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- PublicationAccès libreBiologically induced mineralization in the tree Milicia excelsa (Moraceae) : its causes and consequences to the environmentIroko trees (Milicia excelsa) in Ivory Coast and Cameroon are unusual because of their highly biomineralized tissues, which can virtually transform the trunk into stone. Oxalic acid (C2O4H2) and metal-oxalate play important roles in their ecosystems. In this study, the various forms of oxalate and carbonate mineralization reactions are investigated by using scanning electron microscopy and X-ray diffraction. Calcium oxalate monohydrate is associated with stem, bark and root tissues, whereas calcium oxalate dihydrate is found with wood rot fungi in soils, as well as in decaying wood. Laboratory cultures show that many soil bacteria are able to oxidize calcium oxalate rapidly, resulting in an increase in solution pH. In terms of M. excelsa, these transformations lead to the precipitation of calcium carbonate, not only within the wood tissue, but also within the litter and soil. We calculate that c. 500 kg of inorganic carbon is accumulated inside an 80-year-old tree, and c. 1000 kg is associated with its surrounding soil. Crucially, the fixation of atmospheric CO2 during tree photosynthesis, and its ultimate transformation into calcite, potentially represents a long-term carbon sink, because inorganic carbon has a longer residence time than organic carbon. Considering that calcium oxalate biosynthesis is widespread in the plant and fungal kingdoms, the biomineralization displayed by M. excelsa may be an extremely common phenomena.
- PublicationMétadonnées seulementCycle du carbone et biominéralisation carbonatée en milieu continental: la diagénèse des phases oxalate-carbonateIn terrestrial environments, the carbon sequestration pool in the global carbon cycle is almost entirely attributed to soil and plant organic carbon storage while soil mineral carbon is generally neglected. Nevertheless, significant biologically induced accumulations of carbonate have been observed in soils under the tropical tree "iroko" (Milicia excelsa). Without an input of calcium from carbonate sources in the system, these accumulations constitute a carbon sink by definition. Approximately 1.2. 10-4 to 2.3.10-3 PgC are not sequestered each year due to deforestation of irokos. This calculation is based on an average of 5.76 kg/yr of carbon sequestration by one tree and extrapolated for to the iroko population in Africa. This proposed quantification suggests that the iroko carbon sink is about one or two orders of magnitude less than some environments such as coral reefs or continental shelfes. Furthermore, this carbon sink is highly significant because the residence time of mineral carbon in carbonate-enriched soils associated with the irokos is 102 - 106 years, i.e. 100,000 times longer than soil organic matter residence time. These accumulations of carbonate are the result of the oxalate-carbonate transformation by oxalotrophic bacteria. The iroko tree provides a large amount of oxalate (a photosynthetic by-product), which once released in the soil, can be consumed by soil oxalotrophic bacteria. There are two consequences of this consumption: (i) carbonate ions are produced and available in the soil solution, (ii) and soil pH increases leading to favorable conditions for carbonate precipitation (in a primary acid soil with a pH around 5, the induced pH can reach 9). In the iroko ecosystem, biological agents are present and active at various scales. Termites and saprophytic fungi are involved in the release of oxalate crystals in the soil. Soil oxalotrophic bacteria are able to consume oxalate leading to CO32- production. Carbonate ions can be pumped by iroko roots, circulating into wood vessels in which they can form calcium carbonate crystals in the presence of calcium, when conditions are favorable. They can also precipitate as calcium carbonate in soil pores, depending on local conditions. Soil bacteria and fungi obviously influence precipitation of carbonate inside the soil. Moreover, regarding carbon isotopic signatures and crystallographic properties of needle fiber calcite (NFC) observed in surficial environments, including African soils, they result from a direct biogenic influence. The nomenclature of the various types of NFC has been reinterpreted regarding diagenesis and geochemical fingerprints. The possible biogenic origin of nanorods usually found in the presence of NFC is also discussed from new observations and experiments. In conclusion, this study has demonstrated the potential importance of the oxalate-carbonate pathway in the global carbon cycle, emphasizing the crucial interrelations between geological and biological processes.
