Microbial biomineralization processes forming modern Ca ... - CiteSeerX

Sedimentology (2010) 57, 27–40

doi: 10.1111/j.1365-3091.2009.01083.x

Microbial biomineralization processes forming modern Ca:Mg carbonate stromatolites A LESSANDRA SPADAFORA* , 1 , EDOARDO PERRI, JUDITH A. MCKENZIE* and ´ GONO VASCONCELOS* CRISO *Geological Institute, ETH-Zu¨rich, 8092 Zu¨rich, Switzerland Dipartimento di Scienze della Terra – Universita` della Calabria, 87036 Rende, Italy (E-mail: [email protected]) Associate Editor: Gert-Jan Reichart ABSTRACT

Modern Ca:Mg carbonate stromatolites form in association with the microbial mat in the hypersaline coastal lagoon, Lagoa Vermelha (Brazil). The stromatolites, although showing diversified fabrics characterized by thin or crude lamination and/or thrombolitic clotting, exhibit a pervasive peloidal microfabric. The peloidal texture consists of dark, micritic aggregates of very high-Mg calcite and/or Ca dolomite formed by an iso-oriented assemblage of sub-micron trigonal polyhedrons and organic matter. Limpid acicular crystals of aragonite arranged in spherulites surround these aggregates. Unlike the aragonite crystals, organic matter is present consistently in the dark, micritic carbonate comprising the peloids. This organic matter is observed as submicron flat and filamentous mucus-like structures inside the interspaces of the high-Mg calcite and Ca dolomite crystals and is interpreted as the remains of degraded extracellular polymeric substances. Moreover, many fossilized bacterial cells are associated strictly with both carbonate phases. These cells consist mainly of 0Æ2 to 4 lm in diameter, sub-spherical, rod-like and filamentous forms, isolated or in colony-like clusters. The co-existence of fossil extracellular polymeric substances and bacterial bodies, associated with the polyhedrons of Ca:Mg carbonate, implies that the organic matter and microbial metabolism played a fundamental role in the precipitation of the minerals that form the peloids. By contrast, the lack of extracellular polymeric substances in the aragonitic phase indicates an additional precipitation mechanism. The complex processes that induce mineral precipitation in the modern Lagoa Vermelha microbial mat appear to be recorded in the studied lithified stromatolites. Sub-micron polyhedral crystal formation of high-Mg calcite and/or Ca dolomite results from the coalescence of carbonate nanoglobules around degraded organic matter nuclei. Sub-micron polyhedral crystals aggregate to form larger ovoidal crystals that constitute peloids. Subsequent precipitation of aragonitic spherulites around peloids occurs as micro-environmental water conditions around the peloids change. Keywords Bacterial fossils, biomineralization, microbialite, nanoglobules, peloids, stromatolites.

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Present address: A.R.P.A. Calabria, Dipartimento di Cosenza, 87100 Cosenza, Italy.

Ó 2009 The Authors. Journal compilation Ó 2009 International Association of Sedimentologists

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INTRODUCTION Laminated sedimentary rocks representing microbial mat and/or stromatolite development are remarkable fossils from the early Earth and represent microbial biomineralization processes, which have been active from the Early Archean to the Recent. In modern microbial mats, a complex biological and biochemical organization leads to several zones of photoautotrophic organisms with layers of aerobic and anaerobic heterotrophs metabolizing within variable amounts of extracellular polymeric substances (EPS) (Reid et al., 2000; Visscher & Stolz, 2005; Vasconcelos et al., 2006). Modern lithifying microbial mats produce a range of carbonate precipitates resulting from the interplay of the biological activities of microorganisms and the environmental conditions. The degree of lithification depends on equilibrium between dissolution and precipitation, which are controlled by the balance among photosynthesis, sulphate reduction, respiration and sulphide oxidation (Pinckney & Reid, 1997; Arp et al., 2003; Visscher & Stolz, 2005). Dupraz & Visscher (2005) suggest that the environmental control on organic matter consumption and the saturation state of the solution are the crucial factors that drive precipitation. Even when the stromatolite accretion is influenced strongly by a large amount of trapped grains, the biologically induced mineral precipitation processes follow similar biochemical paths and indisputably are fundamental to the formation of the microbialitic deposit (Visscher et al., 1998; Reid et al., 2000). While the biochemical processes leading to carbonate precipitation in the microbial mat communities have been measured in the field and reproduced in laboratory cultures, the mechanisms of crystal nucleation and growth structures, such as the final texture, are less investigated. In modern lithifying microbial mats from hypersaline environments, the microfabric of carbonate bio-induced precipitates appears to be characterized by autochthonous peloids 20 to 50 lm in size, resulting from the coalescence of nanospherical (200 to 500 nm) structures, surrounded by succeeding microsparite (Dupraz et al., 2004; Riding & Toma´s, 2006). The present paper reports on a study of Recent lithified stromatolites that are forming in Lagoa Vermelha (Brazil) in association with a living microbial mat in which a complex microbial community controls the precipitation of primary Ca:Mg carbonates (Vasconcelos et al., 2006). Based on the results of this research, a genetic

