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Calcification of Blennothrix ganeshii in tropical streams Calcification of the filamentous cyanobacterium Blennothrix ganeshii in calcareous tropical streams of central Mexico region Calcificación de la cianobacteria filamentosa Blennothrix ganeshii en ríos calcáreos tropicales de la región central de México
Abstract
García. 2013. Calcification of the filamentous cyanobacterium Blennothrix ganeshii in calcareous tropical streams of central Mexico region. Hidrobiológica 23 (1): 17-27. ABSTRACT Geochemical, mineralogical and microbiological data from four freshwater streams in central region of Mexico indicate the importance of Blennothrix ganeshii mats (Cyanobacteria, Oscillatoriales) in promoting the formation of calcium car-bonate crystals. The streams were characterized by alkaline waters and relative physicochemical stability during three seasons (cold dry, warm dry and warm rainy). Calcification took the form of a thick, dense layer of calcium carbonate crystals surrounding the extracellular polymeric substances produced by B. ganeshii filaments, giving an appearance of micritic tubes (structures formed by crystallization in the spaces between filaments) along the sheath surfaces. The precipitate was analyzed using X-ray diffraction and energy dispersal X-ray spectrometry, and the calcite crystal habit was determined. The photosynthetic activity of cyanobacterial growth and the presence of abundant extracellular polymeric substances and epiphytic species promote the absorption of ions and mineral nucleation on the surface of the sediment and contribute to the formation of travertine in tropical regions. RESUMEN Los datos geoquímicos, mineralógicos y microbiológicos en cuatro ríos de agua dulce de la región central de México, ponen de manifiesto la importancia de las matas de Blennothrix ganeshii (Cianobacteria, Oscillatoriales) como promo-toras de la formación de cristales de carbonato de calcio. Los ríos se caracterizaron por presentar aguas alcalinas y una relativa estabilidad fisicoquímica durante tres estaciones del año (seca fría, seca templada y lluviosa templada). La calcificación estuvo caracterizada como una densa y gruesa capa de cristales de carbonato de calcio que rodean el mucílago extracelular producido por los filamentos de B. ganeshii, dando la apariencia de tubos micríticos (estructuras formadas por un material cristalizado en las hendiduras existentes entre filamentos) a lo largo de la superficie de la vaina. El precipitado fue identificado como calcita por su hábito cristalino típico y por análisis de difracción de rayos-X y espectrometría de dispersión de energía de rayos X. La actividad fotosintética de los crecimientos de la cianobac-teria y la presencia de abundantes sustancias poliméricas extracelulares y especies epífitas promueven la absorción de iones y nucleación de minerales en la superficie del sedimento y contribuyen a la construcción de travertino en corrientes de regiones tropicales.
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Cyanobacteria have been recognized as key players in the precipitation of calcium carbonate in marine and freshwater systems. These bacteria increase pH, (as a result of photosynthetic activity) and also produce extracellular polysaccharides, which act as binding sites for Ca2+ and CO32- Both processes influence the morphology and the mineralogy of the carbonate minerals. In order to clarify the role of polysaccharides of picocyanobacteria upon calcium carbonate precipitation, both their buffering capacity and ability to induce precipitation need to be investigated. In this experimental study, we characterized the polysaccharides of three unicellular autotrophic picocyanobacterial Synechococcus-type strains by potentiometric titration and infrared spectroscopy. Potentiometric titrations were conducted to determine the total buffering capacity. The nature and concentration of active sites of the polysaccharides was clarified with the aid of potentiometric titration and spectral analysis of an aqueous cellular suspension. Precipitation experiments with polysaccharides of different strains allowed an estimation of their potential to precipitate calcium carbonate. The results presented here indicate that polysaccharides from cyanobacteria have a strong potential to exchange protons with their surrounding environment. Precipitation experiments demonstrated that extracellular polysaccharides of all the strains studied able to precipitate calcium carbonate.
