal III and IV bone (metatarsus) of sheep. ANIMALS: 18 healthy adult sheep. PROCEDURE: Sheep were allocated to 4 groups (3, 3, 5, and 7 sheep in groups 1 to 4, respectively). An ostectomy with various segmental length-to-diaphyseal diameter ratios (0.5, 1.0, 2.0, and 2.0 for groups 1 to 4, respectively) was performed on the left metatarsus of each sheep. The defect was left empty in sheep of groups 1, 2, and 3, whereas the defect was filled with a massive corticocancellous bone autograft in sheep of group 4. RESULTS: All sheep tolerated the surgical procedure well and were able to use the affected limb the day after surgery. Radiographic and histologic examinations conducted 16 weeks after surgery revealed nonunion in all sheep of groups 1, 2, and 3, whereas consistent bone healing with abundant bone formation was observed in all sheep of group 4. CONCLUSIONS AND CLINICAL RELEVANCE: Analysis of these findings suggests that the sheep metatarsal model is a critical-size defect model with low morbidity. It should allow the assessment of new technologies for bone regeneration in conditions closely mimicking the clinical setting. IMPACT FOR HUMAN MEDICINE: Use of this technique in sheep should be of benefit for the preclinical study of osteoconductive, osteoinductive, or osteogenic biomaterials for use in humans.

Interrelations between elastic energy and strain in a tensegrity model: contribution to the analysis of the mechanical response in living cells.

Interactions between the physical and physiological properties of cellular sub-units result in changes in the shape and mechanical behaviour of living tissues. To understand the mechanotransmission processes, models are needed to describe the complex interrelations between the elements and the cytoskeletal structure. In this study, we used a 30-element tensegrity structure to analyse the influence of the type of loading on the mechanical response and shape changes of the cell. Our numerical results, expressed in terms of strain energy as a function of the overall deformation of the tensegrity structure, suggest that changes in cell functions during mechanical stimuli for a given potential energy are correlated to the type of loading applied, which determines the resultant changes in cell shape. The analysis of these cellular deformations may explain the large variability in the response of bone cells submitted to different types of mechanical loading.

The 2.1 Ga old Francevillian biota: biogenicity, taphonomy and biodiversity.

The Paleoproterozoic Era witnessed crucial steps in the evolution of Earth's surface environments following the first appreciable rise of free atmospheric oxygen concentrations ∼2.3 to 2.1 Ga ago, and concomitant shallow ocean oxygenation. While most sedimentary successions deposited during this time interval have experienced thermal overprinting from burial diagenesis and metamorphism, the ca. 2.1 Ga black shales of the Francevillian B Formation (FB2) cropping out in southeastern Gabon have not. The Francevillian Formation contains centimeter-sized structures interpreted as organized and spatially discrete populations of colonial organisms living in an oxygenated marine ecosystem. Here, new material from the FB2 black shales is presented and analyzed to further explore its biogenicity and taphonomy. Our extended record comprises variably sized, shaped, and structured pyritized macrofossils of lobate, elongated, and rod-shaped morphologies as well as abundant non-pyritized disk-shaped macrofossils and organic-walled acritarchs. Combined microtomography, geochemistry, and sedimentary analysis suggest a biota fossilized during early diagenesis. The emergence of this biota follows a rise in atmospheric oxygen, which is consistent with the idea that surface oxygenation allowed the evolution and ecological expansion of complex megascopic life.

Enhancement of the biological activity of BMP-2 by synthetic dextran derivatives.

In the present study, we explored the binding capacity of synthetic heparin-like dextran derivatives to recombinant human bone morphogenetic protein 2 (BMP-2), a heparin-binding osteoinductive growth factor. Affinity electrophoresis analysis provided evidence that carboxymethylated dextran polymers grafted with high amounts of benzylamide groups (named DMCB) interact with BMP-2. The capability of such polysaccharides to potentiate the growth factor biological activity was then investigated. In vitro, DMCB dose-dependently promoted osteoblast differentiation induced by BMP-2 in C2C12 myoblasts more efficiently than heparin. A screening study provided evidence that the potentiating effects of the dextran derivatives on the BMP-2-induced alkaline phosphatase activity improved with their benzylamide groups content and, therefore, with their affinity for the growth factor. The biological activity of BMP-2 was monitored in the culture medium after 6 days using C2C12 cells (containing a BMP sensitive luciferase reporter gene). Like heparin, DMCB sustained the biological activity of the growth factor; this result suggests that the formation of the BMP-2/DMCB complex may protect the protein from being inactivated. In rats in vivo, DMCB also stimulated ectopic calcification mediated by BMP-2. These data indicate that dextran-based polysaccharides prolong the half-life of the growth factor and promote its biological activity.

Benzylaminated dextran-modified hydrogels: a long-term bioactive TGF-beta1 carrier.

Highly porous dextran-based hydrogels [in which various amounts (up to 16.6%, w/w) of a benzylaminated dextran (DMCB) exhibiting high affinity for TGFbeta1 was immobilized] were developed to achieve long-term retention of bioactive TGFbeta1 in situ. Unmodified hydrogels rapidly desorbed 80-90% compared with only 40-60% of the preloaded TGFbeta1 from the DMCB-modified hydrogels during a period of 21 days in PBS in vitro. TGFbeta1 release experiments (performed with high ionic strength solutions) indicated that formation of the complex between TGFbeta1 and functionalized hydrogels was governed by different interactions, depending on the degree of conjugation with DMCB: ionic interactions in the case of weakly conjugated matrices and nonionic interactions in highly conjugated matrices. Using cells containing a TGFbeta-sensitive luciferase reporter gene, weakly DMCB-modified hydrogels sequestered bioactive TGFbeta1 in situ, giving much higher, long-term signaling performance than highly functionalized hydrogels. Because these biocompatible functionalized hydrogels can provide long-term bioactive TGFbeta1, they could be used as scaffolds for cells to stimulate and regulate human tissue repair processes.