model is proposed for the bio-mineral formation and arrangement in the early microbialitic texture, which ultimately comprises the stromatolite structure.

GEOLOGICAL SETTING Lagoa Vermelha is a small, shallow (< 2 m), hypersaline lagoon located along the coast between Saquarema and Arraial do Cabo, about 100 km east of Rio de Janeiro city, Brazil (Fig. 1). Subject to a semi-arid climate due the occurrence of an upwelling zone (Barbie´re, 1985), Lagoa Vermelha is separated from the Atlantic Ocean and the larger neighbouring Lagoa Araruama by a Pleistocene sand dune system through which recharge waters flow (Vasconcelos & McKenzie, 1997). The salinity fluctuates between brackish and hypersaline depending on the ratio of precipitation versus evaporation during the dry and wet seasons. The presence of sulphide in the upper 6 cm of the sediment and positive d34S of sulphate indicates active bacterial sulphate reduction in the lagoon sediment (van Lith et al., 2002). However, the sulphide concentration is not very high and, together with the enriched oxygen isotopic compositions of the sulphate, indicates biotic or abiotic sulphide oxidation. The acidifying effect of sulphide oxidation favours dolomite over Mg calcite formation, as indicated by the saturation states of the two minerals (Moreira et al., 2004). Depending upon location, the bottom surface of the lagoon is dominated by a widespread, flat, lithifying microbial mat that covers low-relief stromatolites in the shallower areas (Fig. 2A). The stromatolites form individual heads averaging 10 cm in height and 30 cm in diameter (Fig. 2B). The living microbial mat caps the underlying lithified laminated structure forming the stromatolite dome (Fig. 3A). The microbial mat is well-

Fig. 1. Location of Lagoa Vermelha east of Rio de Janeiro city (R.J.), Brazil.

Ó 2009 The Authors. Journal compilation Ó 2009 International Association of Sedimentologists, Sedimentology, 57, 27–40

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to determine mineral composition using a Philips PW1730 diffractometer (Philips Analytical, Almelo, The Netherlands). Semi-quantitative elemental compositional analyses of micron-sized spots were obtained using an EDAX energydispersive X-ray spectrometer (EDS; EDAX, Mahwah, NY, USA) during SEM observations. In addition, thin sections were analysed to determine variation in autofluorescence which reveals the distribution of organic matter. These observations were made using the incident light emitted by Hg high-pressure vapours attached to an Axioplan Imaging II microscope (Zeiss, Gottingen, Germany), equipped with high-performance wide band-pass filters.

STROMATOLITE FABRIC, PETROGRAPHY AND COMPOSITION

Fig. 2. (A) Panoramic view of Lagoa Vermelha, on the bottom of which form low-relief stromatolites (arrows). (B) Stromatolite head (approximately 10 cm high and 30 cm in diameter) removed from the water and placed on shore. Note the abundant occurrence of protective extracellular polymeric substances covering the surface of the natural sample.

developed, being 3 to 5 cm thick, with stratified layers of carbonate precipitate alternating with gelatinous organic layers (Fig. 3B; Vasconcelos et al., 2006). The amalgamation of the discrete carbonate laminae in the soft mat with the degradation of the organic matter apparently leads to the development of lithified Ca:Mg carbonate laminae (Fig. 3C), which are the focus of this study.