Bacteria are small, prokaryotic, microorganisms that are ubiquitous in surface and subsurface terrestrial and aquatic habitats. Prokaryotes comprise two Domains (Superkingdoms) in the biological taxonomic hierarchy, the Bacteria and the Archaea. They exhibit remarkable diversity both genetically and metabolically even within the same microenvironment and they are thought to play a major role in the deposition and weathering of minerals in the earth’s crust. The synthesis of minerals by prokaryotes can be grouped into two canonical modes: 1) biologically induced mineralization (BIM) and 2) biologically controlled mineralization (BCM) (Lowenstam 1981; Lowenstam and Weiner 1989). In this chapter, we focus on biologically induced mineralization.
Minerals that form by biologically induced mineralization processes generally nucleate and grow extracellularly as a result of metabolic activity of the organism and subsequent chemical reactions involving metabolic byproducts. In many cases , the organisms secrete one or more metabolic products that react with ions or compounds in the environment resulting in the subsequent deposition of mineral particles. Thus, BIM is a presumably unintended and uncontrolled consequence of metabolic activities. The minerals that form are often characterized by poor crystallinity , broad particle-size distributions, and lack of specific crystal morphologies. In addition , the lack of control over mineral formation often results in poor mineral specificity and/or the inclusion of impurities in the mineral lattice. BIM is, in essence, equivalent to inorganic mineralization under the same environmental conditions and the minerals are therefore likely to have crystallochemical features that are generally indistinguishable from minerals produced by inorganic chemical reactions. In some cases, the metabolic products diffuse away and minerals form from solution. However , bacterial surfaces such as cell walls or polymeric materials (exopolymers) exuded by bacteria, including slimes, sheaths, or biofilms, and even dormant spores, can act as important sites for the adsorption of ions …
Como resultado de un estudio ficoflorístico en tres localidades del estado de Morelos, se registraron 100 especies algales con las siguientes proporciones: 47 Cyanophyceae, 39 Bacillariophyceae, diez Chlorophyceae, tres Rhodophyceae y una Xanthophyceae. Se elaboraron claves dicotómicas para cada una de las clases, y se incluyen las descripciones para todas las especies, acompañadas de fotografías y esquemas.
Twenty-seven contemporary freshwater carbonate deposits were investigated (26 in the British Isles and 1 in S. Australia). The blue-green algae Schizothrix calcicola and Microcoleus vaginatus occurred at 23 of the sites. The remaining sites represented areas where deposition had ceased. About 1% of the dry mass of the deposits consisted of Cyanophyta. The assimilation rates of these algae, measured by 14CO2 uptake, were low. It was estimated that the equilibrium change caused by CO2 uptake due to photosynthesis could account for only 1-2% of the precipitated carbonate. Cultures of S. calcicola failed to deposit carbonates in solid and liquid media. Deposition rates varied between 0.006 and 8 mm a-1 (mean 1.2 mm a-1). The laminations encountered at some sites could be correlated to seasonal differences in cyanophyte growth which enabled some deposits to be dated. Thin sections revealed small crystals adhering to the mucilaginous sheaths of the algae. The transmission of light in some deposits was investigated. The algal layer corresponded to the depth to which light was attenuated to about 1% of the surface intensity.
The genus Blennothrix belongs to the family Oscillatoriaceae (subfamily Oscillatorioideae), differing from others in the subfamily by the presence of several trichomes in a sheath. Freshwater members of Blennothrix have been recently divided into eight species (Komárek 1998), with the taxon B. ganeshii Watanabe et Komárek reported from three sites in two tropical basins of the central region of Mexico. However, information on anatomical (vegetative and reproductive) characters and environmental distribution is scarce and species determination is uncertain. This study analyses the anatomical structure of Blennothrix populations distributed in different sites along the central tropical region of Mexico in order to determine the taxonomic status of the studied populations. Eight populations were sampled, with concurrent environmental data recorded. Morphological characters previously considered to be of taxonomic importance, as well as complementary features such as algal mat length, filament diameter, trichome width, length of cells, and thickness and shape of the sheath were measured in several filaments of each sample. Our results showed that all the observed populations in the central region of Mexico fit within the circumscription of B. ganeshii (Watanabe & Komárek 1989, Komárek 1998). However, two important morphological features were observed in all populations. First, branches were rarely present and varying in frequency from 1 to 3 trichomes per filament. Second, the presence of a transverse lamellation due to constriction in the longitudinal axis of the sheath was observed. Some morphological characters described for this species were extended.
