Structure Of Microbial Mats Research Articles

Neoproterozoic substrate condition vis-à-vis microbial mat structure and its implications: Sonia Sandstone, Rajasthan, India

The present study deals with the Neoproterozoic mat-infested substrate sculptured by inferred bioturbations, excellently preserved within the ca. 600Ma Sonia Sandstone, northwest India. The bioturbations are horizontal, being preserved on the bed-surfaces and have been classified into linear grooves and discoidal structures. The linear grooves are of two types, one group represents a straight to meandering pattern with raised sand ridges on either side, while the other group represents a mostly meandering pattern with changing width along the length, without raised ridges. Both types of grooves resemble trails of advanced organisms. The discoidal structures have five internal lobes emerging from the centre and merging with the outer ring and may represent body impressions of an organism; alternatively may represent variants of Ediacara or other soft bodied organisms. The interaction of the substrate with these possible bioturbators has great significance in establishing the evolutionary history of the substrate and resulting bioturbations during the Proterozoic. Microbial mats possibly provided oxygen and nutrients to the organisms and played an important role in their preservation. The study also tries to extrapolate the life style of these trace-makers. All these traces may raise speculation about the onset-time of some higher order organisms.

Sedimentary facies analyses from nano- to millimetre scale exploring past microbial activity in a high-altitude lake (Lake Son Kul, Central Asia)

AbstractThe fabric of sedimentary rocks in lacustrine archives usually contains long and continuous proxy records of biological, chemical and physical parameters that can be used to study past environmental and climatic variability. Here we propose an innovative approach to sedimentary facies analysis based on a coupled geomicrobiological and sedimentological study using high-resolution microscopic techniques in combination with mineralogical analyses. We test the applicability of this approach on Lake Son Kul, a high alpine lake in central Tien Shan (Kyrgyzstan) by looking at the mineral fabric and microbial communities observed down to the nanoscale. The characterization of microbe–mineral interactions allows the origin of four carbonate minerals (e.g. aragonite, dolomite, Mg-calcite, calcite) to be determined as primary or diagenetic phases in Lake Son Kul. Aragonite was mainly of primary origin and is driven by biological activity in the epilimnion, whereas diagenetic minerals such as Mg-calcite, calcite, dolomite and pyrite were triggered by bacterial sulphate reduction and possibly by methanotrophic archaea. A new morphotype of aragonite (i.e. spherulite-like precursor) occurs in Unit IV (c. 7100–5000 cal. BP) associated with microbial mat structures. The latter enhanced the preservation of viral relics, which have not yet been reported in Holocene lacustrine sediments. This study advocates that microbe–mineral interactions screened down to the nanoscale (e.g. virus-like particles) can be used successfully for a comprehensive description of the fabric of laminated lake sediments. In this sense, they complement traditional facies sedimentology tools and offer valuable new insights into: (1) the study of microbial and viral biosignatures in Quaternary sediments; and (2) palaeoenvironmental reconstructions.

Significance of fossilized microbes from the Cambrian stromatolites in the Tarim Basin, Northwest China

Samples of the Cambrian microbial dolomites were collected from Penglaiba section, a well-exposed stratigraphic section in the northwestern area of the Tarim Basin. This study provides an analogue for mediated dolomites that can precipitate in microbial mats and biofilms. The Cambrian stromatolitic dolomites were studied using high-resolution scanning electron microscopy. The results are as follows: (1) dolomites with 50 nm to 100 nm spherical nanostructures are aggregated into minerals of larger sphericities; (2) nanospherical dolomites of 50 nm to 170 nm diameter are densely arranged as dumbbell-shaped or chained aggregates; (3) silicified filaments, as well as dumbbell-shaped and chain arrangements, are preserved as important microstructures. On the basis of sedimentological, compositional, geochemical, and petrographic data, the microstructures were interpreted as nanoglobules that function as bacteria in the nucleation and filament mineralization stages. The microstructures function as such because they are wrapped in extracellular polymeric substance (EPS) or mucus and mineralized fossils. Silicification accounts for the exceptional preservation of microbial mat structures, including biofilms, as well as filamentous and coccoid microbes. In addition, EPS process is capable of binding different elements, with preference for Si, Mg, and Ca. Such suitable composition favors microbe mineralization and dolomite nucleation on organic substrates. These microscopic structures suggest bacterial mineralization and provide visual evidence for the origin of microbial dolomites.

