台湾地区上下第三系界线划分的孢粉学证据

通过对台湾中部南投县国姓地区北港溪剖面的孢粉样品分析,结合已有的台湾北部基隆地区万里－大武仑露头剖面的孢粉资料,认为台湾地区上下第三系界线置于炭寮地层与十四股层（南投）或公馆凝灰岩与木山层（基隆）之间较为合理。其孢粉组合特征,反映出古气候由晚渐新世经含桤木粉－松粉孢粉组合为特征的寒冷潮湿的北亚热带型向早中新世以含榆科、栎属孢粉组合为特征的温暖湿润的南亚热带型过渡趋势。由于南海北部大陆架北坡的珠海组     

Analysis of growth directions of columnar stromatolites from Walker Lake, western Nevada

Samples of digitate, branching, columnar stromatolites were collected from the steep sides and near horizontal top of four in situ boulders located on the southwestern side of Walker Lake, Nevada, to test the widely held assumption that stromatolite column formation represents a phototropic response. We would predict that the columns on the steeply dipping sides of the boulder would bend upwards toward the light during growth if phototropism was significant during stromatolite morphogenesis. Angle of growth measurements on >300 stromatolites demonstrate that the stromatolites grew nearly normal to their growth surface, regardless of the inclination of their growth surface. No significant differences in the distribution of growth angles between north-, south-, east-, or west-facing samples were observed, and stromatolite lamina thickness did not systematically vary with position on the boulder. The lack of a strong phototropic response does not rule out a biological origin for the Walker Lake structures, but it does suggest that phototropic growth was not a dominant factor controlling stromatolite morphogenesis in these stromatolites and that column formation cannot be uniquely attributed as a phototropic response in stromatolites. It is interesting to note that the morphology of the stromatolites on the top of the boulder is identical to stromatolites on the steep sides. Stromatolite morphogenetic models that predict branching typically require a vertically directed sedimentary component, a feature that would have likely affected the stromatolites on the tops of the boulders, but not the sides, suggesting that other factors may be important in stromatolite morphogenesis.     

Using geographical information techniques to quantify the spatial structure of endolithic boring processes within sediment grains of marine stromatolites

Marine stromatolites are generated through the interactions of environmental parameters and specific microbial processes. The activities of endolithic bacteria, that bore canals through calcium carbonate (CaCO(3)) sand grains (ooids) and reprecipitate the CaCO(3) as a single layer (i.e. micritic laminae) are especially important in the longer term stability of the stromatolite macrostructure. Image analysis and classification approaches have been used previously, but only seldom as a quantitative microscopic tool. Here, we develop a new approach that enables the quantification of microscale (i.e. micrometers to millimeters) spatial structure within marine stromatolites. To demonstrate our approach, images were acquired from two different layers of a stromatolite: "orange layers", where microboring of canals within ooids was relatively abundant, and "white layers" where microboring was greatly reduced or lacking. Images were then transformed into spatial maps. Computation of canal and ooid grain areas within each image was conducted and statistically compared between replicate samples from the two stromatolite layers. This allowed quantification of the areas of ooid grains that were microbored. Based on our results, we suggest that our method could be widely applicable to sedimentary environments, and other areas of fundamental research.     