METHODS Using traditional petrographic methods, the stromatolite microfacies were evaluated. Scanning electron microscopy (SEM) studies of the nanofacies were carried out on polished thin sections and freshly broken surfaces, using an FEI-Philips ESEM-FEG Quanta 200F (FEI-Philips, Brno, Czech Republic). Samples were carbon-coated or gold-coated, respectively, depending on whether they were prepared for a micro-analysis or textural study. X-ray diffraction analysis was conducted

The stromatolite selected for this study exhibits isolated domal growth morphology up to 8 cm high and 13 cm wide (Fig. 3C). Cross-sections through the stromatolite show that the fabric is composed of alternating continuous and regular millimetre-thick, convex upward, laminae intercalated with sub-centimetre massive intervals with only a patchy development of layering disturbed by regular millimetre-size cavities. Flat and elongated larger cavities, up to a few centimetres in width, break up the lateral continuity of the lamination. Allochthonous sand-size particles, including recognizable larger bioclasts, such as small bivalves, gastropods, foraminifera and ostracods, are incorporated locally into the laminae, but these generally are volumetrically insignificant. Thin-section examination of the stromatolite structure reveals a peloidal and aphanitic fabric (Figs 4 and 5A). The peloidal texture originates from the 20 to 50 lm size aggregates of dark micrite surrounded by microspar (layer 1; Fig. 4). Inside the thinner laminae, this texture passes gradually from peloidal to more solid and homogeneous (aphanitic) micrite (layer 2; Fig. 4). Within the poorly laminated layers, a clotted thrombolitic fabric, containing many microcavities, is recognizable (layer 3; Fig. 4). Clots, ranging widely in size with an average of 50 to 100 lm, are composed of aggregated peloids and typically show relatively evenly spaced distribution in a matrix composed of microsparite or sparite. Microcavities are lined by thin (ca 30 to 50 lm) isopachous fringes of primary marine cements. These microcavities locally contain bioclastic sediment or are empty, lacking late diagenetic cements.


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Fig. 3. (A) Lagoa Vermelha stromatolite maintained under hypersaline conditions in an aquarium in the Geomicrobiology Laboratory, ETH-Zu¨rich. Note the actively photosynthesizing microbial mat covering the stromatolite head; scale bar is 2 cm. (B) Cross-section through the microbial mat showing stratified white layers of carbonate precipitates alternating with lithified organic layers. The latter reflects the differentiated microbial composition of the Lagoa Vermelha mat, with green layers of photosynthetic bacteria on top producing oxygen bubbles, followed by intervals of brownish layers, with heterotrophic bacteria, red layers with purple sulphur bacteria and an underlying grey layer with sulphate reducers (Vasconcelos et al., 2006); scale bar is 2 cm. (C) Lithified internal structure beneath the soft microbial mat cover showing the developed Ca:Mg carbonate laminae coalescing into stromatolitic and thrombolitic forms, which are the focus of this study; scale bar is 2 cm.

Scanning electron microscopy observations of the peloidal texture reveal that it is formed by oval-shaped crystal aggregates from 5 to 10 lm in size, together with microsparite that appears as spherulites and fringes of acicular limpid crystals, with a typical thickness of 1 to 3 and 10 to 20 lm in length (Fig. 5B). The oval-shaped masses originate from the iso-oriented assemblage of small trigonal polyhedrons, which are ca 1 lm wide and ca 0Æ3 lm thick (Fig. 5C). Energy-dispersive X-ray spectrometer analysis indicates that the polyhedrons of ovoid crystals are high-Mg calcite with 25 to 35 mol% of Mg and around 1 to 2 mol% of Na. The Mg contents in some laminae reach 40 mol%, which implies the presence of Ca dolomite. The acicular crystals are aragonite with a Ca content of 94 to 96 mol%, and traces of Mg, Na and Sr. X-ray diffraction analysis of bulk samples of the stromatolite confirms that it is composed mainly of two carbonate phases (high-Mg calcite and aragonite) with clay minerals in trace amounts. The presence of a large amount of Mg in the calcite

crystals is also indicated by the anomalous ˚ position of the d104 peak, which is ca 2Æ96 A ˚ , and rerelative to standard calcite 3Æ035 A presents a considerable shift towards the stoi˚ ) (Jones et al., chiometric dolomite peak (2Æ886 A 2001).

