Characterization of bacterial diversity associated with microbial mats,gypsum evaporites and carbonate microbialites in thalassic wetlands: Tebenquiche and La Brava,Salar de Atacama,Chile

In this paper, we report the presence of sedimentary microbial ecosystems in wetlands of the Salar de Atacama. These laminated systems, which bind, trap and precipitate mineral include: microbial mats at Laguna Tebenquiche and Laguna La Brava, gypsum domes at Tebenquiche and carbonate microbialites at La Brava. Microbial diversity and key biogeochemical characteristics of both lakes (La Brava and Tebenquiche) and their various microbial ecosystems (non-lithifying mats, flat and domal microbialites) were determined. The composition and abundance of minerals ranged from trapped and bound halite in organic-rich non-lithifying mats to aragonite-dominated lithified flat microbialites and gypsum in lithified domal structures. Pyrosequencing of the V4 region of the 16s rDNA gene showed that Proteobacteria comprised a major phylum in all of the microbial ecosystems studied, with a marked lower abundance in the non-lithifying mats. A higher proportion of Bacteroidetes was present in Tebenquiche sediments compared to La Brava samples. The concentration of pigments, particularly that of Chlorophyll a, was higher in the Tebenquiche than in La Brava. Pigments typically associated with anoxygenic phototrophic bacteria were present in lower amounts. Organic-rich, non-lithifying microbial mats frequently formed snake-like, bulbous structures due to gas accumulation underneath the mat. We hypothesize that the lithified microbialites might have developed from these snake-like microbial mats following mineral precipitation in the surface layer, producing domes with endoevaporitic communities in Tebenquiche and carbonate platforms in La Brava. Whereas the potential role of microbes in carbonate platforms is well established, the contribution of endoevaporitic microbes to formation of gypsum domes needs further investigation.     

Mineralogical composition and biomass studies of the microbial mats sediments from the Ebro Delta, Spain.

The mineral composition of the microbial mats at La Banya spit was studied. The spit is formed by a narrow sand bar and a peninsula and is located south of the main body of the Ebro Delta (Tarragona, Spain). Although quartz was the predominant mineral component in all sampling sites, clay, feldspars, calcite, aragonite, halite, dolomite and gypsum were also found. An increase in both the fine material (clay) and the halite content was observed in the sites influenced by nearby salterns. The amount of each mineral did not differ significantly along a 55 cm deep profile, except for halite and aragonite, which reached a maximum in the surface and decreased with depth. Dolomite, which ranged from 0.5 to 5% (w/w), is a possible indicator of sulfate-reducing bacteria activity in the past. Organic carbon and total nitrogen were quantified for biomass assessment. Total nitrogen ranged from 0.1 to 0.56% in the uppermost layer, where the microbial mat is active, but was undetectable at deeper layers. Organic carbon ranged from 1 to 5.5% in the active microbial mat layers and decreased to 0.3% at deeper layers. During the summer, both organic carbon and total nitrogen contents (biomass) of the microbial mat samples from some sites increase, whereas other sites show constant concentrations throughout the year, and others have a fluctuant biomass content.     

Metabolic potential of lithifying cyanobacteria-dominated thrombolitic mats

Thrombolites are unlaminated carbonate deposits formed by the metabolic activities of microbial mats and can serve as potential models for understanding the molecular mechanisms underlying the formation of lithifying communities. To assess the metabolic complexity of these ecosystems, high throughput DNA sequencing of a thrombolitic mat metagenome was coupled with phenotypic microarray analysis. Functional protein analysis of the thrombolite community metagenome delineated several of the major metabolic pathways that influence carbonate mineralization including cyanobacterial photosynthesis, sulfate reduction, sulfide oxidation, and aerobic heterotrophy. Spatial profiling of metabolite utilization within the thrombolite-forming microbial mats suggested that the top 5 mm contained a more metabolically diverse and active community than the deeper within the mat. This study provides evidence that despite the lack of mineral layering within the clotted thrombolite structure there is a vertical gradient of metabolic activity within the thrombolitic mat community. This metagenomic profiling also serves as a foundation for examining the active role individual functional groups of microbes play in coordinating metabolisms that lead to mineralization.     