Unraveling the three-dimensional morphology of Archean microbialites

Fenestrate microbialites from the 2521±3 Ma Gamohaan Formation, South Africa, are composed of calcite with traces of kerogen that represent the remains of ancient microbial mats. To delineate the 3-D geometry of these microbialites, specimens were serial-sectioned; sequential slices were polished in 120 μm increments and scanned to yield an image stack, which was rendered into a virtual model of the microbialites. The resulting virtual representation allowed for visualization and characterization of microbial growth geometries that were not visible from 2-D surfaces. Several new insights into the structure of microbialites emerged from characterizing their 3-D structure including the recognition of two new features, linear structures and tubular structures. The long, thin nature of these structures makes them difficult to identify in two dimensions. However, in three dimensions, they can be traced as thin ropes of fossilized microbial communities emerging from more typical microbial mat structures. Overall, these results demonstrate a new set of microbial features in the Gamohaan Formation that were only characterized by reconstructing the full geometry of the microbialites in three dimensions.

COVARIANCE OF MICROFOSSIL ASSEMBLAGES AND MICROBIALITE TEXTURES ACROSS AN UPPER MESOPROTEROZOIC CARBONATE PLATFORM

Early diagenetic chert nodules and beds in the upper Mesoproterozoic Angmaat (formerly Society Cliffs) Formation, Baffin and Bylot islands, preserve microfossils and primary petrofabrics that record microbial mat deposition and lithification across a range of peritidal carbonate environments. Five distinct microfossil assemblages document the distribution of mat-building and mat-dwelling populations across a gradient from restricted, frequently exposed flats to more persistently subaqueous environments. Mats built primarily by thin filamentous or coccoidal cyanobacteria give way to a series of more robust forms that show increasing assemblage diversity with decreasing evidence of subaerial exposure. Distinct fabric elements are associated with each microbial assemblage, and aspects of these petrofabrics are recognizably preserved within unsilicified carbonate in the same beds. These include some features that are distinctly geologic in nature (e.g., seafloor cements) and others that reflect microbial growth and decomposition (e.g., tufted microbialites). A particularly distinctive, micronodular fabric is here interpreted as carbonate infilling of primary voids within microbial mat structures. Such structures mark the co-occurrence of cyanobacterial photosynthesis that produced oxygen gas, filamentous mat builders that imparted the coherence necessary to trap gas bubbles, elevated carbonate saturation required to preserve void fabrics via penecontemporaneous cementation, and a relative paucity of detrital sediment that would have inhibited mat growth. Petrofabrics preserved in Angmaat samples are widespread in upper Paleoproterozoic and Mesoproterozoic carbonate successions but are rare thereafter, perhaps recording, at least in part, the declining carbonate saturation state of seawater. Covariation of microfossil assemblages with petrofabrics in both silicified and unsilicified portions of carbonate beds supports hypotheses that link stromatolite microstructure to the composition and diversity of mat communities.

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.

Vertical structure of bi-layered microbial mats from Byers Peninsula, Maritime Antarctica

AbstractA summer study of the vertical structure of bi-layered microbial mats was carried out on Byers Peninsula (Livingston Island, South Shetland Islands). These benthic communities had a common basic structure that consisted of two distinct layers differing in composition, morphology and colour. Our sampling focused on mats showing more layering, which thrived over moist soils and at the bottom of ponds. The photosynthetic pigments analysis performed by high-performance liquid chromatography demonstrated a major occurrence of cyanobacteria and diatoms on these mats, the former being more abundant in relative terms on the surface and composed by morphospecies grouping into orders Oscillatoriales, Nostocales and Chroococcales. The areal chlorophyll a concentrations were slightly higher in the deeper layer although not significantly. Our microscopic and chemical analyses showed that non-active biomass accumulates at the surface. Hence, the upper layers showed the sheath pigment scytonemin and higher amounts of exopolysaccharides, as a strategy to cope with environmental stress. On the other hand, the basal layer was composed of more active photosynthetic microbiota, which also revealed a more balanced stoichiometry. Here we exemplify how environmental stresses are potentially overcome by physiological mechanisms developed by microbial mats which also shape their vertical structure.