Dendritic calcite forms in an active cold-water tufa system in association with extracellular polymeric substances (EPS) that discontinuously coat bryophytes and cyanobacteria. Dendrites consist of 100–200 nm thick calcite fibres that form 3D lattice-like domains. In each dendrite domain, fibres have three structurally equal orientations, which correspond in disposition to radii from the centre of a calcite unit cell to the convex triple face junctions on its surface. Fibres do not form in the orientation of the c-axis. The external form of each dendrite has the shape of half of a shortened octahedron, with an upper triangular surface parallel to the substrate. Dendrite nucleation takes place on or in microbial EPS, whether microbial cells are present or not, and is probably effected by attraction of Ca2+ cations to negatively charged EPS, together with CO2-degassing and concomitant pH increase of supersaturated spring water in stream splash zones. Ensuing dendrite growth is abiogenic and controlled by diffusion. Dendrite c-axes are perpendicular to the substrate, probably because the negative charge of EPS forces the orientation of Ca2+ and CO planes within the developing dendrite crystal to be parallel to the EPS film surface. Dendrites are eventually filled and overgrown by solid, syntaxial calcite, which gradually and completely obliterates the dendrites as more familiar calcite crystal forms develop. No trace of the dendritic nucleus remains in the rock record. Calcite crystal nucleation may take place by this mechanism in many marine and meteoric settings, given that microbial EPS is now assumed to be virtually ubiquitous in these environments. This phenomenon could contribute to the development of familiar fabrics such as marine micrite cement and fibrous calcite cement, radial ooids, peloids, ‘abiogenic’ stromatolites, sea floor precipitates, microbialites, tufa, travertine, speleothems, and some meteoric cements. It may also contribute to the substrate-normal orientation of c-axes of common cement fabrics.
RESUMEN El Marjal de Pego-Oliva es una de las zonas húmedas más importantes del litoral mediterráneo español y sirve de refugio a comunidades vegetales y animales, muy amenazadas en otros lugares. Las actividades humanas en la zona se remontan a la edad del bronce y todavía quedan restos de los asentamientos romanos y musulmanes. En la actualidad se trata de una impor-tante zona del Levante Español en el cultivo del arroz y de cítricos. Este humedal presenta un gradiente de salinidad desde aguas dulces a salobres que se manisfiesta en el cambio de las comunidades de algas y plantas. La diversidad de macro y microalgas en la zona es alta, a pesar de que nunca ha sido estudiada de forma intensiva, y se desarrollan especies de afini-dad tropical. El grado de singularidad de este enclave se hace patente al compararlo con los restantes humedales litorales de la vertiente española. Se comparan los datos obtenidos en este estudio con datos previos, los más antiguos de los cuales pro-ceden de hace 20 años, en un intento de mostrar la evolución de la flora y hacer una llamada de atención sobre el interés de la conservación de la zona, hoy ya declarada parque natural, pero sometida todavía a un gran número de presiones. El número total de especies reconocido en la actualidad está probablemente muy por debajo del real, debido a que el enorme número de ambientes diferentes existente en la zona, muchos de ellos temporales, hacen su muestreo bastante complicado y a que se care-ce por completo de datos de algunos de los grupos algales.
Cyanobacteria have affected major geochemical cycles (carbon, nitrogen, and oxygen) on Earth for billions of years. In particular,
they have played a major role in the formation of calcium carbonates (i.e., calcification), which has been considered to be
an extracellular process. We identified a cyanobacterium in modern microbialites in Lake Alchichica (Mexico) that forms intracellular
amorphous calcium-magnesium-strontium-barium carbonate inclusions about 270 nanometers in average diameter, revealing an unexplored
pathway for calcification. Phylogenetic analyses place this cyanobacterium within the deeply divergent order Gloeobacterales.