Photosynthesis versus Exopolymer Degradation in the Formation of Microbialites on the Atoll of Kiritimati,Republic of Kiribati,Central Pacific

Aragonitic microbialites, characterized by a reticulate fabric, were discovered beneath lacustrine microbial mats on the atoll of Kiritimati, Republic of Kiribati, Central Pacific. The microbial mats, with cyanobacteria as major primary producers, grow in evaporated seawater modified by calcium carbonate and gypsum precipitation and calcium influx via surface and/or groundwaters. Despite the high aragonite supersaturation and a high photosynthetic activity, only minor aragonite precipitates are observed in the top parts of the microbial mats. Instead, major aragonite precipitation takes place in lower mat parts at the transition to the anoxic zone. The prokaryotic community shows a high number of phylotypes closely related to halotolerant taxa and/or taxa with preference to oligotrophic habitats. Soil- and plant- inhabiting bacteria underline a potential surface or subsurface influx from terrestrial areas, while chitinase-producing representatives coincide with the occurrence of insect remains in the mats. Strikingly, many of the clones have their closest relatives in microorganisms either involved in methane production or consumption of methane or methyl compounds. Methanogens, represented by the methylotrophic genus Methanohalophilus, appear to be one of the dominant organisms in anaerobic mat parts. All this points to a significant role of methane and methyl components in the carbon cycle of the mats. Nonetheless, thin sections and physicochemical gradients through the mats, as well as the ¹²C-depleted carbon isotope signatures of carbonates indicate that spherulitic components of the microbialites initiate in the photosynthesis-dominated orange mat top layer, and further grow in the green and purple layer below. Therefore, these spherulites are considered as product of an extraordinary high photosynthesis effect simultaneous to a high inhibition by pristine exopolymers. Then, successive heterotrophic bacterial activity leads to a condensation of the exopolymer framework, and finally to the formation of crevice-like zones of partly degraded exopolymers. Here initiation of horizontal aragonite layers and vertical aragonite sheets of the microbialite occurs, which are considered as 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 leads to thin fibrous aragonite veneers. While sulfate reduction, methylotrophy, methanogenesis and ammonification play an important role in element cycling of the mat, there is currently no evidence for a crucial role of them in CaCO₃ precipitation. Instead, photosynthesis and exopolymer degradation sufficiently explain the observed pattern and fabric of microbialite formation.     

Phylogenetic Analysis of a Microbialite-Forming Microbial Mat from a Hypersaline Lake of the Kiritimati Atoll,Central Pacific

On the Kiritimati atoll, several lakes exhibit microbial mat-formation under different hydrochemical conditions. Some of these lakes trigger microbialite formation such as Lake 21, which is an evaporitic, hypersaline lake (salinity of approximately 170‰). Lake 21 is completely covered with a thick multilayered microbial mat. This mat is associated with the formation of decimeter-thick highly porous microbialites, which are composed of aragonite and gypsum crystals. We assessed the bacterial and archaeal community composition and its alteration along the vertical stratification by large-scale analysis of 16S rRNA gene sequences of the nine different mat layers. The surface layers are dominated by aerobic, phototrophic, and halotolerant microbes. The bacterial community of these layers harbored Cyanobacteria (Halothece cluster), which were accompanied with known phototrophic members of the Bacteroidetes and Alphaproteobacteria. In deeper anaerobic layers more diverse communities than in the upper layers were present. The deeper layers were dominated by Spirochaetes, sulfate-reducing bacteria (Deltaproteobacteria), Chloroflexi (Anaerolineae and Caldilineae), purple non-sulfur bacteria (Alphaproteobacteria), purple sulfur bacteria (Chromatiales), anaerobic Bacteroidetes (Marinilabiacae), Nitrospirae (OPB95), Planctomycetes and several candidate divisions. The archaeal community, including numerous uncultured taxonomic lineages, generally changed from Euryarchaeota (mainly Halobacteria and Thermoplasmata) to uncultured members of the Thaumarchaeota (mainly Marine Benthic Group B) with increasing depth.     

