Automicrites in modern cyanobacterial stromatolitic deposits of Rangiroa, Tuamotu Archipelago, French Polynesia: Biochemical parameters underlaying their formation (original) (raw)
Related papers
Microbial carbonate crusts-a key to the environmental analysis of fossil spongiolites?
Facies, 1993
Morphological and geochemical comparisons between modern cryptic microbialites from Lizard Island/Great Barrier Reef and fossil counterparts in the Upper Jurassic (Southern Germany, Dobrogea/Romania) and late Lower Cretaceous (Aptian/ Albian from Cantabria/Spain) spongiolitic environments show that there are common factors controlling the crust formations mostly independent of light despite of diverging (paleo-) oceanographic positions as well as relationships of competitors. Factors such as increased alkalinity ,oligotrophy, and reduced allochthonous deposition are of major importance. Thrombolitic microbialites are interpreted as biologically induced and therefore calcified in isotopic equilibrium with the surrounding sea water. Corresponding with shallowing upward cycles, microbial mats which produce stromatolitic peloidal crusts become more important. Different biomarkers are introduced for the first time extracted and analyzed from spongiolitic limes tones ofLower Kimmeridgian age from Southern Germany.
Geobiology, 2008
A previously published hydrothermal brine-river water mixing model driven by ocean crust production suggests that the molar Mg/Ca ratio of seawater (m Mg/Ca sw) has varied significantly (~1.0-5.2) over Precambrian time, resulting in six intervals of aragonite-favouring seas (m Mg/Ca sw > 2) and five intervals of calcite-favouring seas (m Mg/Ca sw < 2) since the Late Archaean. To evaluate the viability of microbial carbonates as mineralogical proxy for Precambrian calcite-aragonite seas, calcifying microbial marine biofilms were cultured in experimental seawaters formulated over the range of Mg/Ca ratios believed to have characterized Precambrian seawater. Biofilms cultured in experimental aragonite seawater (m Mg/Ca sw = 5.2) precipitated primarily aragonite with lesser amounts of high-Mg calcite (m Mg/Ca calcite = 0.16), while biofilms cultured in experimental calcite seawater (m Mg/Ca sw = 1.5) precipitated exclusively lower magnesian calcite (m Mg/Ca calcite = 0.06). Furthermore, Mg/ Ca calcite varied proportionally with Mg/Ca sw. This nearly abiotic mineralogical response of the biofilm CaCO 3 to altered Mg/Ca sw is consistent with the assertion that biofilm calcification proceeds more through the elevation of , via metabolic removal of CO 2 and/or H + , than through the elevation of Ca 2 + , which would alter the Mg/Ca ratio of the biofilm's calcifying fluid causing its pattern of CaCO 3 polymorph precipitation (aragonite vs. calcite; Mg-incorporation in calcite) to deviate from that of abiotic calcification. If previous assertions are correct that the physicochemical properties of Precambrian seawater were such that Mg/Ca sw was the primary variable influencing CaCO 3 polymorph mineralogy, then the observed response of the biofilms' CaCO 3 polymorph mineralogy to variations in Mg/Ca sw , combined with the ubiquity of such microbial carbonates in Precambrian strata, suggests that the original polymorph mineralogy and Mg/Ca calcite of well-preserved microbial carbonates may be an archive of calcite-aragonite seas throughout Precambrian time. These results invite a systematic evaluation of microbial carbonate primary mineralogy to empirically constrain Precambrian seawater Mg/Ca.
Carbonate factories: A conundrum in sedimentary geology
Earth-Science Reviews, 2008
Describing, characterizing and interpreting the nearly infinite variety of carbonate rocks are conundrumsintricate and difficult problems having only conjectural answersthat have occupied geologists for more than two centuries. Depositional features including components, rock textures, lithofacies, platform types and architecture, all vary in space and time, as do the results of diagenetic processes on those primary features. Approaches to the study of carbonate rocks have become progressively more analytical. One focus has evolved from efforts to build reference models for specific Phanerozoic windows to scrutinize the effect of climate and long-term oscillations of the ocean-atmosphere system in influencing the mineralogy of carbonate components. This paper adds to the ongoing lively debates by attempting to understand changes in the predominant types of carbonate-producing organisms during the Mesozoic-Cenozoic, while striving to minimize the uniformitarian bias. Our approach integrates estimates of changes in Ca 2+ concentration in seawater and atmospheric CO 2 , with biological evolution and ecological requirements of characteristic carbonate-producing marine communities. The underlying rationale for our approach is the fact that CO 2 is basic to both carbonates and organic matter, and that photosynthesis is a fundamental biological process responsible for both primary production of organic matter and providing chemical environments that promote calcification. Gross photosynthesis and hypercalcification are dependent largely upon sunlight, while net primary production and, e.g., subsequent burial of organic matter typically requires sources of new nutrients (N, P and trace elements). Our approach plausibly explains the changing character of carbonate production as an evolving response to changing environmental conditions driven by the geotectonic cycle, while identifying uncertainties that deserve further research. With metazoan consumer diversity reduced by the end-Permian extinctions, excess photosynthesis by phytoplankton and microbial assemblages in surface waters, induced by moderately high CO 2 and temperature during the Early Mesozoic, supported proliferation of nontissular metazoans (e.g., sponges) and heterotrophic bacteria at the sea floor. Metabolic activity by those microbes, especially sulfate reduction, resulted in abundant biologically-induced geochemical carbonate precipitation on and within the sea floor. For example, with the opening of Tethyan seaways during the Triassic, massive sponge/microbe boundstones (the benthic automicrite factory) formed steep, massive and thick progradational slopes and, locally, mud-mounds. As tectonic processes created shallow epicontinental seas, photosynthesis drove lime-mud precipitation in the illuminated zone of the water column. The resulting neritic lime-mud component of the shallow-water carbonate factory became predominant during the Jurassic, paralleling the increase in atmospheric pCO 2 , while the decreasing importance of the benthic automicrite factory parallels the diversification of calcifying metazoans, phytoplankton and zooplankton. With atmospheric pCO 2 declining through the Cretaceous, the potential habitats for neritic lime-mud precipitation declined. At the same time, peak oceanic Ca 2+ concentrations promoted biotically-controlled calcification by the skeletal factory. With changes produced by extinctions and turnovers at the Cretaceous-Tertiary boundary, adaptations to decreasing Ca 2+ and pCO 2 , coupled with increasing global temperature gradients (i.e., high-latitude and deep-water cooling), and strategies that efficiently linked photosynthesis and calcification, promoted successive changes of the dominant skeletal factory through the Cenozoic: larger benthic foraminifers (protist-protist symbiosis) during the Paleogene, red algae during the Miocene and modern coral reefs (metazoan-protist symbiosis) since Late Miocene.