Light-dependant biostabilisation of sediments by stromatolite assemblages

For the first time we have investigated the natural ecosystem engineering capacity of stromatolitic microbial assemblages. Stromatolites are laminated sedimentary structures formed by microbial activity and are considered to have dominated the shallows of the Precambrian oceans. Their fossilised remains are the most ancient unambiguous record of early life on earth. Stromatolites can therefore be considered as the first recognisable ecosystems on the planet. However, while many discussions have taken place over their structure and form, we have very little information on their functional ecology and how such assemblages persisted despite strong eternal forcing from wind and waves. The capture and binding of sediment is clearly a critical feature for the formation and persistence of stromatolite assemblages. Here, we investigated the ecosystem engineering capacity of stromatolitic microbial assemblages with respect to their ability to stabilise sediment using material from one of the few remaining living stromatolite systems (Highborne Cay, Bahamas). It was shown that the most effective assemblages could produce a rapid (12-24 h) and significant increase in sediment stability that continued in a linear fashion over the period of the experimentation (228 h). Importantly, it was also found that light was required for the assemblages to produce this stabilisation effect and that removal of assemblage into darkness could lead to a partial reversal of the stabilisation. This was attributed to the breakdown of extracellular polymeric substances under anaerobic conditions. These data were supported by microelectrode profiling of oxygen and calcium. The structure of the assemblages as they formed was visualised by low-temperature scanning electron microscopy and confocal laser microscopy. These results have implications for the understanding of early stromatolite development and highlight the potential importance of the evolution of photosynthesis in the mat forming process. The evolution of photosynthesis may have provided an important advance for the niche construction activity of microbial systems and the formation and persistence of the stromatolites which came to dominate shallow coastal environments for 80% of the biotic history of the earth.     

Formation of micro‐spherulitic barite in association with organic matter within sulfidized stromatolites of the 3.48 billion‐year‐old Dresser Formation,Pilbara Craton

The shallow marine and subaerial sedimentary and hydrothermal rocks of the ~3.48 billion‐year‐old Dresser Formation are host to some of Earth's oldest stromatolites and microbial remains. This study reports on texturally distinctive, spherulitic barite micro‐mineralization that occur in association with primary, autochthonous organic matter within exceptionally preserved, strongly sulfidized stromatolite samples obtained from drill cores. Spherulitic barite micro‐mineralization within the sulfidized stromatolites generally forms submicron‐scale aggregates that show gradations from hollow to densely crystallized, irregular to partially radiating crystalline interiors. Several barite micro‐spherulites show thin outer shells. Within stromatolites, barite micro‐spherulites are intimately associated with petrographically earliest dolomite and nano‐porous pyrite enriched in organic matter, the latter of which is a possible biosignature assemblage that hosts microbial remains. Barite spherulites are also observed within layered barite in proximity to stromatolite layers, where they are overgrown by compositionally distinct (Sr‐rich), coarsely crystalline barite that may have been sourced from hydrothermal veins at depth. Micro‐spherulitic barite, such as reported here, is not known from hydrothermal systems that exceed the upper temperature limit for life. Rather, barite with near‐identical morphology and micro‐texture is known from zones of high bio‐productivity under low‐temperature conditions in the modern oceans, where microbial activity and/or organic matter of degrading biomass controls the formation of spherulitic aggregates. Hence, the presence of micro‐spherulitic barite in the organic matter‐bearing Dresser Formation sulfidized stromatolites lend further support for a biogenic origin of these unusual, exceptionally well‐preserved, and very ancient microbialites.     

Microbialites and micro-encrusters in shallow coral bioherms (Middle to Late Oxfordian, Swiss Jura mountains)