STROMATOLITE BIOGENIC STRUCTURES

Mineral structures Mineralized bacteria-like fossil remains were found closely associated with the previously described mineral phases. These remains consist mainly of filamentous and sub-spherical coccoid forms preserved both as moulds and mineralized bodies within all microfacies and mineral phases. Filamentous and rod-like fossils are slightly curved, apparently solid tubes (Fig. 6). These fossils have a uniform diameter of ca 2 lm and


Biomineralization processes forming stromatolites

Fig. 4. Thin-section photomicrograph showing microfacies of Lagoa Vermelha stromatolite. Dashed lines distinguish the different trends of lamination formation. Thin laminae (layers 1 and 2) are intercalated with massive thrombolitic intervals with large cavities (layer 3). Bright white areas are of primary porosity. Microfabrics are peloidal ‘P’ or aphanitic ‘A’; peloids are aggregates of dark micrite ‘p’ surrounded by microspar ‘s’ (see also Fig. 5A). Inside the coarsely laminated layer 3, clots composed of aggregates of peloids ‘pp’ are distinguishable in a microspar matrix ‘s’. Cavities ‘c’ are lined with isopachous fringes of primary marine cements ‘cm’.

have been observed as isolated fragments or even closely associated, appearing to form colony-like clusters (Fig. 6A). The observed mineralized filaments apparently have retained their original size, as the tubular moulds, sometimes preserved with the mineralized bodies, show an internal diameter a little larger than the filaments (Fig. 6B). This observation could also indicate an early diagenetic process, which could help to preserve the microbial structure. The coccoid fossils consist mainly of spherical and ovoid solid structures (Fig. 7) and, according to their dimensions, it is possible to recognize two distinct classes, which are, respectively, around 2Æ0 and 4Æ0 lm in diameter, and are often seen as isolated forms (Fig. 7A and C). These coccoids correspond in size with the observed

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moulds (Fig. 7B). Another type of sub-micronsized coccoid mould with a diameter of 200 to 300 nm is recognizable, forming colony-like clusters, closely stacked or thinly grouped (Fig. 7D). Both filamentous and coccoid fossils are composed of a fine granular texture formed by nanocrystal aggregates of carbonate. It is commonly observed that the coccoids are often the nucleation centres for the crystal growth of aragonite spherulites or fans (Fig. 7A and B). Higher magnification observations of the crystal surfaces show the presence of much smaller spherical structures or nanoglobules, 50 to 100 nm in size, aggregated to form a granular texture (Fig. 8). These finer-scale structures are distributed patchily, but they are abundant particularly in the dark, micritic high-Mg calcite and Ca dolomite; they are, however, absent in the aragonitic microsparite. Finally, further mineral structures composed of framboidal aggregates of pyrite with spherical morphology < 5 lm in diameter have been observed (Fig. 9). These structures are formed by a variable number of sub-idiomorphic to idiomorphic cubic microcrystals (ca 500 nm).

Organic structures The dark, micritic, high-Mg calcite and Ca dolomite that constitute peloids, isolated or aggregated, are characterized by a strong autofluorescence indicating the diffuse presence of organic matter (Fig. 10). By contrast, aragonite crystals that surround the peloids show very low autofluorescence. Moreover, autofluorescence reveals delicate folded layers lining the cavities and tubular forms, which are otherwise almost transparent in transmitted light. Scanning electron microscopy analyses of the mineral microstructures confirm the diffuse presence of organic matter, the majority of which consists of planar or sheet-like features (Fig. 11). The 0Æ5 to 1 lm thick, gently folded sheets line the cavities and/or envelop the crystal aggregates extending for several tens of microns. These sheets locally form a sub-polygonal honeycomblike network and/or gradually pass from flat sheets to filaments indicating an original viscous mucus-like behaviour (Fig. 11A). Moreover, filamentous tubular organic moulds of bacteria-like bodies are associated with the mucus-like planar sheets (Fig. 11B). Sub-micron flat and filamentous organic structures frequently occur between the polyhedrons of oval-shaped crystals of highMg calcite, suggesting a close association with