The formation of micritic limestones and the development of limestone-marl alternations in the Silurian of Gotland,Sweden

Summary Micritic limestone-marl alternations make up the major part of the Silurian strata on Gotland (Sweden). Their position on the stable Baltic Shield protected them from deep burial and tectonic stress and allowed the preservation of early stages of burial diagenesis, including lithification. In the micritic limestones certain characteristics have been preserved (e.g., pitted microspar crystals, sharp boundaries between microspar and components, lack of deformation phenomena) that offer insights into their formation. We suppose the formation of these micritic limestones and limestone-marl alternations to be based on a rhythmic diagenesis within an aragonite solution zone (ASZ) close below the sediment surface. The micritic limestones are the product of a poikilotopic cementation of carbonate muds which consisted of varying portions of aragonitic, calcitic and terrigenous matter. Their microspar crystals show the primary size and shape of the cements lithifying the original carbonate mud. Dissolution of aragonite in the marls provided the carbonate for the lithification of the limestones. By cementation, the limestone beds evaded further compaction. The marls, which already underwent a volume decrease by aragonite depletion, lacked cement and became more and more compacted due to increasing sedimentary overburden. Although field observations show that primary differences in material influence the development of limestone-marl alternations they are not required for their formation.     

Biogeochemical cycling and microbial diversity in the thrombolitic microbialites of Highborne Cay,Bahamas

Thrombolites are unlaminated carbonate build‐ups that are formed via the metabolic activities of complex microbial mat communities. The thrombolitic mats of Highborne Cay, Bahamas develop in close proximity (1–2 m) to accreting laminated stromatolites, providing an ideal opportunity for biogeochemical and molecular comparisons of these two distinctive microbialite ecosystems. In this study, we provide the first comprehensive characterization of the biogeochemical activities and microbial diversity of the Highborne Cay thrombolitic mats. Morphological and molecular analyses reveal two dominant mat types associated with the thrombolite deposits, both of which are dominated by bacteria from the taxa Cyanobacteria and Alphaproteobacteria. Diel cycling of dissolved oxygen (DO) and dissolved inorganic carbon (DIC) were measured in all thrombolitic mat types. DO production varied between thrombolitic types and one morphotype, referred to in this study as ‘button mats’, produced the highest levels among all mat types, including the adjacent stromatolites. Characterization of thrombolite bacterial communities revealed a high bacterial diversity, roughly equivalent to that of the nearby stromatolites, and a low eukaryotic diversity. Extensive phylogenetic overlap between thrombolitic and stromatolitic microbial communities was observed, although thrombolite‐specific cyanobacterial populations were detected. In particular, the button mats were dominated by a calcified, filamentous cyanobacterium identified via morphology and 16S rRNA gene sequencing as Dichothrix sp. The distinctive microbial communities and chemical cycling patterns within the thrombolitic mats provide novel insight into the biogeochemical processes related to the lithifying mats in this system, and provide data relevant to understanding microbially induced carbonate biomineralization.     

Calcium dynamics in microbialite‐forming exopolymer‐rich mats on the atoll of Kiritimati,Republic of Kiribati,Central Pacific

Microbialite‐forming microbial mats in a hypersaline lake on the atoll of Kiritimati were investigated with respect to microgradients, bulk water chemistry, and microbial community composition. O₂, H₂S, and pH microgradients show patterns as commonly observed for phototrophic mats with cyanobacteria‐dominated primary production in upper layers, an intermediate purple layer with sulfide oxidation, and anaerobic bottom layers with sulfate reduction. Ca²⁺ profiles, however, measured in daylight showed an increase of Ca²⁺ with depth in the oxic zone, followed by a sharp decline and low concentrations in anaerobic mat layers. In contrast, dark measurements show a constant Ca²⁺ concentration throughout the entire measured depth. This is explained by an oxygen‐dependent heterotrophic decomposition of Ca²⁺‐binding exopolymers. Strikingly, the daylight maximum in Ca²⁺ and subsequent drop coincides with a major zone of aragonite and gypsum precipitation at the transition from the cyanobacterial layer to the purple sulfur bacterial layer. Therefore, we suggest that Ca²⁺ binding exopolymers function as Ca²⁺ shuttle by their passive downward transport through compression, triggering aragonite precipitation in the mats upon their aerobic microbial decomposition and secondary Ca²⁺ release. This precipitation is mediated by phototrophic sulfide oxidizers whose action additionally leads to the precipitation of part of the available Ca²⁺ as gypsum.     