The chemical composition and structure of the intracellular precipitates suggest some level of cellular control on the biomineralization
process. This discovery expands the diversity of organisms capable of forming amorphous calcium carbonates.
A review is given of the occurrence, ecological significance and physiological mechanism of bicarbonate utilization by aquatic photosynthetic organisms. Following a short description of the carbon dioxide water system, the significance of the unstirred layer and the slow diffusion rate of CO2 in water is discussed. Three experimental procedures are discussed to establish the use of bicarbonate: establishing the relationship between pH and rate of carbon assimilation; the kinetic-reaction approach, which uses the slow equilibration between CO2 and HCO3− in water; the difference in discrimination between 12C and 13C during CO2 and HCO3− assimilation. The ability to use HCO3− occurs in many different groups of aquatic photosynthetic organisms, with the notable exception of Bryophytes and Pteridophytes, and is related to growth conditions. Polarity, that is the spatial separation of HCO3− influx and resulting OH− efflux, is observed in higher plant species and Characeae that assimilate HCO3−. These polar leaves or banded Characean cells are often used to study the mechanism. Three mechanisms for HCO3− assimilation apparently exist: H+HCO3− symport, external acidification of HCO3− into CO2 and an increased rate of conversion of HCO3− into CO2 by the action of the enzyme carbonic anhydrase. The role of transfer cells or plasmalemmasomes, often observed in HCO3− assimilating leaves, is still unclear. Their role may be to create sufficient space for membrane-bound enzyme systems by increasing the membrane surface area or to confine an extracellular space with a low pH value. The ecological significance of photosynthetic HCO3− utilization seems to be that it compensates for slow supply of CO2 by diffusion and/or suppresses photorespiration by creating a high intracellular CO2 concentration.
Five test runs were performed to assess possible bias when performing the loss on ignition (LOI) method to estimate organic matter and carbonate content of lake sediments. An accurate and stable weight loss was achieved after 2 h of burning pure CaCO₃ at 950 °C, whereas LOI of pure graphite at 530 °C showed a direct relation to sample size and exposure time, with only 40–70% of the possible weight loss reached after 2 h of exposure and smaller samples losing weight faster than larger ones. Experiments with a standardised lake sediment revealed a strong initial weight loss at 550 °C, but samples continued to lose weight at a slow rate at exposure of up to 64 h, which was likely the effect of loss of volatile salts, structural water of clay minerals or metal oxides, or of inorganic carbon after the initial burning of organic matter. A further test-run revealed that at 550 °C samples in the centre of the furnace lost more weight than marginal samples. At 950 °C this pattern was still apparent but the differences became negligible. Again, LOI was dependent on sample size. An analytical LOI quality control experiment including ten different laboratories was carried out using each laboratory’s own LOI procedure as well as a standardised LOI procedure to analyse three different sediments. The range of LOI values between laboratories measured at 550 °C was generally larger when each laboratory used its own method than when using the standard method. This was similar for 950 °C, although the range of values tended to be smaller. The within-laboratory range of LOI measurements for a given sediment was generally small. Comparisons of the results of the individual and the standardised method suggest that there is a laboratory-specific pattern in the results, probably due to differences in laboratory equipment and/or handling that could not be eliminated by standardising the LOI procedure. Factors such as sample size, exposure time, position of samples in the furnace and the laboratory measuring affected LOI results, with LOI at 550 °C being more susceptible to these factors than LOI at 950 °C. We, therefore, recommend analysts to be consistent in the LOI method used in relation to the ignition temperatures, exposure times, and the sample size and to include information on these three parameters when referring to the method.