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Ar 25 µm Fig. 5. Crystalline microfabric of Lagoa Vermelha stromatolites. (A) Thin-section photomicrograph under plane polarized light showing dark micrite (white arrows) of peloids, formed by cloudy irresolvable crystals and microsparite (black arrows) composed of acicular fans of limpid crystals. The central homogeneous area is an empty cavity. (B) SEM photomicrograph of oval-shaped aggregates of crystals of high-Mg calcite ‘Cal’, constituting micritic peloids, surrounded by spherulites of acicular crystals composed of aragonite ‘Ar’, corresponding to microsparite. (C) Close-up view of oval-shaped aggregate of high-Mg calcite showing iso-oriented assemblage of trigonal polyhedrons (white arrow: c-axis view; black arrow: a/b-axis view). In the background, prismatic acicular crystals of aragonite are visible. Ó 2009 The Authors. Journal compilation Ó 2009 International Association of Sedimentologists, Sedimentology, 57, 27–40


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Fig. 6. Scanning electron microscopy photomicrographs of filamentous bacterial fossils. (A) Closely associated slightly curved mineralized (high-Mg calcite) tubes (white arrows) forming a colony-like cluster. (B) Close-up view of a filament (white arrow) embedded in a mucus-like organic membrane (black arrow).

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Fig. 7. Scanning electron microscopy photomicrographs of coccoid bacterial fossils. (A) Single mineralized ovoidal bacteria (arrow) in the centre of aragonitic crystals. (B) Empty moulds of single (white arrows) and possibly colonial (black arrow) coccoid bacterial forms in the nucleus of aragonitic spherulites. (C) Mineralized spherical form (arrow) encased in high-Mg calcite. (D) Dwarf empty moulds (arrows) in high-Mg calcite.

this mineral phase (Fig. 11C). Aragonite crystals, which are sometimes partially coated with large sheets, lack these small-scale organic structures. Thin organic membranes and filaments cover the mineralized tubular and coccoid fossils (Figs 6B and 7C). Organic matter remains were also observed around and within pyrite microcrystals.

DISCUSSION Lagoa Vermelha stromatolites show distinctive fabrics, characterized by thin or crude lamination and/or thrombolitic clotting, and exhibit a pervasive peloidal microfabric. This peloidal texture consists of dark, micritic aggregates of Ca:Mg carbonate originating from the iso-oriented


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Fig. 8. Scanning electron microscopy photomicrographs of surface texture. (A) Granular nanometre-scale texture (black arrows) on the surface of high-Mg calcite crystals. White arrows indicate extracellular polymeric substance remains. See also Fig. 11C. (B) Close-up view of the granular texture on the surface of a Ca dolomite crystal which forms by the coalescence of nanoglobules.

Fig. 9. Scanning electron microscopy photomicrograph of isolated framboidal aggregates of pyrite associated with Ca: Mg carbonate mineral phases.

assemblage of sub-micron trigonal polyhedrons and organic matter remains. Limpid acicular crystals of aragonite arranged in spherulites surround these aggregates (Fig. 5). The crystal assemblage characterized by the aggregation of nanocrystals, the widespread presence of benthonic micro-organism fossils clearly embedded in the peloidal microstructure and the antigravity assemblage of peloids all indicate the autochthonous origin of the peloids that constitute Lagoa Vermelha laminae. Thus, their origin is related to some in situ process of precipitation that also probably represents the main mechanism for the stromatolite formation. The purely descriptive term ‘peloid’ refers to micritic aggregates of uncertain origin (McKee & Gutschick, 1969), including allochthonous grains with various origins (Macintyre, 1985; Flu¨gel, 2004). These peloids have often been regarded as

being autochthonous benthonic microbial in origin as they constitute the most common microfabric of modern and ancient microbialites (Gebelein, 1974; Bertrand-Sarfati, 1976; Monty, 1976; Kennard & James, 1986; Dupraz & Strasser, 1999; Riding, 2000, 2002a). However, it has been demonstrated that very similar micromorphologies can be created by purely abiotic mechanisms (Bosak et al., 2004). Chafetz (1986) and Riding (2002b) suggested that peloids could be calcified aggregates resembling bacterial microcolonies in Phanerozoic fossil biofilms. Following studies on modern microbial mat lithification, the formation of peloidal carbonate precipitates has been associated with the metabolic activities of bacteria (Paerl et al., 2001; Dupraz et al., 2004; Riding & Toma´s, 2006). Peloidal microfabric may form in situ during very early diagenesis, due to degradation and calcification of organic matter (mainly EPS) driven by heterotrophic bacteria; inter-peloid spaces can be infilled successively by abiotic precipitation of microsparite, as the organic matter is removed. The dark, micritic Ca:Mg carbonates that constitute peloids in Lagoa Vermelha stromatolites show the remains of organic matter to be present consistently, as indicated by their strong autofluorescence in comparison with aragonite crystals (Fig. 10). These organic matter remains are observed in SEM as sub-micron flat and filamentous mucus-like structures inside the interspaces of the high-Mg calcite crystals and are interpreted as the remains of degraded EPS (Fig. 11). Such mucus-like structures closely resemble the subpolygonal honeycomb arrangement of the natural EPS (De´farge et al., 1996), possibly after drying in the absence of degrading organisms. In fact, EPS