Modern carbonate microbialites from an asbestos open pit pond, Yukon, Canada

Microbialites were discovered in an open pit pond at an abandoned asbestos mine near Clinton Creek, Yukon, Canada. These microbialites are extremely young and presumably began forming soon after the mine closed in 1978. Detailed characterization of the periphyton and microbialites using light and scanning electron microscopy was coupled with mineralogical and isotopic analyses to investigate the mechanisms by which these microbialites formed. The microbialites are columnar in form (cm scale), have an internal spherulitic fabric (mm scale), and are mostly made of aragonite, which is supersaturated in the subsaline pond water. Initial precipitation is seen as acicular aragonite crystals nucleating onto microbial biomass and detrital particles. Continued precipitation entombs benthic diatoms (e.g. Brachysira vitrea), filamentous algae (e.g. Oedogonium sp.), dinoflagellates, and cyanobacteria. The presence of phototrophs at spherulite centers strongly suggests that these microbes play an important initial role in aragonite precipitation. Substantial growth of individual spherulites occurs abiotically through periodic precipitation of aragonite that forms concentric laminations around spherulite centers while pauses in spherulite growth allow for colonization by microbes. Aragonite associated with biomass (δ(13)C = -4.6‰ VPDB) showed a (13)C-enrichment of 0.8‰ relative to aragonite exhibiting no biomass (δ(13)C = -5.4‰ VPDB), which suggests a modest removal of isotopically light dissolved inorganic carbon by phototrophs. The combination of a low sedimentation rate, high calcification rate, and low microbial growth rate appears to result in the formation of these microbialites. The formation of microbialites at an historic mine site demonstrates that an anthropogenically constructed environment can foster microbial carbonate formation.     

Active eukaryotes in microbialites from Highborne Cay,Bahamas, and Hamelin Pool (Shark Bay), Australia

Microbialites are organosedimentary structures that are formed through the interaction of benthic microbial communities and sediments and include mineral precipitation. These lithifying microbial mat structures include stromatolites and thrombolites. Exuma Sound in the Bahamas, and Hamelin Pool in Shark Bay, Western Australia, are two locations where significant stands of modern microbialites exist. Although prokaryotic diversity in these structures is reasonably well documented, little is known about the eukaryotic component of these communities and their potential to influence sedimentary fabrics through grazing, binding and burrowing activities. Accordingly, comparisons of eukaryotic communities in modern stromatolitic and thrombolitic mats can potentially provide insight into the coexistence of both laminated and clotted mat structures in close proximity to one another. Here we examine this possibility by comparing eukaryotic diversity based on Sanger and high-throughput pyrosequencing of small subunit ribosomal RNA (18S rRNA) genes. Analyses were based on total RNA extracts as template to minimize input from inactive or deceased organisms. Results identified diverse eukaryotic communities particularly stramenopiles, Alveolata, Metazoa, Amoebozoa and Rhizaria within different mat types at both locations, as well as abundant and diverse signatures of eukaryotes with <80% sequence similarity to sequences in GenBank. This suggests the presence of significant novel eukaryotic diversity, particularly in hypersaline Hamelin Pool. There was evidence of vertical structuring of protist populations and foraminiferal diversity was highest in bioturbated/clotted thrombolite mats of Highborne Cay.     

The Primary Lower Gypsum in the Mediterranean: A new facies interpretation for the first stage of the Messinian salinity crisis