The global carbonate cycle has been controlled and maintained by life processes for at least 3.5 billion years. Within the exogenic carbonate cycle and in very different environments cyanobacteria appear in ‘key positions’ in that they actively and passively influence carbonate cycling. In the carbonate cycle, cyanobacteria play an important and sometimes decisive role. Cycling of carbon and carbonate is linked to biological processes. Some build up specific carbonate structures, some destroy carbonate substrates and others do both simultaneously. All these processes take place from the high mountains down to the sea in various terrestrial and freshwater as well as marine environments. The photosynthetic activity of cyanobacteria, their extracellular polymeric substances and possibly also their adherent heterotrophic bacteria are responsible for the construction of various carbonate structures and the ability to penetrate carbonate material. Boring activity of euendoliths results in biological corrosion and disintegration of carbonate surfaces. Grazing organisms on carbonate surfaces colonized by epi- and endolithic cyanobacteria produce specific biokarst forms and specific grains which can contribute to nearshore sedimentation. Biological corrosion and abrasion together constitute bioerosion. The results of all these processes are calcareous crusts, typical traces and biokarst forms which in many cases have a high fossilization potential, and therefore can be powerful ecological, palaeoecological and facies indicators in recent as well as in fossil environments.
Freshwater carbonates (tufas) develop today from the Arctic to the tropics, many being localized about springs and upper water courses. Some Quaternary tufas, especially in the Mediterranean region, extend over tens of square kilometres and exceed 30 m in thickness. Radiometric dating of Holocene deposits shows that many have accumulated at an average rate of 1 mm year−1. However, local precipitation may be much faster and some Holocene deposits may even have outpaced their tropical marine carbonate counterparts. Recently, the study of active sites has attempted to quantify the precipitation mechanisms which lead to tufa deposition. However, field observation and sampling procedures suffer from the inherent disadvantages of uncontrolled fluctuations in environmental conditions during the study programme. These disadvantages compromise any interpretations, particularly where controls on spar versus micrite precipitation are concerned. Many of these problems have been overcome in the current study by the construction and operation of laboratory mesocosm flumes which simulate the natural conditions (e.g. pH, flow rate, ambient temperature and daylight) in which freshwater carbonate (tufa) is deposited. Three mesocosms were supplied with natural river water from tufa precipitating streams and two mesocosms were supplied with UV-treated (sterile) river water from the same source. One of the untreated flume mesocosms was linked with a calcium reactor, which replaced calcium ions removed during the precipitation process in order to maintain tufa growth over extended experimental runs. Low-magnesium calcite precipitates (both rhombic sparite grown from long-crystallite dendrites and short-crystallite dendrite triad precursors) and micrite peloids (grown from spherulitic precursors) were precipitated in intimate association with biofilm (extracellular polymeric substances) within the four mesocosms supplied with natural river water. Virtually, no tufa-like precipitate was obtained from the flumes supplied with UV-treated river water. A second extended run flume experiment was also carried out for comparison purposes using a calcium hydroxide solution in deionized water. Collectively, these experiments provide convincing evidence confirming that the presence of a microbial biofilm strongly influences the precipitation of carbonates in riverine freshwater settings. In particular, experimental results show that micro-peloidal micrite and short-crystallite calcite dendrites are only produced in the presence of microbial extracellular polymeric substances.