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Fig. 10. Thin-section photomicrographs of peloidal microfabric of Lagoa Vermelha stromatolites. (A) Under transmitted light, dark, micritic peloids form clots composed of high-Mg calcite (black arrows) surrounded by lighter microsparitic aragonite crystals (white arrows). (B) Epifluorescent image of the field in (A). High-Mg calcite is strongly autofluorescent (green fluorescence) compared with the less intense response of aragonite (white arrows). Note also the thin organic fluorescent remains in the cavities, invisible in plane polarized light (dashed arrow).

from hot springs are stable to temperatures of at least 72°C and can partially retain the threedimensional geometry as the water cools (Allen et al., 2000). A variety of elements and compounds can preserve EPS, and the textures and structures, as well as some of its organic breakdown products, can be mineralized (Camoin et al., 1999; Westall et al., 2000; Perri & Tucker, 2007). The presence of fossil EPS strictly associated with their polyhedrons of high-Mg calcite implies that organic matter played a fundamental role in the precipitation of the mineral that forms the peloids. At the same time, EPS basically are absent in the aragonitic phase indicating a different mechanism of precipitation. Although the specific origin of the peloidal fabric that characterizes the stromatolites remains an unresolved question, the relationship between

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organic matter and carbonate precipitation has been described in several papers (Pentecost, 1985; Reitner et al., 1995; Trichet & De´farge, 1995; Arp et al., 1999, 2003; Neuweiler et al., 1999). The processes of EPS degradation are significant particularly in modern microbial mats because they commonly lead to carbonate precipitation, whereas the bacterial bodies themselves are less involved (Trichet et al., 2001; Dupraz et al., 2004; Gautret et al., 2004). Carbonate precipitation processes associated with the EPS degradation progressively infill the space left by the degraded EPS structure. The initial products of calcification (or dolomitization) are nanoglobules (60 to 200 nm) released from the microbial cell wall in the surrounding aquatic environment. These nanoglobules subsequently are merged into larger spheres (200 to 500 nm), which apparently play a primary role as centres of nucleation for succeeding crystal growth (Aloisi et al., 2006; Bontognali et al., 2008). It has been shown that nanospheres with a high organic matter content replace both the EPS structure (Dupraz et al., 2004) and bacterial bodies (Sprachta et al., 2001; van Lith et al., 2003b). Moreover, nanospheres, in a more advanced stage of mineralization, coalesce to form larger (20 to 50 lm) peloids. The mineralogy of the precipitates varies between high-Mg calcite (Mg ca 15 mol%) and Ca dolomite (Mg ca 40 mol%) (Trichet et al., 2001; van Lith et al., 2003a). The discontinuous replacement of EPS by peloids is followed by physico-chemical precipitation of microsparite and sparite in the inter-peloidal space. This process results in the formation of a structure that has an intimate mixture of peloidal micrite and microspar comparable with microstructures of many fossil stromatolitic and thrombolitic microbialites (Dupraz et al., 2004; Riding & Toma´s, 2006). A similar mechanism of EPS degradation and calcification is inferred to explain the peloidal structure observed in the Lagoa Vermelha stromatolites because of the presence of fossil EPS and the close association only with high-Mg carbonates, which is characterized by a granular texture closely resembling nanoglobules (Fig. 8). Such spheroids, less than 200 nm in diameter, have been described extensively from sedimentary environments and have controversially been interpreted as nanobacteria (Folk, 1993, 1999) or as mineralized macromolecules (Southam & Donald, 1999). Very similar nanoscale globules have been observed in microbial carbonate precipitation experiments (Vasconcelos et al., 1995; Warthmann et al., 2000; Aloisi et al.,


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2006; Bontognali et al., 2008) and in ancient microbial carbonates (Perri & Tucker, 2007). These globules also form as a result of enzymedriven bacterial decaying of organic tissues