Summary Benthic microbial crusts (microbialites or microbolites) are an important component of Middle to Upper Oxfordian shallow-water coral bioherms in the Swiss Jura. They display stromatolitic (laminated), thrombolitic (clotted), and leiolitic (structureless) fabrics, which are distributed heterogeneously throughout the studied sections. The bioherms can be subdivided into coral-microbialite facies, microbialite-dominated facies, and sediment matrix. Macroscopic and microscopic study reveals that microbialitic encrustations commonly occur in two layers. The first one is directly in contact with the substrate and composed of leiolite (locally stromatolite) and a well-diversified micro-encruster fauna; the second one fills the remaining porosity partly or completely with thrombolite and low-diversity micro-encrusters. The growth of the first layer accompanies the growth of the coral reef and thus formed under the same environmental conditions. The second layer is the result of a moving encrustation front filling the remaining porosity (micro- and macrocavities) inside the reef, below the living surface. Both layers play an important role in early cementation. Phototrophic cyanobacteria probably intervene in the formation of the first encrustation zone, whereas heterotrophic bacteria associated to acidic, Ca²⁺-binding macromolecules in biofilms are thought to contribute to the thrombolite inside the reef body. When coral growth cannot take pace with microbialite development, the thrombolite from reaches the surface of the construction and finally covers the reef. The result is a thick interval of thrombolite, which can be interpreted as being related to an ecological crisis in coral-reef evolution. A semi-quantitative analysis of the relative abundance of microbialite types and associated micro-encrusters permits to better constrain the processes leading to a reef crisis. Four micro-encruster associations can be distinguished, and each follows an evolutionary trend in the studied section:Terebella-Tubiphytes dominated,Serpula-Berenicea dominated,Litho-codium dominated, andBacinella dominated. These trends are interpreted to reflect changes in environmental conditions. Bioerosion generally is at its maximum before and after abundant growth of microbialite. According to microbialite-bioerosion relationships and shifts in micro-encruster associations, we propose that the evolution towards a coral-reef crisis involves four main phases: (1) An oligotrophic to low mesotrophic phase when low water turbidity and good oxygenation allow phototrophic metabolisms. This leads to a maximum of coral diversity and development of light-dependent micro-encrusters. (2) A low-mesotrophic phase when increased turbidity and slack water circulation reduce the photic zone and favor heterotrophic micro- and macrofauna. Bioerosion through bivalves increases. (3) A high-mesotrophic phase when environmental conditions are so bad that only microbiatite can be produced. (4) A eutrophic phase when carbonate production is inhibited by high nutrient input and clay flocculation as a result of increased terrestrial run-off. The observed evolutionary trends are not directly linked to changes in bathymetry, but sea-level fluctuations played an important role in opening and closing the depositional environments on the shallow platform. Climatic changes contributed in modulating the influx of siliciclastics and nutrients, and the alkalinity of the water. Demise of coral reefs generally coincides with low sea level and humid climate. Sea-level and climatic fluctuations and, consequently, the crises in reef growth are linked to orbital cycles in the Milandkovitch frequency band.     

A unique algal ridge system in the Exuma Cays, Bahamas

 An algal ridge system discovered along the Exuma Cays, Bahamas constructs bioherms to a thickness of at least 1.5 m and is associated with modern intertidal stromatolites. These algal ridges are unique because they grow in atypical environments characterized by relatively low wave energy, high rates of sedimentation and low rates of herbivory. They also are composed primarily of the branching crustose coralline alga, Neogoniolithon strictum, which heretofore was not known to form algal ridges. Lateral growth rates of crusts, vertical growth rates of branches and survivorship of transplanted N. strictum were greatest in the shallow fore reef zone of the algal ridge. The alga is also capable of surviving and growing when covered with sediment for at least 100 days. Under such conditions it transforms from a branched to an unbranched morphology. Parrotfish grazing, which is said to limit the abundance of branched corallines and algal ridges, was two orders of magnitude lower than in published accounts from other reef systems of the Caribbean and one order of magnitude less than that found on nearby coral reefs of the west Exuma Sound. Neogoniolithon strictum, a delicate and open-branched coralline, persisted for over a year without grazing damage when transplanted to a depth of 2.3 m. This algal ridge-building coralline becomes a well-indurated limestone following submarine lithification of sediment that infills the open branch framework. As a result, N. strictum ridges are comparable to the dense frameworks associated with most algal ridges. Observations of N. strictum -associated bioherms along Central America suggest this ridge system may exist elsewhere under conditions similar to those described for the Bahamas. Accepted: 23 March 1996     