Actively forming travertine and tufa sites are widespread in temperate and warm climates, such as the Mediterranean region, but also develop in other climatic regimes from the Arctic to equatorial latitudes. The terms travertine and tufa are sometimes applied indiscriminately today; however, each has a specific definition. Most studied deposits are active or of Quaternary (especially Holocene) age. There are fewer Tertiary and even less late Mesozoic examples on record. In part, this is because of a recognition problem as these deposits commonly preserve as fragmentary erosional remnants. Physico-chemical precipitation of calcium carbonate is commonly considered to be the main mechanism contributing to tufa and travertine deposition. Rapid cooling of waters further aids the process in thermal (travertine) sites. However, biomediation of calcium carbonate, associated with prokaryote–microphyte biofilms, is now recognized to be equally important in slow flowing and near static sites. Much of this precipitation occurs in close association with cyanobacterial, heterotrophic bacterial and diatoms. The Mediterranean region figures significantly in the development of travertine and tufa research. Consequently, this article employs key literature on Mediterranean examples to illustrate developments and concepts in freshwater carbonate research. Modern work started in the 1950s and defined many previously unknown outcrops. Many such sites became the targets for early radiocarbon dating studies. Since the late 1970s, a number of laboratory-based and field-based classification schemes have been proposed. Some schemes were based on associated macrovegetation types; others were based on petrological characteristics or on geometry and sedimentary facies characteristics. Of late, the sedimentological modelling has benefited greatly from the use of shallow geophysical techniques in order to better define the internal geometries of the deposits. Two distinct research strands were developed during the 1990s, both aimed at a better understanding of environmental change. The first considered biocomponents. The research was multifaceted and ranged from exploring the role of micro-organisms in cement and lime mud precipitation to biostratigraphic analysis of contained pollen and invertebrate remains; these provided proxy environmental information which has helped to define past climate variability in the region. The second research strand focussed on geochemical signatures as a proxy tool for defining environmental change. This definition was made possible by new petrological studies which have greatly aided the recognition of primary depositional fabrics. This improved knowledge has reduced the sampling dangers of diagenetic contamination inherent in earlier geochemical studies. Lithofacies modelling has also brought to light important early post-depositional neomorphic changes, which obscure primary fabrics, and complex diagenetic histories have been revealed. Stable isotope signatures of carbon and oxygen have also been used to define palaeo-water temperature. Fine-tuning of depositional time frames has also been achieved from absolute dating techniques employing radiocarbon and uranium/thorium.
Deposits produced by microbial growth and metabolism have been important components of carbonate sediments since the Archaean. Geologically best known in seas and lakes, microbial carbonates are also important at the present day in fluviatile, spring, cave and soil environments. The principal organisms involved are bacteria, particularly cyanobacteria, small algae and fungi, that participate in the growth of microbial biofilms and mats. Grain‐trapping is locally important, but the key process is precipitation, producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation. Various metabolic processes, such as photosynthetic uptake of CO 2 and/or HCO 3 – by cyanobacteria, and ammonification, denitrification and sulphate reduction by other bacteria, can increase alkalinity and stimulate carbonate precipitation. Extracellular polymeric substances, widely produced by microbes for attachment and protection, are important in providing nucleation sites and facilitating sediment trapping.
Microbial carbonate microfabrics are heterogeneous. They commonly incorporate trapped particles and in situ algae and invertebrates, and crystals form around bacterial cells, but the main component is dense, clotted or peloidal micrite resulting from calcification of bacterial cells, sheaths and biofilm, and from phytoplankton‐stimulated whiting nucleation. Interpretation of these texturally convergent and often inscrutable fabrics is a challenge. Conspicuous accumulations are large domes and columns with laminated (stromatolite), clotted (thrombolite) and other macrofabrics, which may be either agglutinated or mainly composed of calcified or spar‐encrusted microbes. Stromatolite lamination appears to be primary, but clotted thrombolite fabrics can be primary or secondary. Microbial precipitation also contributes to hot‐spring travertine, cold‐spring mound, calcrete, cave crust and coated grain deposits, as well as influencing carbonate cementation, recrystallization and replacement. Microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water, and thus relies uniquely on a combination of biotic and abiotic factors. This overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas, and in the past by temporal variation in abundance of marine microbial carbonates. Patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry.
Stromatolites appeared at ∼3450 Ma and were generally diverse and abundant from 2800 to 1000 Ma. Inception of a Proterozoic decline variously identified at 2000, 1000 and 675 Ma, has been attributed to eukaryote competition and/or reduced lithification. Thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the Palaeozoic, and reappeared in the Late Devonian. Microbial carbonates retained importance through much of the Mesozoic, became scarcer in marine environments in the Cenozoic, but locally re‐emerged as large agglutinated domes, possibly reflecting increased algal involvement, and thick micritic reef crusts in the late Neogene. Famous modern examples at Shark Bay and Lee Stocking Island are composite coarse agglutinated domes and columns with complex bacterial–algal mats occurring in environments that are both stressed and current‐swept: products of mat evolution, ecological refugia, sites of enhanced early lithification or all three?