The detailed facies and physical stratigraphic analysis of the Primary Lower Gypsum in the Mediterranean indicates a surprising bed-by-bed correlation at basin-scale (Spain, Italy, Hellenic arc and Cyprus arc), that is tuned to the orbital calibration for the first stage of the Messinian salinity crisis from 5.96 to 5.61 Ma ago. A total of 16, precessionally-controlled, gypsum cycles were deposited rapidly in less than 350 ka, forming sequences up to 300 m thick. The lack of subaerial exposure features and the common facies associations and stacking pattern for sections located thousands of kilometers apart in different geological settings indicates a modest depositional depth, not extremely shallow. Selenite deposition occurred only at the bottom of restricted marginal basins less than 200 m deep, while no gypsum could precipitate in the deeper euxinic Mediterranean portions where only thin and barren shale/dolostone couplets formed. The lowermost selenite beds pass laterally to dolomite-rich limestones interbedded with barren euxinic shales in poorly oxygenated settings, indicating that the gypsum sedimentation was diachronous and did not necessarily mark the onset on the Messinian salinity crisis.Evaporite facies sequences (EF1 to 8) within individual gypsum beds show small-scale, subaqueous sedimentary cycles that mimic regressive-transgressive cycles: a) initial evaporite precipitation at relatively low supersaturation produced the massive selenite (facies EF3) in a relatively deep setting; b) continuous evaporation and drawdown by oscillating brine level formed the banded selenite (EF4) at the aridity acme of the precessionally-controlled cycle; c) general progressive brine level rise with strong brine flow led to the formation of large selenite supercones branching laterally (supercones in Spain and branching selenite, EF5, in the rest of the Mediterranean); and d) flooding by undersaturated continental water terminated gypsum precipitation with the deposition of argillaceous sediments (EF1, Northern Apennines), and/or limestone (EF2, Sicily and Spain) during the humid phase in the precession climate cycle.The stacking pattern and selenite facies associations suggest an overall shallowing-upward trend with a basin-wide hydrologic change starting from the 6th cycle (5.84 Ma), which is marked by the appearance of the branching selenite facies (supercones) in Spain and indicates that the brines became current-dominated. The Sr-isotope stratigraphy suggests that in the Northern Apennines the brines were strongly modified by continental waters (⁸⁷Sr/⁸⁶Sr = 0.708893 to 0.708998), and received direct pulses of Atlantic seawater (⁸⁷Sr/⁸⁶Sr = 0.70900 to 0.709024) only in the upper part of the section. In areas away from the mainland, such as Sicily, the continental input was by far less important.     

Community structure and physiological characterization of microbial mats in Byers Peninsula, Livingston Island (South Shetland Islands, Antarctica)

The community structure and physiological characteristics of three microbial mat communities in Byers Peninsula (Livingston Island, South Shetland Islands, Antarctica) were compared. One of the mats was located at the edge of a stream and was dominated by diatoms (with a thin basal layer of oscillatorian cyanobacteria), whereas the other two mats, located over moist soil and the bottom of a pond, respectively, were dominated by cyanobacteria throughout their vertical profiles. The predominant xanthophyll was fucoxanthin in the stream mat and myxoxanthophyll in the cyanobacteria-dominated mats. The sheath pigment scytonemin was absent in the stream mat but present in the soil and pond mats. The stream mat showed significantly lower delta13C and higher delta15N values than the other two mats. Consistent with the delta15N values, N2 fixation was negligible in the stream mat. The soil mat was the physiologically most active community. It showed rates of photosynthesis three times higher than in the other mats, and had the highest rates of ammonium uptake, nitrate uptake and N2 fixation. These observations underscore the taxonomic and physiological diversity of microbial mat communities in the maritime Antarctic region.     

Two-dimensional mapping of photopigment distribution and activity of Chloroflexus-like bacteria in a hypersaline microbial mat

Pigment analysis in an intact hypersaline microbial mat by hyperspectral imaging revealed very patchy and spatially uncorrelated distributions of photopigments Chl a and BChl a/c, which are characteristic photopigments for oxygenic (diatoms and cyanobacteria) and anoxygenic phototrophs (Chloroflexaceae). This finding is in contrast to the expectation that these biomarker pigments should be spatially correlated, as oxygenic phototrophs are thought to supply the Chloroflexaceae members with organic substrates for growth. We suggest that the heterogeneous occurrence is possibly due to sulfide, whose production by sulfate-reducing bacteria may be spatially heterogeneous in the partially oxic photic zone of the mat. We furthermore mapped the near-infra-red-light controlled respiration of Chloroflexaceae under light and dark conditions and found that Chloroflexaceae are responsible for a major part of oxygen consumption at the lower part of the oxic zone in the mat. The presence of Chloroflexaceae was further confirmed by FISH probe and 16S rRNA gene clone library analysis. We assume that species related to the genera Oscillochloris and 'Candidatus Chlorothrix', in contrast to those related to Chloroflexus and Roseiflexus, depend less on excreted photosynthates but more on the presence of free sulfide, which may explain their presence in deeper parts of the mat.     

