Christian Défarge

Texture of Microbial Sediments Revealed by Cryo-Scanning Electron Microscopy

ABSTRACT Textures of modern lacustrine stromatolites on Kiritimati (Line Islands, Central Pacific Ocean), and of buried layers in the stromatolitic carbonate sediments from French Polynesian atoll lakes (kopara), have been studied using cryo-scanning electron microscopy (SEM equipped with a freeze-drying sample preparation device). This study confirms that microscopic three-dimensional organic networks built through reorganization of polysaccharide fibers inherited from sheaths of dead cyanobacteria, and from other extracellular polymer secretions, are common components of microbial sediments, of which they may form the framework, In addition to this role in sediment cohesion and formation of microstructure, the organic framework appears to be involved in carbonate precipitation with n the stromatolites, through chemical (nucleating), steric, and hydrodynamical controls. The role of the dead organic constituents of the stromatolites is not only confirmed in micrometer-size crystal precipitation but extended to the post mortem internal mineralization of cyanobacterial filaments, and to the formation of peloids that evolve into spherulites. Bacterial, including nannobacterial, carbonate bodies, and carbonate-impregnated cyanobacterial sheaths are also shown to form in the stromatolites studied. All these carbonate precipitation processes may cooperate in lithification of a given sediment.

Role of a cyanobacterial cover on structural stability of sandy soils in the Sahelian part of western Niger

Microbiotic soil crusts, mostly formed by cyanobacteria, are widespread on the surface of fallow land in western Niger. They lie adjacent to completely bare soils. We have investigated the structural stability of these soils by testing aggregate breakdown under fast-wetting, slow-wetting and mechanical breakdown. The tests were effected on aggregates ranging from 3 to 5 mm in size. These experiments were completed by micromorphological examinations under light and scanning electron microscope. For all tests, the fragment size distribution and the mean weight diameter (MWD) revealed the great stability of aggregates from soils with a cyanobacterial cover (MWD 1.82 to 3.10 mm), compared to those from soils devoid of cyanobacterial cover (MWD 0.25 to 1.26 mm). Fast- and slow-wetting of microbiotic soil crust material induced a lesser disaggregation of aggregates compared to mechanical breakdown. On the contrary, fast-wetting and mechanical breakdown of aggregates from soils devoid of cyanobacterial cover induced a greater disaggregation than slow-wetting. Microscopic examination of microbiotic crusts revealed an intricate network of filamentous cyanobacteria and extracellular polymer secretions, which binds and entraps mineral particles on the soil surface. Organo-mineral aggregates ranging from 0.10 to 0.12 mm were observed. Below the superficial crusts, filaments and residual organic matter bind tightly soil particles, thus giving a compact structure. The great stability of aggregates of soil with cyanobacterial cover is likely related to the binding and gluing effect of cyanobacteria and derived organic matter. This is consistent with the positive correlation between MWD values and organic carbon content. The present results thereby confirm the resistance to erosion of soil with microbial cover as indicated by field measurements.

Christmas Island lagoonal lakes, models for the deposition of carbonate{evaporite{organic laminated sediments

The atoll of Christmas Island (now known as Kiritimati) in the Kiribati Republic (Central Pacific) lies at about 28N in the intertropical convergence zone. Much of the surface area of the atoll (ca. 360 km 2 ) is occupied by numerous lakes in which carbonate, evaporite (calcium sulfate, halite) and organic layers are deposited. Observations suggest that deposition of these different laminae is controlled by climatic and biologic factors. It is thought that periodic climatic variations, such as El Nino- Southern Oscillations (ENSO) events which bring heavy rainfall to the atoll, result in the succession of the precipitation of carbonate minerals (during periods after dilution of hypersaline waters by heavy rains), followed by evaporitic minerals (carbonate, calcium sulfate, halite) when salinity increases through evaporation. Thick (up to 5 cm) microbial (essentially cyanobacterial) mats develop continuously on the lake bottom surfaces providing the sediment with an important (total organic carbon 2-5%) organic contribution in the form of an internal, geometrically structured, network in which the authigenic minerals precipitate. The high bioproductivity of these microbial populations is reflected in low d 13 C values of sedimentary organic carbon (214 to 217‰), interpreted as being the result of high atmospheric CO2 demand (Geochim. Cosmochim. Acta, 56 (1992) 335). The well-laminated organic layers present in the sediment profile result from the death and burial of microbial populations at the time of severe climatic events (storms, heavy rainfall). These lagoonal lakes provide a model for the deposition of carbonate and organic matter in an evaporitic environment. The high ratio of deposited carbonate vs. sulfate 1 chloride, when compared to low ratio in evaporitic salinas, results from both a lack of limitation of calcium, magnesium and carbonate ions (in a carbonate reef environment) and active processes of high-Mg calcite precipitation (organomineralization). q 2001 Elsevier Science B.V. All rights reserved.