- PublicationAccès libreBiomineralization in plants as a long-term carbon sinkCarbon sequestration in the global carbon cycle is almost always attributed to organic carbon storage alone, while soil mineral carbon is generally neglected. However, due to the longer residence time of mineral carbon in soils (102–106 years), if stored in large quantities it represents a potentially more efficient sink. The aim of this study is to estimate the mineral carbon accumulation due to the tropical iroko tree (Milicia excelsa) in Ivory Coast. The iroko tree has the ability to accumulate mineral carbon as calcium carbonate (CaCO3) in ferralitic soils, where CaCO3 is not expected to precipitate. An estimate of this accumulation was made by titrating carbonate from two characteristic soil profiles in the iroko environment and by identifying calcium (Ca) sources. The system is considered as a net carbon sink because carbonate accumulation involves only atmospheric CO2 and Ca from Ca-carbonate-free sources. Around one ton of mineral carbon was found in and around an 80-year-old iroko stump, proving the existence of a mineral carbon sink related to the iroko ecosystem. Conservation of iroko trees and the many other biomineralizing plant species is crucial to the maintenance of this mineral carbon sink.
- PublicationMétadonnées seulementBacterially induced mineralization of calcium carbonate in terrestrial environments: the role of exopolysaccharides and amino acids
- PublicationMétadonnées seulement
- PublicationAccès libreBiologically induced accumulations of CaCO3 in orthox soils of Biga, Ivory CoastBiologically induced accumulations of calcium carbonate have been found inside orthox soils, under and around the native iroko tree Milicia excelsa (Moraceae) in Biga (Ivory Coast). The nature of these accumulations and their origin were studied in two soil profiles, directly under the tree and at a distance of 30 cm from the trunk. Microscale forms of CaCO3 include: (1) wood pseudomorphic structures such as parenchyma cells, cellulose fibers, and calcitic vessel infillings; (2) three types of rhombohedra; and (3) needle fiber calcite (NFC). In addition, large scale blocks exhibit three types of textures: (1) micritic calcite, which seems to be the original material; (2) light-colored sparite in moldic voids; and (3) asymmetrical radiaxial laminated fibrous cement. Some micritic aggregates and hemi-spherulites (vaterite) were found in the sap on the trunk as well as in soils on silica grains and the wood itself. The mineralogy of all these carbonate forms is mainly a stoichiometric calcite or a moderately enriched Mg calcite. However, some samples contain monohydrocalcite, as well as two polymorphs of calcium oxalate (weddellite and whewellite). Calcite precipitation is facilitated by the oxidation of oxalate by soil bacteria that contributes to the increase in pH in Biga soils. This is in contrast to conventional orthox soils. Therefore, three conditions are necessary for biologically induced precipitation of calcium carbonate in orthox soils associated with iroko trees: the presence of a large amount of oxalate (originating from the tree and fungi), the existence of an oxalotrophic flora for oxalate oxidation into carbonate, and a dry season.
- PublicationAccès libreFungi, bacteria and soil pH: the oxalate–carbonate pathway as a model for metabolic interactionThe oxalate–carbonate pathway involves the oxidation of calcium oxalate to low-magnesium calcite and represents a potential long-term terrestrial sink for atmospheric CO2. In this pathway, bacterial oxalate degradation is associated with a strong local alkalinization and subsequent carbonate precipitation. In order to test whether this process occurs in soil, the role of bacteria, fungi and calcium oxalate amendments was studied using microcosms. In a model system with sterile soil amended with laboratory cultures of oxalotrophic bacteria and fungi, the addition of calcium oxalate induced a distinct pH shift and led to the final precipitation of calcite. However, the simultaneous presence of bacteria and fungi was essential to drive this pH shift. Growth of both oxalotrophic bacteria and fungi was confirmed by qPCR on the frc (oxalotrophic bacteria) and 16S rRNA genes, and the quantification of ergosterol (active fungal biomass) respectively. The experiment was replicated in microcosms with non-sterilized soil. In this case, the bacterial and fungal contribution to oxalate degradation was evaluated by treatments with specific biocides (cycloheximide and bronopol). Results showed that the autochthonous microflora oxidized calcium oxalate and induced a significant soil alkalinization. Moreover, data confirmed the results from the model soil showing that bacteria are essentially responsible for the pH shift, but require the presence of fungi for their oxalotrophic activity. The combined results highlight that the interaction between bacteria and fungi is essential to drive metabolic processes in complex environments such as soil.
- PublicationAccès libreBacterially Induced Mineralization of Calcium Carbonate in Terrestrial Environments: The Role of Exopolysaccharides and Amino Acids(2003)
;Braissant, Olivier ; ;Dupraz, ChristopheVerrecchia, Eric P.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.
- PublicationAccès libreTracing soil carbon cycle and the origin of needle fibre calcite