Halimeda bioherms of the northern Great Barrier Reef

The reefless tract directly behind the ribbon reefs on the outer shelf off Cooktown supports a luxuriant growth of Halimeda that, during the Holocene, has developed into bioherms. These mounded biodies of unconsolidated sediment have formed banks that vary in height between 2 and 20 m. Combined shallow, high-resolution seismic reflection profiles and side-scan sonar have diferentiated three areas of biohermal complexes behind the ribbon reefs of Cooktown. Observations by SCUBA and submersible plus the sedimentology of the bioherms indicate that they are in situ accumulations. Evidence from dating of cores suggests that the Halimeda bioherms began to grow about 10 000 years B.P. and their growth has continued to the present time, even though their tops are presently restricted to a depth of -20 m. It is suggested that the origin and morphology of the bioherms are related to a specific hydrodynamic phenomenon, involving jets of nutrient-rich, upwelled oceanic water intruding onto the outer shelf via the narrow passes between the ribbon reefs, and forming eddies behind the ribbons.     

Coral reefs in a clastic sedimentary environment: Fossil (Miocene,S.W. Turkey) and modern (Recent,Red Sea) analogues

Summary The Miocene sequences of southwestern Turkey and the Recent Red Sea are characterised by an unusual and distinct sedimentary association. In both cases near-shore patch and fringing reefs occur along the margins of alluvial fans and braidplains developed under semiarid climatic conditions. In such an environment reef location, morphology and internal structure are controlled primarily by the clastic sedimentary system rather than the independent growth of the framework builders as in normal reef environments. The primary control is a morphological one, the reefs adopting the overall geometry of the sediment body forming a fan shaped arcuate belt. Irregularities in reef morphology are clearly related to the presence of fluvial channels on the fans. The coarse grained gravel of the fans provide an ideal substrate for coral colonisation, this is reflected in the lack of a pioneer community in the Miocene reefs. Periodic rapid burial of the reefs terminates the production of carbonate debris by mechanical and biological erosion resulting in preservation of a relatively unaltered primary framework. The inter-reef sediment consists exclusively of terrigenous sediment.     

Hot spring siliceous stromatolites from Yellowstone National Park: assessing growth rate and laminae formation

Stromatolites are commonly interpreted as evidence of ancient microbial life, yet stromatolite morphogenesis is poorly understood. We apply radiometric tracer and dating techniques, molecular analyses and growth experiments to investigate siliceous stromatolite morphogenesis in Obsidian Pool Prime (OPP), a hot spring in Yellowstone National Park. We examine rates of stromatolite growth and the environmental and/or biologic conditions that affect lamination formation and preservation, both difficult features to constrain in ancient examples. The "main body" of the stromatolite is composed of finely laminated, porous, light-dark couplets of erect (surface normal) and reclining (surface parallel) silicified filamentous bacteria, interrupted by a less-distinct, well-cemented "drape" lamination. Results from dating studies indicate a growth rate of 1-5 cm year(-1) ; however, growth is punctuated. (14)C as a tracer demonstrates that stromatolite cyanobacterial communities fix CO(2) derived from two sources, vent water (radiocarbon dead) and the atmosphere (modern (14)C). The drape facies contained a greater proportion of atmospheric CO(2) and more robust silica cementation (vs. the main body facies), which we interpret as formation when spring level was lower. Systematic changes in lamination style are likely related to environmental forcing and larger scale features (tectonic, climatic). Although the OPP stromatolites are composed of silica and most ancient forms are carbonate, their fine lamination texture requires early lithification. Without early lithification, whether silica or carbonate, it is unlikely that a finely laminated structure representing an ancient microbial mat would be preserved. In OPP, lithification on the nearly diurnal time scale is likely related to temperature control on silica solubility.     