Kopara in Polynesian atolls: early stages of formation of calcareous stromatolites

Widespread modern calcareous stromatolites known as kopara in the vernacular language on the Tuamotu Archipelago have been found accreting on the bottom of shallow (generally < 1 m depth) lakes located on the rims of these French Polynesian atolls. They are flat, gelatinous sediments, several tens of centimetres thick, with lamination comprised of vertically alternating red organic-rich, and white carbonate-rich, millimetre-thick laminae. They originate from benthic microbial communities mainly composed of cyanobacteria, generally dominated by the genus Phormidium. The living cyanobacteria occupy only the top 1–3 cm of the deposit, the red organic matter below being made of remnants of the dead microbes, essentially of polysaccharide microfibrils inherited from the sheaths and arranged into a three-dimensional network enclosing pores ranging from a few tens of angstroms to a few micrometres wide. The carbonates are predominantly high-Mg calcite (9–19 mole% MgCO3) which precipitated as micron-size bunches, within the pores, on walls of the organic network. It is sometimes accompanied by aragonite (less than 22% of the total mineral fraction when present), and lesser high-Mg calcite, allochthonous bioclasts, which occasionally form up to few centimetres-thick detrital intercalations. The pore waters are variable mixtures of freshwater (rain or ground water) and sea water from the ocean or lagoon which are feeding waters of the lakes. Their chemistry deviate from a straight mixture gradient with respect to Ca2+, Mg2+, and alkalinity under the influence of local calcification and bacterial processes (e.g. ammonia production, sulphate reduction) within the sediments. It is hypothesized that the calcification is initiated at carboxylic sites on the walls of the polysaccharide network, the pores acting as confining organic compartments with increased internal supersaturation. Lamination is suggested to be due to the combined effects of the stratification of the microbial activity of the deposits, and alternations of fresh and saline periods within the lakes. Lacustrine environments on oceanic atolls are found to be important habitats of modern microbialite formation.

Experimental silicification of the extremophilic Archaea Pyrococcus abyssi and Methanocaldococcus jannaschii: applications in the search for evidence of life in early Earth and extraterrestrial rocks

Hydrothermal activity was common on the early Earth and associated micro-organisms would most likely have included thermophilic to hyperthermophilic species. 3.5-3.3 billion-year-old, hydrothermally influenced rocks contain silicified microbial mats and colonies that must have been bathed in warm to hot hydrothermal emanations. Could they represent thermophilic or hyperthermophilic micro-organisms and if so, how were they preserved? We present the results of an experiment to silicify anaerobic, hyperthermophilic micro-organisms from the Archaea Domain Pyrococcus abyssi and Methanocaldococcus jannaschii, that could have lived on the early Earth. The micro-organisms were placed in a silica-saturated medium for periods up to 1 year. Pyrococcus abyssi cells were fossilized but the M. jannaschii cells lysed naturally after the exponential growth phase, apart from a few cells and cell remains, and were not silicified although their extracellular polymeric substances were. In this first simulated fossilization of archaeal strains, our results suggest that differences between species have a strong influence on the potential for different micro-organisms to be preserved by fossilization and that those found in the fossil record represent probably only a part of the original diversity. Our results have important consequences for biosignatures in hydrothermal or hydrothermally influenced deposits on Earth, as well as on early Mars, as environmental conditions were similar on the young terrestrial planets and traces of early Martian life may have been similarly preserved as silicified microfossils.