Community Structure and Function of High-Temperature Chlorophototrophic Microbial Mats Inhabiting Diverse Geothermal Environments

Six phototrophic microbial mat communities from different geothermal springs (YNP) were studied using metagenome sequencing and geochemical analyses. The primary goals of this work were to determine differences in community composition of high-temperature phototrophic mats distributed across the Yellowstone geothermal ecosystem, and to identify metabolic attributes of predominant organisms present in these communities that may correlate with environmental attributes important in niche differentiation. Random shotgun metagenome sequences from six phototrophic communities (average ∼53 Mbp/site) were subjected to multiple taxonomic, phylogenetic, and functional analyses. All methods, including G + C content distribution, MEGAN analyses, and oligonucleotide frequency-based clustering, provided strong support for the dominant community members present in each site. Cyanobacteria were only observed in non-sulfidic sites; de novo assemblies were obtained for Synechococcus-like populations at Chocolate Pots (CP_7) and Fischerella-like populations at White Creek (WC_6). Chloroflexi-like sequences (esp. Roseiflexus and/or Chloroflexus spp.) were observed in all six samples and contained genes involved in bacteriochlorophyll biosynthesis and the 3-hydroxypropionate carbon fixation pathway. Other major sequence assemblies were obtained for a Chlorobiales population from CP_7 (proposed family Thermochlorobacteriaceae), and an anoxygenic, sulfur-oxidizing Thermochromatium-like (Gamma-proteobacteria) population from Bath Lake Vista Annex (BLVA_20). Additional sequence coverage is necessary to establish more complete assemblies of other novel bacteria in these sites (e.g., Bacteroidetes and Firmicutes); however, current assemblies suggested that several of these organisms play important roles in heterotrophic and fermentative metabolisms. Definitive linkages were established between several of the dominant phylotypes present in these habitats and important functional processes such as photosynthesis, carbon fixation, sulfur oxidation, and fermentation.

Bacteria and diatom co-occurrence patterns in microbial mats from polar desert streams.

The ephemeral stream habitats of the McMurdo Dry Valleys of Antarctica support desiccation and freeze-tolerant microbial mats that are hot spots of primary productivity in an otherwise inhospitable environment. The ecological processes that structure bacterial communities in this harsh environment are not known; however, insights from diatom community ecology may prove to be informative. We examined the relationships between diatoms and bacteria at the community and taxon levels. The diversity and community structure of stream microbial mats were characterized using high-throughput pyrosequencing for bacteria and morphological identification for diatoms. We found significant relationships between diatom communities and the communities of cyanobacteria and heterotrophic bacteria, and co-occurrence analysis identified numerous correlations between the relative abundances of individual diatom and bacterial taxa, which may result from species interactions. Additionally, the strength of correlations between heterotrophic bacteria and diatoms varied along a hydrologic gradient, indicating that flow regime may influence the overall community structure. Phylogenetic consistency in the co-occurrence patterns suggests that the associations are ecologically relevant. Despite these community- and taxon-level relationships, diatom and bacterial alpha diversity were inversely correlated, which may highlight a fundamental difference between the processes that influence bacterial and diatom community assembly in these streams. Our results therefore demonstrate that the relationships between diatoms and bacteria are complex and may result from species interactions as well as niche-specific processes.

Antagonistic interactions between heterotrophic bacteria as a potential regulator of community structure of hypersaline microbial mats

Microbial mats are laminae of self-sustaining microbial communities with a high level of competition for resources. We tested the hypothesis that chemically mediated antagonism is a potential mechanism for structuring the bacterial community. In the co-culturing assay, 57% of the isolates expressed antagonistic behavior toward one or more isolates and 5% of the isolates inhibited more than 80% of the isolates. We observed greater levels of antagonism between isolates from adjacent laminae than within. The bacterial isolate library derived from the mat was predominately Gram-positive, and inhibition within this group was greater than against the few Gram-negative isolates. Microdiversity of 16S rRNA gene was observed for Bacillus marisflavi isolates, which represented 23 of the 75 isolates in the library. Within this and other groups, the patterns of inhibition and sensitivity varied greatly, suggesting rapid gain and loss of the ability to produce antagonistic secondary metabolites and resistance toward such molecules. Our observations are consistent with the hypothesis that antagonistic interactions are a potential mechanism in addition to physiochemical properties that regulate the vertical distribution of aerobic heterotrophic bacteria in hypersaline microbial mats.