Sedimentation on Rasdhoo and Ari Atolls, Maldives, Indian Ocean

A first systematic study of composition, texture, and distribution of modern sediments in two Maldivian atolls reveals the predominance of skeletal carbonates. Fragments of corals, calcareous algae, mollusks, benthic foraminifera, and echinoderms are identified in the grain-size fraction >125 μm. Non-skeletal grains such as cemented fecal pellets and aggregate grains only occur in small percentages. Fragments of skeletal grains, aragonite needles, and nanograins (<1 μm) are found in the grain-size fraction <125 μm. Needles and nanograins are interpreted to be largely of skeletal origin. Five sedimentary facies are distinguished (1–5), for which the Dunham-classification is applied. Fore reef, reef, back reef, as well as lagoonal patch reef and faro areas in both atolls are characterized by the occurrence of coral grainstones (1), which also contain fragments of red coralline algae, the codiacean alga Halimeda, and mollusks. On reef islands, coral-rich sediment is cemented to form intertidal beachrock and supratidal cayrock. Skeletal grains in atoll-interior lagoons are mainly mollusks and foraminifera. The lagoon of Rasdhoo Atoll is covered in the west by mudstones (2), in the center by mollusk packstones (3) and mollusk wackestones (4), and by hard bottoms with corals in the east adjacent to channels through the atoll reef margin. The interior lagoon of Ari Atoll contains mollusk wackestones (4) in the center and mollusk-foraminifer packstones (5). Marginal lagoon areas are characterized by hard bottoms with corals. Facies distribution appears to be an expression of depositional energy, which decreases from the atoll margin towards the center in Ari Atoll, and towards the west in Rasdhoo Atoll. Predominant sediment mineralogies include aragonite and high-magnesium calcite. Mean aragonite content decreases from 90% in coral grainstone to 70–80% in mollusk packstone, mollusk wackestone, and mudstone, and to 50% in mollusk-foraminifer packstone. Stable isotopes of oxygen and carbon in bulk samples range from −3 to −1.5 (δ¹⁸O) and from +0.4 to +3.2 (δ¹³C). It is not possible to delineate facies based on O- and C-isotopes.     

Quaternary dune carbonates from the Mediterranean Coast of Egypt: Petrography and diagenesis

Summary The Quaternary carbonates of the Mediterranean coast of Egypt between Alexandria and Salum appear as parallel limestone ridges rising up to 100 m above sea level. These ridges are dominated by dunal carbonates which differ not only in their primary composition but also by distinct grades of meteoric water diagenesis. Oolitic facies dominates the younger aeolianites of the first and second ridges. Bioclastic facies with abundant coralline algae, benthonic foraminifers, molluscs, echinoderms and intraclasts represents the major rock type in the older aeolianites. Features of meteoric water diagenesis include precipitation of increasing amounts of avoid-filling low Mg-calcite spar, dissolution of aragonite and stabilization of aragonite and high Mg-calcite to low Mg-calcite. Aeolianites below paleosol horizons contain abundant calcrete cements, micritized fossils and detrital grains which are commonly corroded and replaced by calcite. Three stages of progressive meteoric diagenesis are recognised in the Egyptian Quaternary aeolianites. In stage 1 minor precipitation of low Mg-calcite occurs at the grain boundaries. Stage 2 is marked by partial dissolution of aragonite, partial loss of high Mg-calcite and precipitation of low Mg-calcite in some pore spaces. In stage 3, most of the remaining pores are occluded by cementation. At the end of stage 3, Mg is removed from high Mg-calcite but some aragonite still persists. The early vadose cements are represented by miniscus, bridge and pendant cements. The phreatic cements were precipitated as bladed spar in the isopachous rims and equant spar in the intergranular and mouldic porosity. The late vadose cements are represented by micritic cements that were related to calcrete formation. Elemental behaviour during meteoric water diagenesis indicates that it leads to a gradual decrease in bulk Sr along with Na in progressively altered aeolianites. Mn distribution is controlled by the carbonate mineralogy (aragonite versus calcite) as well as the proximity of the aeolianites to the overlying paleosol horizons.     

Bacterial, archaeal and eukaryotic diversity of smooth and pustular microbial mat communities in the hypersaline lagoon of Shark Bay