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(Schieber & Arnott, 2003). In any case, nanoscale calcified spheres commonly occur both in nature and in nucleation experiments, suggesting that nanoglobules represent an important early stage in microbial calcification. Isolated framboidal aggregates of pyrite (< 5 lm) also indicate microbial activity. The formation of low-temperature microbial pyrite is linked indirectly to sulphate-reducing bacteria and the framboidal shape is also considered a morphological biomarker (Popa et al., 2004). In Lagoa Vermelha stromatolites, many fossil bacterial cells are associated strictly with both aragonitic and Mg carbonate mineral phases (Fig. 7). Cell morphology typically is sub-spherical, rodlike and filamentous, either isolated or occurring in colony-like clusters. Based on their dimension and shape, four cell types are recognizable, ranging from 0Æ2 to 4 lm in diameter, possibly corresponding to different bacterial species. The observed dimensions of the mineralized bodies should correspond to the original width of the bacterial cell as the moulds, sometimes preserved with the mineralized bodies, show a slightly larger internal diameter (Fig. 6B). This observation could also indicate that mineralization was an early process, affecting bacterial bodies before their dimensions were reduced by fluid loss. Fossilization of the bacterial bodies is possible, as, during entombment, the bacterial cells adsorb Mg2+and Ca2+ ions that then combine with bicarbonate to precipitate crystals on the surface of the cellular membrane (van Lith et al., 2003b). Larger micrometre forms are easily referable to the bacterial species well-documented in the living mat closely associated with such stromatolites (Vasconcelos et al., 2006) and particularly with the carbonate-producing layer, which is constituted mainly of sulphate-reducing bacteria. Several other studies have proposed bacterial mediation for the precipitation of Ca dolomite or high-Mg calcite in microbial mats, linking sulphate-reducing bacteria and carbonate precipFig. 11. Scanning electron microscopy photomicrographs of fossilized organic matter. (A) Thick sheets of organic matter mucus-like remains (EPS) forming a subpolygonal honeycomb-like network (white arrows). (B) Empty filamentous tubular organic structure (white arrow), possibly a mould of filamentous bacteria associated with mucus-like planar sheets (black arrow). (C) Flat and filamentous mucus-like organic structures (white arrows) between the polyhedrons of oval-shaped crystals of high-Mg calcite. Note the granular texture which is particularly abundant on the surface of the polyhedrons of high-Mg calcite.


Biomineralization processes forming stromatolites itation both in modern environments and the laboratory (Visscher & Stolz, 2005; Wright & Wacey, 2005). Nevertheless, not all mats lithify and several models for lithification are known, ranging from completely microbial to purely A

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chemically mediated mechanisms (Arp et al., 2003). Dupraz & Visscher (2005) indicate two crucial factors driving precipitation: (i) the environmental control on organic matter consumption (mainly EPS); and (ii) the saturation state of the solution with respect to the carbonate minerals, depending upon ion activity product, solubility product and pH. Furthermore, the mineralization processes of sulphate-reducing bacteria have been described as being the consequence of carbonate precipitation around bacterial cells within EPS during the formation of modern dolomitic stromatolites that, in turn, produce bacterial fossils (van Lith et al., 2003b). The complex processes that induce mineral precipitation in the Lagoa Vermelha living microbial mat are well-recorded in the studied lithified stromatolites and can be presented schematically (Fig. 12). The bacterial metabolism and EPS degradation promote the precipitation of early Mg-rich carbonate with an ovoidal shape. These ovoidal aggregates comprise sub-micron polyhedron crystals composed of Ca dolomite and/or high-Mg calcite, which result from the agglutination of nanoglobules forming during the degradation processes. The continuation of this aggregation process forms larger ovoidal crystals, which subsequently merge to form peloids. All these steps may represent the first stage of stromatolitic laminae formation. The precipitation of aragonitic spherulites around peloids may occur after all of the organic matter is degraded by sulphate-reducing bacteria, as this degradation increases bicarbonate ion concentration providing ideal conditions for purely inorganic precipitation (Dupraz et al., 2004; Riding & Toma´s, Fig. 12. Model illustrating stages of the peloidal microfabric formation in the Lagoa Vermelha stromatolites, from the initial microbial mat calcification (A) to the formation of the stromatolite body (F). (A) In the microbial mats, the EPS degradation processes lead to Ca:Mg carbonate precipitation, forming discrete layers of carbonate. (B) Carbonate precipitation initiates with the formation of polyhedral crystals (possibly after coalescence of nanoglobules) that mineralize organic matter (including replacement of bacterial bodies). (C) Advancing the process of mineralization, polyhedral crystals form oval crystal aggregates. (D) Ovoid formation and clustering, which produce the peloids, is followed by the precipitation of aragonitic microsparite in the remaining porosity. (E) The peloidal microfabric (micrite and microsparite) constitutes the main texture in all laminae of Lagoa Vermelha stromatolites. (F) Later, early primary marine cements eventually fill the remaining cavities to complete the lithification of the stromatolite structure.