Life (Basel, Switzerland), 2015
Marine cyanobacterial mats were cultured on coastal sediments (Nivå Bay, Øresund, Denmark) for over three years in a closed system. Carbonate particles formed in two different modes in the mat: (i) through precipitation of submicrometer-sized grains of Mg calcite within the mucilage near the base of living cyanobacterial layers, and (ii) through precipitation of a variety of mixed Mg calcite/aragonite morphs in layers of degraded cyanobacteria dominated by purple sulfur bacteria. The d13C values were about 2‰ heavier in carbonates from the living cyanobacterial zones as compared to those generated in the purple bacterial zones. Saturation indices calculated with respect to calcite, aragonite, and dolomite inside the mats showed extremely high values across the mat profile. Such high values were caused by high pH and high carbonate alkalinity generated within the mats in conjunction with increased concentrations of calcium and magnesium that were presumably stored in sheaths and extr...
Crystal Growth & Design, 2019
The importance of amorphous calcium carbonate (ACC) as a potential precursor phase in the biomineralization of marine calcifiers is increasingly being reported, particularly as the presence of ACC has been observed or inferred in several major groups. Here, we investigate the structure and conditions required to precipitate ACC from seawater-based solutions, with emphasis on the coinfluence of the carbonate system (pH, dissolved inorganic carbon (DIC) concentration), seawater Mg/Ca ratio and the presence of amino acids. We find that Mg 2+ and the presence of aspartic acid, glutamic acid, and glycine strongly inhibit ACC precipitation. Moreover, we were unable to precipitate ACC from seawater with a carbonate chemistry within the range of that thought to characterise the calcification site of certain marine calcifiers (i.e. DIC <6 mM, pH <9.3), although substantial modification of the seawater Mg/Ca ratio (Mg/Ca sw) allowed precipitation at a reduced DIC, with the implication that this could be an important component of utilising an ACC pathway. Finally, the degree to which Mg/Ca sw and the addition of amino acids influence the structure of ACC and the necessary seawater [CO 3 2-] for precipitation is strongly pH dependent. At lower, more biologically relevant pH than that typical of much inorganic work, decreasing Mg/Ca sw can result in greater long-range order and less water of crystallisation, but facilitates precipitation at a considerably lower [CO 3 2-] than at higher pH.
Experimental evidence for condensation reactions between sugars and proteins in carbonate skeletons
Geochimica et Cosmochimica Acta, 1992
Melanoidins, condensation products formed from protein and polysaccharide precursors, were once thought to be an important geological sink for organic carbon. The active microbial recycling of the precursors, coupled with an inability to demonstrate the formation of covalent linkages between amino acids and sugars in melanoidins, has shaped a powerful argument against this view. Yet, melanoidins may still be an abundant source of macromolecules in fossil biominerals such as shells, in which the proteins and polysaccharides are well protected from microbial degradation. We have modelled diagenetic changes in a biomineral by heating at 90°C mixtures of protein, polysaccha~des, and finely ground calcite crystals in sealed glass vials. Changes to the protein bovine serum albumin (BSA, fraction V) were monitored by means of gel electrophoresis and immunology. In the presence of water, BSA was rapidly hydrolyzed and remained immunologically reactive for less than 9 h. Under anhydrous conditions the protein was immunologically reactive for the whole period of the experiment ( 128 1 h), unless mono-or disaccharide sugars were also present. In the presence of these reactive sugars, browning, a discrete increase in molecular weight of the protein, and a concomitant loss of antigenicity confirmed that the sugars were attaching covalently to the protein, forming melanoidins. The de now formation of products crossreactive with antibodies raised against organic matter isolated from the shells of a fossil mollusc (Mercenariu mercenaria) indicated that at least in part the model simulated natural diagenesis. We roughly estimate that, at the global scale, 2.4 X lo6 tonnes of calcified tissue matrix glycoproteins is processed annually through the melanoidin nathwav. This amount would be equivalent to 7 per mil of the total flux of organic carbon into marine sediments.