The bacterial, archaeal and eukaryotic populations of nonlithifying mats with pustular and smooth morphology from Hamelin Pool, Shark Bay were characterised using small subunit rRNA gene analysis and microbial isolation. A highly diverse bacterial population was detected for each mat, with 16S rDNA clones related to Actinobacteria, Bacteroidetes, Chloroflexi, Cyanobacteria, Gemmatimonas, Planctomycetes, Alphaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, Verrucomicrobia and candidate division TM6 present in each mat. Spirochaetes were detected in the smooth mat only, whereas candidate division OP11 was only detected in the pustular mat. Targeting populations with specific primers revealed additional cyanobacterial diversity. The archaeal population of the pustular mat was comprised purely of Halobacteriales, whereas the smooth mat contained 16S rDNA clones from the Halobacteriales, two groups of Euryarchaea with no close characterised matches, and the Thaumarchaea. Nematodes and fungi were present in each mat type, with diatom 18S rDNA clones only obtained from the smooth mat, and tardigrade and microalgae clones only retrieved from the pustular mat. Cultured isolates belonged to the Firmicutes, Gammaproteobacteria, Alphaproteobacteria, Bacteroidetes, Actinobacteria, Cyanobacteria, and Halobacteriales. The mat populations were significantly more diverse than those previously reported for Hamelin Pool stromatolites, suggesting specific microbial populations may be associated with the nonlithifying and lithifying microbial communities of Hamelin Pool.     

Middle Miocene carbonates from the Northern Plateau of the Western Desert (Egypt): Petrography and geochemistry

Summary An extensive carbonate platform of predominately Middle Miocene age (Marmarica Formation) occupies the larger part of the northern plateau of the Western Desert of Egypt. The Marmarica Formation (up to 150m thick) exposed on the cliffs facing the Mediterranean coast consists mainly of alternating limestones and dolostones. Deposition took place in a shallow and normal marine environment. The limestones are dominated by algal boundstone, crossbedded packstone and bioturbated wackestone facies. The occurrence of the crossbedded packstone facies throughout the Marmarica Formation indicates that a shallow marine environment prevailed. Lithification of the precursor carbonates took place mainly in a meteoric environment. Replacement dolomitization ranged from fabric destructive to retentative and from fabric selective to pervasive. The presence of an abundant open marine fauna, the lack of evaporites, coupled with the contents of Sr and Na suggests that dolomitization took place in solutions more dilute than normal sea water. The limestone and dolostone facies cannot be separated into distinct types based on their major or trace element chemistry. Only Mn and Sr seem to be facially controlled. Both elements are particularly enriched in the lagoonal facies compared with the organic buildup facies. The difference in the Mn content is attributed to their proximity to continental areas at the time of deposition. The differences in Sr values are interpreted by an originally differing mineralogy (calcite versus aragonite) and different rates of diagenesis. Dolomitization did not appear to influence the Mn content as substantially as the Sr content. The amount of the acid insoluble residue is controlled by the distribution of Si, Ti, Al, Fe, k, Rb and Zr.     

An actualistic perspective into Archean worlds - (cyano-)bacterially induced sedimentary structures in the siliciclastic Nhlazatse Section, 2.9 Ga Pongola Supergroup, South Africa

Extensive microbial mats colonize sandy tidal flats that form along the coasts of today's Earth. The microbenthos (mainly cyanobacteria) respond to the prevailing physical sediment dynamics by biostabilization, baffling and trapping, as well as binding. This biotic-physical interaction gives rise to characteristic microbially induced sedimentary structures (MISS) that differ greatly from both purely physical structures and from stromatolites. Actualistic studies of the MISS on modern tidal flats have been shown to be the key for understanding equivalent fossil structures that occur in tidal and shelf sandstones of all Earth ages. However, until now the fossil record of Archean MISS has been poor, and relatively few specimens have been found. This paper describes a study location that displays a unique assemblage with a multitude of exceptionally preserved MISS in the 2.9-Ga-old Pongola Supergroup, South Africa. The 'Nhlazatse Section' includes structures such as 'erosional remnants and pockets', 'multidirected ripple marks', 'polygonal oscillation cracks', and 'gas domes'. Optical and geochemical analyses support the biogenicity of microscopic textures such as filamentous laminae or 'orientated grains'. Textures resembling filaments are lined by iron oxide and hydroxides, as well as clay minerals. They contain organic matter, whose isotope composition is consistent with carbon of biological origin. The ancient tidal flats of the Nhlazatse Section record four microbial mat facies that occur in modern tidal settings as well. We distinguish endobenthic and epibenthic microbial mats, including planar, tufted, and spongy subtypes. Each microbial mat facies is characterized by a distinct set of MISS, and relates to a typical tidal zone. The microbial mat structures are preserved in situ, and are consistent with similar features constructed today by benthic cyanobacteria. However, other mat-constructing microorganisms also could have formed the structures in the Archean tidal flats.