The relict Fairmont Hot Springs deposit, formed largely of carbonates, covers an area of 0·5 km2, and is up to 16 m thick. The triangle-shaped discharge apron, which broadens down-valley, is divided into a proximal part with beds dipping at <10° and a distal part with beds dipping at 10° to 15°. The deposit is formed of the: (1) Basal Macrophyte; (2) Lower Carbonate; (3) Middle Clastic; (4) Upper Carbonate; and (5) Upper Clastic Sequences. Two charcoal samples embedded in the Lower Carbonate Sequence yielded dates of 8690 ± 90 and 8270 ± 70 cal yr bp, indicating that much of the deposit formed post-glacially during the Early to Mid-Holocene. Deposit aggradation ceased in the Mid to Late Holocene when the Fairmont Creek valley was incised. The Lower and Upper Carbonate Sequences, which are the thickest sequences, are composed of nearly equal parts of travertine (abiotic) and tufa (biotic), with feather dendrite travertine, radiating dendrite travertine and stromatolite tufa dominating. Competition between calcite precipitation rates and biotic growth rates controlled the distribution of tufa and travertine across the discharge apron. Calcite and biotic growth rates were controlled largely by flow velocity across the apron which, in turn, was controlled by topography and regular fluctuations in spring water discharge volume. During times of high spring discharge, slow sheet flow over the proximal part of the apron promoted stromatolite growth, whereas fast, turbulent flow on the distal part of the apron induced rapid feather dendrite formation. During times of low spring discharge, quiescent, shallow evaporative pools, conducive to radiating dendrite formation, formed on the proximal part of the apron, whereas slow flow on the distal part promoted stromatolite growth. Facies with high calcite supersaturation experienced rapid abiotic dendrite growth that precluded most biotic growth.
In freshwater environments such as river and stream bottoms, rocks and submerged vegetation are covered with a biological felt (also called a periphyton, microbial mat, biofilm, etc.) that is susceptible to calcification. Compilation of an extensive bibliography and our own observations have allowed the identification of 44 species of Coccogonophyceae, 122 Hormogonophyceae, 2 Chrysophyceae, 35 Chlorophyceae, 3 Xanthophyceae, 2 diatoms, and 3 Rhodophyceae that grow on calcareous tufa and coat vegetation. Diverse genera include species that are also calcified but impossible to determine because they lack reproductive organs. Crystals have been described from 74 species in the literature and we have observed 53 others. They can be classified into 10 groups: (1) platelets on cell walls (Volvocales, analogues of coccolithophorids) (2) crystals in mucilage (Synechococcus, diatoms, Hydrurus) and calcified stalks (Oocardium) (3) sheaths containing crystals in the form of simple or three-branched needles, dendritic crystals, and crystals with box-work fabric (Geitleria, Scytonema) (4) sheaths containing calcite spherulites (5) stalks intersecting a large crystal (Cymbella) (6) micrite tubes (Phormidium, Schizothrix) (7) isolated rhombohedra (Zygnema, Scytonema), rhombohedra in clusters or chains (Nostoc parmelioides) (8) sparite platelets (Vaucheria) or isodiametric crystals (Scytonema, Chaetophora) (9) large crystals crosscut by many parallel filaments (Rivularia, Batrachospermum), and (10) fan-like crystals (Phormidium). These crystals can be arranged in clusters or form regular laminations. They can transform into isodiametric sparite crystals to form fan-like or radial palisadic structures. Knowledge of primary crystals and their diagenetic transformations is necessary to correctly interpret freshwater stromatolites. The latter always result from intense calcification and are a diagenetic transformation of a biological felt made of many prokaryotic and eukaryotic algal species, small invertebrates, and organic and mineral debris.