Early Neoproterozoic well-preserved stromatolites from southern Liaoning,North China: characteristics and paleogeographic implications

We report morphology and microstructure of the stromatolites of the Ganjingzi Formation in southern Liaoning. Sedimentologic and morphologic analyses indicate that the lower stromatolite mounds formed in a transgressive succession, while the stromatolite columns in the more complex upper biostrome changed vertically from dispersed growth to dense clumping. Biostratigraphic analysis shows that the stromatolites in the Ganjingzi Formation are similar to those from coeval strata in the Xuzhou-Huainan Region and in southern Jilin. Comparisons of the morphotype genera of stromatolites and the sedimentary setting between different areas, imply that sea-level was fluctuating in the east of the North China Craton (NCC) during the Ganjingzi interval and that the transgressions were beneficial to stromatolite growth, as indicated by the increased number of stromatolites in the study area.     

Influence of hyporheic zone characteristics on the structure and activity of microbial assemblages

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Seasonal and spatial diversity of microbial communities in marine sediments of the South China Sea

This study was conducted to characterize the diversity of microbial communities in marine sediments of the South China Sea by means of 16S rRNA gene clone libraries. The results revealed that the sediment samples collected in summer harboured a more diverse microbial community than that collected in winter, Deltaproteobacteria dominated 16S rRNA gene clone libraries from both seasons, followed by Gammaproteobacteria, Acidobacteria, Nitrospirae, Planctomycetes, Firmicutes. Archaea phylotypes were also found. The majority of clone sequences shared greatest similarity to uncultured organisms, mainly from hydrothermal sediments and cold seep sediments. In addition, the sedimentary microbial communities in the coastal sea appears to be much more diverse than that of the open sea. A spatial pattern in the sediment samples was observed that the sediment samples collected from the coastal sea and the open sea clustered separately, a novel microbial community dominated the open sea. The data indicate that changes in environmental conditions are accompanied by significant variations in diversity of microbial communities at the South China Sea.     

Facies and paleoecology of the late ordovician (Caradoc-Ashgill) stromatoporoid bioherms of Tasmania, Australia

Summary Stromatoporoids, together with other sedentary organisms, form bioherms in the Ordovician Gordon Group which were deposited on a carbonate platform of the Western Tasmanian Terrane. The shallow marine carbonates of the older formations show monotonous lithofacies and biota. The variety of the lithofacies and the diversity of sedentary organisms increases in the younger formations which exhibit evidence of subaerial exposure (fissure fillings and mud-cracks). These phenomena partly reflect the tectonic history of the Western Tasmania Terrane, and probably indicate a general increase in amplitude of sea-level change during the late Ordovician (Caradoc-Ashgill). The bioherms are most frequent in the uppermost horizons (the Den Formation—late Caradoc to early Ashgill?), where the outerops exhibit floatstone and bindstone fabrics. Stromatoporoids and corals construct generally small-scale (less than several meters in width and less than 1 m in height) binding structure. Based on growth forms, stromatoporoid genera are assigned to two morphotypes. Morphotype A generally shows laminar to low domical forms (low height/width ratio) exhibiting ragged margins and sediment inclusions within skeletons. In contrast, morphotype B consists of high domical growth forms (high height/width ratio) and lacks sediment inclusions. These differences in growth forms are interpreted to reflect different modes of biomineralization, together with environmental preferences of the individual organisms. Results of this study and previous publications, overall suggest a progressive development and diversification of biohermal biota in the middle to upper Ordovician of Tasmania. The scale and diversity of the Tasmanian bioherms are probably much smaller than the bioherms and reefs of the younger ages (Silurian and Devonian), and for stromatoporoids, the tendency of diversification is consistent with those of the other Ordovician sections. The Gordon Group provides useful information regarding the early evolutionary history of the Ordovician-Devonian reef-forming communities.     