Microbial mat structures in profile: The Neoproterozoic Sonia Sandstone, Rajasthan, India

Ubiquitous microorganisms, especially cyanobacteria preferably grow on the sediment surface thereby producing microbial mats. In the absence of grazers and bioturbators, microbial mat is a unique feature of the Proterozoic. Most of the papers so far published described a wide variety of bed surface microbial mat structures with rare illustrations from sections perpendicular to bedding. Nonetheless, bed surface exposures are relatively rare in rock records. This limitation of bed surface exposures in rock records suggest that a study of microbial mats in bed-across sections is needed. The 60 m thick coastal marine interval of the Sonia Sandstone Formation is bounded between two terrestrial intervals, a transgressive lag at the base and an unconformity at the top, and has been chosen for exploration of microbial mat structures in bed-across sections. A wide variety of microbial mat-induced structures in bed-across sections are preserved within the coastal interval of the Sonia Sandstone. Though many of these structures are similar in some aspects with bed surface structures, some of those presented here are new. The palaeogeographic range of these microbial structures extends from supralittoral to neritic. Diagenetic alterations of microbial mats produce pyrite and those zones are suitable for the preservation of microbial remains. SEM and EDAX analyses show fossil preservation of filamentous microbial remains that confirm the presence of microbial mats within the coastal interval of the Sonia Sandstone. Effects of proliferation of microbial mats in the siliciclastic depositional setting are numerous. The mat-cover on sediment surfaces hinders reworking and/or erosion of the sediments thereby increases the net sedimentation rate. Successive deposition and preservation of thick microbial mat layer under reducing environments should have a great potential for hydrocarbon production and preservation and therefore these Proterozoic formations could be a target for exploration.

The genesis of actively growing siliceous stromatolites: Evidence from Lake Specchio di Venere, Pantelleria Island, Italy

This study documents the attributes of siliceous stromatolites growing in the Lake Specchio di Venere, on the volcanic island of Pantelleria, Italy, in a setting characterized by very shallow cold waters and pools and by scattered hydrothermal activity, which exhales mainly CO 2, at emission point temperatures of 34 to 58 °C. The saturation indexes indicate that the lake waters are saturated with respect to tridymite, cristobalite, chalcedony and quartz, and slightly undersaturated with respect to amorphous silica. Common roughly laminated and poorly lithified stromatolites show scanning electron microscope (SEM) evidence for silicified microbial mat structures, including biofilms, filamentous and coccoid cells, and extracellular polymeric substances (EPS). The screening of bacterial 16S rRNA libraries indicates that autotrophic and heterotrophic bacterial communities colonize surface and core levels of the stromatolites. Locally the stromatolites show a granular non-porous fabric, where filaments and silica sheets are not apparent. Inhomogeneity in the stromatolite fabric corresponds with varying DNA content and different structures of the colonizing bacterial communities, non-porous stromatolite levels being microbially colonized to a lesser extent. Based on Fourier Transform (FT)–Raman and FT-infrared investigations, the laminated stromatolite contains early diagenetic tridymite in addition to amorphous silica (opal-A), whereas the non-porous stromatolite shows an essentially amorphous character. The laminated stromatolite is thought to form at relatively low rates of silica precipitation and with a possible microbial mediation in terms of microbial cells and their EPS accelerating the onset of amorphous silica nucleation. It is suggested that high porosity probably favoured a consistent flux of silica-rich fluids that triggered the opal-A to tridymite transformation, thus strengthening the preservation of biosignals. Non-porous stromatolite growth could reflect temporal or localised changes in environmental conditions that caused variations in the degree of silica saturation in the lake waters inducing abiotic silica accumulation. Accelerated opal-A deposition could have obscured primary filamentous fabrics and limited the flux of pore fluid needed for sustaining the process of silica maturation.

Dolomite–silica stromatolites in Miocene lacustrine deposits from the Duero Basin, Spain: the role of organotemplates in the precipitation of dolomite