Calcareous freshwater streams near Bad Urach, southwest Germany, were studied to determine the environmental limits to cyanobacterial calcification. Daily variations in temperature, pH, calcium concentration, and alkalinity were measured at seasonal intervals from September 1993 to January 1995 in two small woodland streams with lengths of 1.0 and 1.5 km. The principal cause of supersaturation in these fast-flowing streams is inorganic carbon dioxide outgassing from resurging groundwater, locally enhanced by turbulence at waterfalls and cascades. Photosynthetic uptake of carbon dioxide and temperature effects are negligible. Nonetheless, organic substrates, particularly cyanobacteria-dominated microbial mats and biofilm, significantly localize precipitation by providing suitable nucleation sites. Precipitation rates on artificial substrates, up to 2.2 mm/yr on limestone, correlate with high Saturation Index values. Copper substrates inhibited microbial colonization and received negligible encrustation. Tufa formation and external calcium carbonate encrustation of cyanobacteria are conspicuous where the annual WATEQ4F Saturation Index exceeds 0.8, and are slightly below 0.7. Calcium carbonate impregnation of cyanobacterial sheaths has not been observed. We infer that in these fast-flowing streams cyanobacteria utilize CO2 in photosynthesis whereas elsewhere, in sluggish freshwater, cyanobacteria utilize HCO3−, leading to sheath impregnation by calcium carbonate even where Saturation Index is only 0.2–0.3. Thus, photosynthetic influence on cyanobacterial calcification appears to be negligible in fast-flowing CO2-rich streams and cyanobacterial calcification takes the form of external encrustation. In slow-flowing CO2-poor streams and lakes cyanobacteria preferentially utilize bicarbonate and sheath impregnation can result. Modern tropical marine carbonate shelves have saturation indices commonly in the range 0.62–0.82 and cyanobacterial calcification is negligible. Extrapolation suggests that cyanobacterial calcification could occur in present-day seas if Saturation Index exceeded 0.8. This level, corresponding with heavy calcification in the freshwater streams studied, may have been widespread in marine environments in the geological past at times when calcified cyanobacteria and environmentally influenced non-skeletal calcium carbonate precipitates were abundant.
This study stresses the role of specific bacterial outer structures (such as glycocalix and parietal polymers) on calcium carbonate crystallization in terrestrial environments. The aim is to compare calcium carbonate crystals obtained in bacterial cultures with those obtained during abiotically mediated synthesis to show implications of exopolysaccharides and amino acids in the mineralogy and morphology of calcium carbonate crystals produced by living bacteria. This is done using various amounts of purified exopolysaccharide (xanthan EPS) and L-amino acids with a range of acidities. Amino acids and increasing xanthan content enhance sphere formation in calcite and vaterite. Regarding calcite, the morphology of crystals evolves from rhombohedral to needle shape. This evolution is characterized by stretching along the c axis as the amino acid changes from glutamine to aspartic acid and as the medium is progressively enriched in EPS. Regarding vaterite, the spherulitic habit is preserved throughout the morphological sequence and starts with spheres formed by the agglomeration of short needles, which are produced in a xanthan-free medium with glutamine. Monocrystals forming spheres increase in size as xanthan is added and the acidity of amino acids (glutamic and aspartic acids) is increased. At high xanthan concentrations, amino acids, and mainly aspartic and glutamic acids, induce vaterite precipitation. The role of the carboxyl group is also probably critical because bacterial outer structures associated with peptidoglycan commonly contain carboxyl groups. This role, combined with the results presented here, clearly demonstrate the influence of bacterial outer structure composition on the morphology and mineralogy of bacterially induced calcium carbonate. This point should not be neglected in the interpretation of calcite cements and carbonate accumulations in terrestrial environments.
rAMoS, n. ce-niceroS, o. cruz, A. AguAyo & F. ArcegA-cAbrerA Water chemistry of lakes related to active and inactive Mexican volca-noes
- M A Armienta
- G Vilaclara
- S De La Cruz-Reina
ArMienTA, M. A., g. VilAclArA, S. de lA cruz-reinA, S. rAMoS, n. ce-niceroS, o. cruz, A. AguAyo & F. ArcegA-cAbrerA. 2008. Water chemistry of lakes related to active and inactive Mexican volca-noes. Journal of Volcanology and Geothermal Research 178: 249- 258
Cyanobacteria tufa calcification in two freshwater streams: ambient environment, chemical thresholds and biological processes Algal communities from calcareous springs and streams in La Huasteca, central Mexico: A synthesis
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