‘Stromatolites’ built by sponges and microbes – a new type of Phanerozoic bioconstruction

Two ‘stromatolites’ from Carboniferous and Triassic carbonates previously regarded as microbial bioconstructions are analysed and reinterpreted as sponge‐microbial build‐ups. The automicritic aggregations in these build‐ups are similar to the previously reported fossils of keratose demosponges in showing moulded anastomosing filamentous structures. All the studied columnar or domal constructions were formed in turbulent water with high sedimentation rate. The Carboniferous build‐ups were constructed in the shallow subtidal zone of an open shelf or a ramp. The laminations within the stromatolite‐like columns are composed of alternating dark micritic laminae of sponge fossils and pale laminae of neomorphic microspars. The accretion of these columns is probably related to the repeated cycles of sponge growth, rapid lithification after burial, re‐exposure and erosion, and settlement of new generations. The Triassic rocks are presumed to have been precipitated in a slightly evaporitic environment based on lithological features. They show a transition from planar laminae, which were formed under the influence of microbial mats, to stromatolitic columnar or domal build‐ups, which are dominated by stacked micritic clumps of probable sponge fossils. The sponge–microbe alternation may have been controlled by variation of salinity. Comparable with a recent study, this work shows that sponge‐related bioconstructions can be morphologically similar to microbialites in the level of mega‐ and mesostructures.     

Sequence stratigraphy and paleo-oceanography of an open-marine mixed carbonate/siliciclastic succession (Late Jurassic,Southern Germany)

Summary The Late Jurassic epicontinental sea of South Germany protruded far to the North forming a wide bay which was rimmed by shallow-water platforms (Swiss and French Jura). This wide shelf is characterized by extensive downslope mud accumulations including siliceous sponge buildups. The bioherms are aligned along the more pericontinental parts of this shelf, which graded to the South into the Helvetic Basin of the Tethys Ocean. Five sedimentary cycles of Oxfordian to Middle Kimmeridgian age (ox. 2–ox. 3, ki.1.–ki.1.2, ki.3, ki.1.3–ki.2.1, ki. 2.1–ki. 2.2) were used for interregional correlation. Each cycle is characterized by a vertical suite from marl to pure limestone. The bases of the marls are characterized by abundant open-marine fossils, glauconite and phosphate (fish teeth and pellets) and interpreted as condensed sections. Deepening is indicated by bioherms changing their growth form before demise from large structures into small isolated buildups, which commonly occur within deeper water. Sequence boundaries, are present at the transition from marl into limestone. Two phases (middle ki. 1.3 and ki. 1/2) of debris-flow deposition, one accompanied by the sudden spreadout of biostromes and basinward shift of bioherms, are interpreted as lowstand phases associated with sequence boundaries. The remaining sedimentary cycles described here lack such lowstand deposits. Instead, corresponding positions in the cycles are characterized by omission features. This problem is still unresolved. It may either be due to sediment trapping on the ramp or to a changing origin of the cycles compared. It is proposed here that highstand sediments, having higher contents of fine-grained siliciclastics than lowstand deposits, formed during a humid and warm climate with high rates of continued runoff and sediment transport. Lowstand deposits consist of pure offbank carbonates, because the associated drier climate reduced fluvial input of terrigenous material. Corresponding patterns of climatic change are also seen in platform sediments from the Swiss Jura Range; however, climatic cycles correlate with eustatic sea level fluctuations in only about 50% of the cases. This misfit may partially result from problems with biostratigraphic correlation (boreal— Tethyan). Small-scale sedimentary cycles with an average duration of 66’000 years (Mutabilis chron, ki. 2) to 95’000 years (Planula chron, ox. 3) provide a tool for detailed stratigraphic correlation in biostromes, small scale lenticular bioherms (1–2 m thickness) and large bedded bioherms (many tens of metres thickness). Interruptions of bioherm growth are due to temporal oxygen deficiency related to plankton blooms. The bathymetrically deepest bioherms-small and lenticular in shape—therefore suffered the highest number of ecological break-downs, whereas massive bioherms continuously remained above the critical level.     