Nitrogen fixation by microbial crusts from desiccated Sahelian soils (Niger)

Cyanobacterial crusts developing on the sandy and loamy soils of fallow lands in the Sahel (Niger) were investigated for their potential to fix nitrogen. Three sites were selected in this arid environment, differing in sediment type and species composition. In the sandy sites heterocystous nitrogen-fixing cyanobacteria were present, whereas the loamy site did not contain such species. All sites showed light-dependent nitrogenase activity, starting within 2 h after re-wetting of the desiccated crust samples. Inhibition of photosystem II caused a decrease of nitrogenase activity in the samples with heterocystous cyanobacteria, but was stimulatory in the non-heterocystous crust. The results suggest that cyanobacterial crusts may be important for the improvement of the soil by enriching it with nitrogen.

On the appearance of cyanobacterial calcification in modern stromatolites

Abstract In August 1991 calcified (magnesian calcite) filamentous cyanobacteria ( Scytonema mirabile ) appeared on the surface of a modern lacustrine stromatolite (Lake R2, Rangiroa Atoll, Tuamotu Archipelago, French Polynesia). Previously, the topmost photosynthetic layer of this sediment was devoid of such microbial biomineralisers. The calcified cyanobacteria are accompanied by magnesian calcite grains which are evocative of bacterial precipitates. Although the appearance of cyanobacterial calcification might be related to environmental changes in the lake (in particular a decrease in water salinity), the persistence of non-calcified specimens of a filamentous cyanobacteria ( Phormidium crossbyanum ), which was once a dominant species of the lake benthos, shows that calcification and environmental conditions might be only indirectly related. Thus, attempts to use the fossil record of calcareous cyanobacterial skeletons to infer changes in the chemical composition of past oceans cannot be made without due regard of biological processes like the appearance of new competitors.

Structure and Dynamics of Exopolymers in an Intertidal Diatom Biofilm

Diatom biofilms growing at the surface of the intertidal mudflat of Marennes Oléron, France, were incubated for 48 h in the laboratory under simulated conditions of high- and low tide (immersed and emersed in seawater) and day and night (illuminated or dark conditions). The biofilms were subsequently sampled using the cryolander technique, without disturbing the structure. The samples were kept in liquid nitrogen until they were transferred to the cooled stage of a field-emission cryo-scanning electron microscope, which was used to study the structural relationships between the sediment particles, the diatoms and the different types of extracellular polymeric substances (EPS) produced by these organisms. The diatoms were most abundant at the sediment surface when incubated in the light under emersed conditions. In the dark or when immersed, the diatoms migrated into the sediment. In the light, the diatoms were coated with EPS, while this was not the case when incubated in the dark. When immersed, the sediment surface appeared smooth as the result of the deposition of mud. Under emersed conditions, the coarser silt grains were prominently present. These grains were wrapped with organic matter and bound together through threads of EPS. This was the case both in light and in dark incubated sediment. It is proposed that this latter type of EPS contributes to the increased erosion threshold of intertidal mudflats colonized by biofilms of diatoms.

Investigating Physical Processes Leading to Sinkhole Occurrence in Val d’Orléans (France)

Sinkholes in Val d’Orleans occur regularly and can have significant socio-economic impacts. They are due to the presence of a karstified limestone under 5–15-m-thick alluvial deposits. Intense karstification within the area is caused by infiltration of large amount of Loire River water in the karst aquifer through swallow holes. Two mechanisms for the formation of sinkholes are proposed: (1) suffosion of alluvial deposits towards the karstic conduits leading to the occurrence of a void at the base of the alluvium that will progressively enlarge up to reach the surface; (2) karst conduit ceiling breakdown as a consequence of conduit size evolution and/or hydrostatic pressure changes within the karst aquifer. In order to better understand the mechanisms leading to sinkhole occurrence, an experimental site comprising groundwater and surface deformation monitoring has been set up in an area known to be regularly impacted by new sinkholes. First results show similar groundwater level variations in the alluvial and karstic aquifers with, however, a small difference at the beginning of recession limbs when the alluvial aquifer shows higher water levels compared to the karstic aquifer. This situation may favour suffosion and/or sloughing. The comparison of water chemistry between Loire River and karst groundwater seems to indicate very active dissolution processes directly downgradient of the swallow holes and a potential rapid evolution of karst conduit sizes. Such an evolution can contribute to conduit ceiling breakdown resulting in collapse sinkholes. Obtained results bring new insights that will be useful for adequate sinkhole risk management (e.g. hazard mapping and surveillance methodology, etc.).