AbstractThis research provides an ancient analogue for biologically mediated dolomite precipitation in microbial mats and biofilms, and describes the involvement of highly structured extracellular polymeric secretion (EPS) templates in dolomite nucleation. The structure of EPS is shown to match the hexagonal–trigonal lattice geometry of dolomite, which favoured the epitaxial crystallization of dolomite on the organic substrate. This structure of EPS also matches the arrangement of silica nanospheres in opal, which further accounts for the organically‐templated formation of opal enabling the non‐replacive co‐existence of dolomite and silica. The study is focused on a 50 m thick dolomite succession that is exposed in central areas of the Tertiary Duero Basin and was deposited in a mudflat‐saline lake sedimentary complex during the Middle to Late Miocene (9 to 15 Ma). In the intermediate intervals of the succession, poorly indurated dolomite beds pass gradually into silica beds. On the basis of sedimentological, compositional, geochemical and petrographic data, silica and dolomite beds have been interpreted as mineralized microbial mats. The silica beds formed in marginal areas of the lake in response to intense evaporative concentrations; this resulted in the rapid and early precipitation of opal. Silicification accounted for the exceptional preservation of the microbial mat structure, including biofilms, filamentous and coccoid microbes, and EPS. Extracellular polymeric secretions have a layered structure, each layer being composed of fibres which are arranged in accordance with a reticular pattern, with frequent intersection angles at 120° and 60°. Therefore, the structure of EPS matches the lattice geometry of dolomite and the arrangement of silica nanospheres in opal. Additionally, EPS binds different elements, with preference to Si and Mg. The concurrence of suitable composition and surface lattice morphologies in the EPS favoured the crystallization of dolomite on the substrate. In some cases, dolomite nucleation took place epicellularly on coccoid micro‐organisms, which gave way to spheroid crystals. Organic surfaces enable the inorganic mineral precipitation by lowering the free energy barrier to nucleation. Most of the microbial mats probably developed on the lake floor, under sub‐aqueous conditions, where the decomposition of organic matter took place. The subsequent formation of openly packed dolomite crystals, with inter‐related Si‐enriched fibrils throughout, is evidence for the pre‐existence of fibrillar structures in the mats. Miocene dolomite crystals are poorly ordered and non‐stoichiometric, with a slight Ca‐excess (up to 5%), which is indicative of the low diagenetic potential the microbial dolomite has towards a more ordered and stoichiometric structure; this confirms that microbial imprints can be preserved in the geological record, and validates their use as biosignatures.

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.

Microbial mat mediated structures in the Ediacaran Sonia Sandstone, Rajasthan, India, and their implications for proterozoic sedimentation

A strong influence of microbial mats on the physics of Neoproterozoic sedimentation is explored within a 60 m-thick stratigraphic interval within the Sonia Sandstone, Jodhpur Group, western India. This marine interval is bounded by two terrestrial units, its base being marked by a transgressive lag and the top by an unconformity. Progradation from upper neritic, above fair-weather wave base to supralittoral settings was later terminated by a transgression; deposits of lower shoreface-upper shoreface transition thus overlie supralittoral aeolian sandstones, the basal contact of the former deposits being marked by another transgressive lag. A wide spectrum and abundant examples of microbial mat or mat-derived structures supports unusual cohesiveness within granular sand deposited in a high-energy marine environment. The cohesiveness is manifested in abundant preservation of several delicate primary structures and also in their replication in overlying beds, even after they were subjected to high-energy currents. A low rate of sedimentation and severely restricted sediment reworking resulted from prolific mat growth, and consequent depletion in the sediment budget was manifested in bedform migration and evolution. These factors could also have influenced the sequence-building pattern in a distinctive way, as is already reported from several Meso- and Neoproterozoic successions. Ubiquitous mat growth thus had the potential to impart significant distinctions to Proterozoic clastic sedimentary successions.

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.

Characterization of Phototrophic Purple Nonsulfur Bacteria Forming Colored Microbial Mats in a Swine Wastewater Ditch

The community structure of pink-colored microbial mats naturally occurring in a swine wastewater ditch was studied by culture-independent biomarker and molecular methods as well as by conventional cultivation methods. The wastewater in the ditch contained acetate and propionate as the major carbon nutrients. Thin-section electron microscopy revealed that the microbial mats were dominated by rod-shaped cells containing intracytoplasmic membranes of the lamellar type. Smaller numbers of oval cells with vesicular internal membranes were also found. Spectroscopic analyses of the cell extract from the biomats showed the presence of bacteriochlorophyll a and carotenoids of the spirilloxanthin series. Ubiquinone-10 was detected as the major quinone. A clone library of the photosynthetic gene, pufM, constructed from the bulk DNA of the biomats showed that all of the clones were derived from members of the genera Rhodobacter and Rhodopseudomonas. The dominant phototrophic bacteria from the microbial mats were isolated by cultivation methods and identified as being of the genera Rhodobacter and Rhodopseudomonas by studying 16S rRNA and pufM gene sequence information. Experiments of oxygen uptake with lower fatty acids revealed that the freshly collected microbial mats and the Rhodopseudomonas isolates had a wider spectrum of carbon utilization and a higher affinity for acetate than did the Rhodobacter isolates. These results demonstrate that the microbial mats were dominated by the purple nonsulfur bacteria of the genera Rhodobacter and Rhodopseudomonas, and the bioavailability of lower fatty acids in wastewater is a key factor allowing the formation of visible microbial mats with these phototrophs.