Importance of the Sedimentary Matrix for Anaerobic Oil Degradation by Marine Bacterial Assemblages

The microbial conversion of petroleum hydrocarbons is increasingly employed in bioremediation efforts. In marine sediments, oxygen levels are characteristically depleted, so anaerobic degradation coupled to sulfate reduction dominates. Prior studies have noted that anaerobic degradation is much reduced in the absence of sediments. In this study, a simple centrifugation protocol was used to extract sediment porewaters to obtain a sediment-free bacterial assemblage capable of anaerobic hydrocarbon degradation. In addition, the factors in sediment that were important to degradation rates were determined. Experiments were designed to differentiate among the effects of increased surface area associated with individual grains in sediments, differing levels of organic constituents in sediments and water, and disparate microbiota within the sedimentary matrix and those free living. Anaerobic alkane degradation, sulfate levels and bacterial community structure were monitored over 90 days in five treatments consisting of Bonny Light crude oil added to (1) intact sediment, (2) sediment-free supernate from centrifuged sediment, (3) supernate plus autoclaved sediment, (4) supernate plus organic-free (combusted) sediment, and (5) a control, with autoclaved supernate plus autoclaved sediment. Lack of surface area associated with sediment grains had little effect on degradation. Separation of porewaters from the sedimentary matrix resulted in loss of bacterial biomass, although this had only a temporary negative effect on degradation rates. Reduction of organic matter due to sediment removal had the largest effect, resulting initially in lower degradation rates. However, sulfate depletion in low organic treatments was also reduced so that long-term loss of alkanes was enhanced.     

Reliability of CARD-FISH Procedure for Enumeration of Archaea in Deep-Sea Surficial Sediments

The enumeration of Archaea in deep-sea sediment samples is still limited, although different methodological procedures have been applied. Among these, catalysed reporter deposition-fluorescence in situ hybridisation (CARD-FISH) technique is a promising tool for estimation of archaeal abundance in deep-sea sediment samples. Comparing different permeabilisation treatments, the best results obtained both on archaeal pure cultures and on natural assemblages were with hydrochloric acid (0.1 M) and proteinase K (0.004 U/ml) treatments. The application of CARD-FISH on deep-sea sediments revealed that Archaea reach up to 41% of total prokaryotic cells. Specific probes for planktonic Archaea showed that marine Crenarchaea dominated archaeal seafloor communities. No clear bathymetric trends were observed for archaeal abundances and the morphology of continental margin (slope vs. canyon) seems not to have a direct influence on archaeal relative abundances. The site-specific sediment habitat—both abiotic environmental setting and sedimentary organic matter quality—explain up to 65% of variance of archaeal, crenarchaeal and euryarchaeal relative abundance, suggesting a wide ecophysiological adaptation to deep-sea benthic ecosystems. The findings demonstrate that Archaea are an important component of benthic microbial assemblages so far neglected, and hence they lay the groundwork for more focused research on their ecological importance in the functioning of deep-sea benthic ecosystems.     

Microbial diversity in modern marine stromatolites, Highborne Cay, Bahamas

Living marine stromatolites at Highborne Cay, Bahamas, are formed by microbial mat communities that facilitate precipitation of calcium carbonate and bind and trap small carbonate sand grains. This process results in a laminated structure similar to the layering observed in ancient stromatolites. In the modern marine system at Highborne Cay, lamination, lithification and stromatolite formation are associated with cycling between three types of microbial communities at the stromatolite surface (Types 1, 2 and 3, which range from a leathery microbial mat to microbially fused sediment). Examination of 923 universal small-subunit rRNA gene sequences from these communities reveals that taxonomic richness increases during transition from Type 1 to Type 3 communities, supporting a previous model that proposed that the three communities represent different stages of mat development. The phylogenetic composition also changes significantly between these community types and these community changes occur in concert with variation in biogeochemical rates. The dominant bacterial groups detected in the stromatolites include Alphaproteobacteria , Planctomycetes , Cyanobacteria and Bacteroidetes . In addition, the stromatolite communities were found to contain novel cyanobacteria that may be uniquely associated with modern marine stromatolites. The implications of these findings are discussed in the context of current models for stromatolite formation.     

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.