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2006). The fact that spherulitic nuclei have been associated with many fossilized bacterial bodies could indicate that subsequent metabolic activities have occurred in a distinct bacterial community inducing aragonite precipitation (Arp et al., 2003). As a granular detrital component is basically absent, the biologically induced mineral precipitation, which leads to the formation of the Lagoa Vermelha stromatolites, represents the unique inferable mechanism for the growth of these microbialites. Such biochemically dominated stromatolites are very common in the geological record, but it seems that their modern counterparts are less common relative to the trapping and binding examples (Riding, 2000). As a rule, recent stromatolite accretion is influenced strongly by the incorporation of a large amount of trapped grains. However, the biologically induced mineral precipitation processes are indisputably important even for those microbialites formed predominantly by a trapping–binding mechanism (Visscher et al., 1998; Reid et al., 2000).

CONCLUSIONS Stromatolites from Lagoa Vermelha present a unique mineralogical pattern formed exclusively by in situ microbial biomineralized high-Mg calcite and Ca dolomite associated with aragonite; this pattern distinguishes them from the other modern grain-rich examples, such as those found in the Bahamas or Shark Bay, Australia. The carbonate precipitation is associated with microbial metabolism and EPS degradation inside the living mat, mainly corresponding to the heterotrophic degradation of the organic matter by sulphate-reducing bacteria. The stromatolites show diversified fabrics, characterized by thin or crude lamination and/ or thrombolitic clotting and exhibit a pervasive peloidal microfabric. The peloidal texture consists of dark, micritic aggregates of oval-shaped Ca dolomite and/or very high-Mg calcite formed by the iso-oriented assemblage of sub-micron trigonal polyhedrons and organic matter remains. Limpid acicular crystals of aragonite arranged in spherulites surround the aggregates. The co-existence of sub-micron flat and filamentous, mucus-like remains of degraded EPS and fossilized rod-like and filamentous bacteria, strictly associated with the polyhedrons of Ca dolomite and high-Mg calcite, implies that the organic matter and the microbial metabolism

played a fundamental role in the precipitation of the minerals that form the peloids. At the same time, the lack of EPS in the aragonitic phase indicates an additional precipitation mechanism. The process initiates inside the microbial mats, where the EPS degradation processes lead to carbonate precipitation, forming discrete layers of calcite and Mg calcite (Fig. 12A). The initial products of calcification are nanoglobules (60 to 200 nm) that subsequently are merged into larger spheres (200 to 500 nm), playing a primary role as centres of nucleation for succeeding crystal growth. Sub-micron polyhedral crystals of Ca dolomite and/or very high-Mg calcite result(s) from the coalescence of carbonate nanoglobules around degraded organic matter (EPS and/or bacterial bodies) nuclei (Fig. 12B). Polyhedral crystals grow, forming larger ovoidal crystalline aggregates that constitute dark peloids (Fig. 12C and D). Precipitation of aragonitic spherulites around peloids then occurs, filling inter-peloidal space (Fig. 12D). Ultimately, early primary marine cements eventually fill the remaining cavities (Fig. 12E). The final product is the formation of the peloidal microfabric texture documented in the microstructures of Lagoa Vermelha stromatolites (Fig. 12F). Based on the results of this study of a modern carbonate microbialite, the recognition of an autochthonous peloidal microtexture could be an important step in the interpretation of possible microbial metabolism being involved in stromatolite formation throughout the geological record.

ACKNOWLEDGEMENTS The authors are grateful to two anonymous reviewers and P. Swart for the thoughtful remarks on the manuscript.

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Manuscript received 30 October 2008; revision accepted 14 May 2009