Anaerobic Methane Oxidation and Sulfate Reduction in Bacterial Mats on Coral-Like Carbonate Structures in the Black Sea

A detailed study of the processes of anaerobic methane oxidation and sulfate reduction in the bacterial mats occurring on coral-like carbonate structures in the region of methane seeps in the Black Sea, as well as of the phenotypic diversity of sulfate-reducing bacteria developing in this zone, has been performed. The use of the radioisotopic method shows the microbial mat structure to be heterogeneous. The peak activity of the two processes was revealed when a mixture of the upper (dark) and underlying (intensely pink) layers was introduced into an incubation flask, which confirms the suggestion that methanotrophic archaea and sulfate-reducing bacteria closely interact in the process of anaerobic methane oxidation. Direct correlation between the rate of anaerobic methane oxidation and the methane and electron acceptor concentrations in the medium has been experimentally demonstrated. Several enrichment and two pure cultures of sulfate-reducing bacteria have been obtained from the near-bottom water and bacterial mats. Both strains were found to completely oxidize the substrates to CO2 and H2S. The bacteria grow at temperatures ranging from -1 to 18 (24) degrees C, with an optimum in the 10-18 degrees C range, and require the presence of 1.5-2.5% NaCl and 0.07-0.2% MgCl2 x 6H2O. Regarding the aggregate of their phenotypic characteristics (cell morphology, spectrum of growth substrates, the capacity for complete oxidation), the microorganisms isolated have no analogues among the psychrophilic sulfate-reducing bacteria already described. The results obtained demonstrate the wide distribution of psychrophilic sulfate-reducing bacteria in the near-bottom water and bacterial mats covering the coral-like carbonate structures occurring in the region of methane seeps in the Black Sea, as well as the considerable catabolic potential of this physiological group of psychrophilic anaerobes in deep-sea habitats.

Microbial diversity of benthic mats along a tidal desiccation gradient

We investigated the influence of desiccation frequency, indicated by tidal position, on microbial community structure, diversity and richness of microbial mats. We independently characterized cyanobacterial, bacterial and archaeal communities, and their spatial variability for two distinct microbial mat systems: subtidal hypersaline mats and intertidal sand flat mats. Community fingerprints based on 16S rDNA were obtained via denaturing gradient gel electrophoresis using polymerase chain reaction primers specific for each group. Fingerprints for all three groups were consistently similar [> or =85% according to Weighted Pair Group with Arithmetic Mean (WPGMA) analysis] along a 1-km-long transect in subtidal mats. Here, pair-wise comparison analysis yielded minimal variation in diversity and richness for all groups. Fingerprints of three sites along an intertidal transect were heterogenous (> or =32% similarity according to WPGMA analysis) with clear shifts in community structure in all three microbial groups. Here, all groups exhibited statistically significant decreases in richness and diversity with tidal height (as desiccation frequency increases). Regression analysis yielded a strong correlation between diversity or richness estimates and position along the tidal gradient, for both Archaea and Bacteria, with Cyanobacteria exhibiting a weaker correlation. These results suggest that desiccation frequency can shape the structure of microbial mat communities, with Archea being least tolerant and Cyanobacteria most tolerant.

Fe-Si biominerals in the Vilyuchinskie hot springs, Kamchatka Peninsula, Russia.

The micromorphological structure of microbial mats (biomats) from the hot springs of the Vilyuchinskaya hydrothermal system, Kamchatka Peninsula, Russia, were investigated. The Vilyuchinskie hot springs had a discharge temperature of 55-56 degrees C and Na-Ca-HCO3-type waters rich in silicic and boric acids. Water and biomats had high concentrations of Fe, Mn, Sr, and As. Enumeration of total bacterial abundance (TBA) demonstrated a low density of bacterial populations. However, the fractions of metabolically active bacteria and respiring iron-oxidizing bacteria in the hot-spring water were high, comprising 68 and 21% of TBA, respectively. Scanning electron microscopy equipped with an energy dispersive X-ray spectrometer (SEM-EDX) showed that unicellular rod-shaped bacteria about 5-microm long predominated in the brown biomats. The mineral capsules of these bacteria contained large amounts of Fe and Si. Extracellular and intracellular particles were observed by transmission electron microscopy. Fe-oxidizing bacteria were isolated from the biomats on agar plates with selective medium. Therefore, it can be concluded that microorganisms inhabiting the biomats of the Vilyuchinskie hot springs are essential for the deposition of Fe-minerals at neutral pH.