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The oxalate-carbonate pathway: measuring biological interactions and dynamics in a natural C sink ecosystem
Titre du projet
The oxalate-carbonate pathway: measuring biological interactions and dynamics in a natural C sink ecosystem
Description
The formation of calcite in otherwise carbonate-free acidic soils through the biological degradation of oxalate is a mechanism termed oxalate-carbonate pathway, which occurs during interaction between biological and geological systems. In this pathway, atmospheric CO2 is fixed by the photosynthetic activity of plants, part of which is destined to the production of oxalate to control the intracellular Ca2+ concentrations. An additional source of calcium oxalate is fungi, which are able to produce this organic acid to cope with elevated concentrations of metals. The decay of plant material results in a source of calcium oxalate for other trophic levels. In spite of its abundance as a substrate, oxalate is a very stable organic anion that can be metabolized only by a group of bacteria that use it as a carbon and energy source. These bacteria close the biological cycle by degrading calcium oxalate, releasing Ca2+ and changing the local soil pH. If the conditions are adequate, the geological part of the pathway begins because this biological process will indirectly lead to the precipitation of secondary calcium carbonate (calcite) under unexpected geological settings.
The activity of the oxalate-carbonate pathway has now been demonstrated in several places around the world. Furthermore, it can constitute an important, although underestimated, soil mineral carbon sink. This is because due to the initial acidic soil conditions and the absence of geological carbonate in the basement, it is unexpected to find C in the form of calcite. By its global scale and its stability through a long period of time, this terrestrial C sink is of a crucial interest as the sustainability of other C sequestering processes (e.g. sinking of CO2 in the ocean) is under question.
The study of the oxalate-carbonate pathway constitutes a multidisciplinary research that brings together competences in biology (botany, physiology, microbiology) and geology (geochemistry, mineralogy, soil science). Thus, from its inception, this research has been carried out by a multidisciplinary team and by combining two crucial aspects: field expeditions and laboratory work using several tropical soils as models. Our most recent results show that biological interactions between bacteria and fungi, that have been underestimated, are essential for reproducing the pathway in vitro. Also, we have observed that Ca budget/availability has a direct impact on pedogenic carbonate accumulations.
Therefore, the aims of this proposal are twofold: first, we aim at elucidating the nature of the interaction between bacteria and fungi and its importance for the oxalate-carbonate pathway. Second, we aim at understanding how the Ca cycle (pools, fluxes and limitations in the concentration of Ca2+) can drive an “ecosystem induced-C sink”, in our case the oxalate-carbonate pathway system.
By pursuing these two objectives, we expect to contribute essential information within two of the current “black boxes” of the system, which will allow the establishment of a model of the dynamics of carbon accumulation associated with the oxalate-carbonate pathway. This may have an enormous impact due to the potential importance of this ecosystem (or equivalent ecosystems) in tackling the increasing atmospheric CO2 concentrations and their direct effect over global climate.
The activity of the oxalate-carbonate pathway has now been demonstrated in several places around the world. Furthermore, it can constitute an important, although underestimated, soil mineral carbon sink. This is because due to the initial acidic soil conditions and the absence of geological carbonate in the basement, it is unexpected to find C in the form of calcite. By its global scale and its stability through a long period of time, this terrestrial C sink is of a crucial interest as the sustainability of other C sequestering processes (e.g. sinking of CO2 in the ocean) is under question.
The study of the oxalate-carbonate pathway constitutes a multidisciplinary research that brings together competences in biology (botany, physiology, microbiology) and geology (geochemistry, mineralogy, soil science). Thus, from its inception, this research has been carried out by a multidisciplinary team and by combining two crucial aspects: field expeditions and laboratory work using several tropical soils as models. Our most recent results show that biological interactions between bacteria and fungi, that have been underestimated, are essential for reproducing the pathway in vitro. Also, we have observed that Ca budget/availability has a direct impact on pedogenic carbonate accumulations.
Therefore, the aims of this proposal are twofold: first, we aim at elucidating the nature of the interaction between bacteria and fungi and its importance for the oxalate-carbonate pathway. Second, we aim at understanding how the Ca cycle (pools, fluxes and limitations in the concentration of Ca2+) can drive an “ecosystem induced-C sink”, in our case the oxalate-carbonate pathway system.
By pursuing these two objectives, we expect to contribute essential information within two of the current “black boxes” of the system, which will allow the establishment of a model of the dynamics of carbon accumulation associated with the oxalate-carbonate pathway. This may have an enormous impact due to the potential importance of this ecosystem (or equivalent ecosystems) in tackling the increasing atmospheric CO2 concentrations and their direct effect over global climate.
Chercheur principal
Statut
Completed
Date de début
1 Janvier 2012
Date de fin
31 Décembre 2013
Chercheurs
Identifiant interne
14564
identifiant
1 Résultats
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- PublicationMétadonnées seulementDetection of active oxalate-carbonate pathway ecosystems in the Amazon Basin: global implications of a natural potential C sink(2013-12-24)The oxalate-carbonate pathway (OCP) is a biogeochemical process, which has been described in Milicia excelsa tree ecosystems of Africa. This pathway involves biological and geological parameters at different scales: oxalate, as a by-product of photosynthesis, is oxidized by oxalotrophic bacteria leading to a local pH increase, and eventually to carbonate accumulation through time in previously acidic and carbonate-free tropical soils. Former studies have shown that this pedogenic process can potentially lead to the formation of an atmospheric carbon sink. Considering that 80 % of plant species are known to produce oxalate, it is reasonable to assume that Milicia excelsa is not the only tree that can support OCP ecosystems. The search for similar conditions on another continent led us to South America, in an Amazon forest ecosystem (Alto Beni, Bolivia). This area was chosen because of the absence of local inherited carbonate in the bedrock, as well as its expected acidic soil conditions. Eleven tree species and associated soils were tested positive for the presence of carbonate with a more alkaline soil pH close to the tree than at distance from it. A detailed study of Pentaplaris davidsmithii and Ceiba speciosa trees showed that oxalotrophy impacted soil pH in a similar way to at African sites (at least with 1 pH unit increasing). African and South American sites display similar characteristics regarding the mineralogical assemblage associated with the OCP, except for the absence of weddellite. The amount of carbonate accumulated is 3 to 4 times lower than the values measured in African sites related to Milicia excelsa ecosystems. Still, these secondary carbonates remain critical for the continental carbon cycle, as they are unexpected in the acidic context of Amazonian soils. Therefore, the present study demonstrates the existence of an active OCP in South America. The three critical components of an operating OCP are the presence of : i) local alkalinisation, ii) carbonate accumulations, and iii) oxalotrophic bacteria, which were identified associated to the oxalogenic tree Ceiba speciosa. If the question of a potential carbon sink related to oxalotrophic-oxalogenic ecosystems in the Amazon Basin is still pending, this study highlights the implication of OCP ecosystems on carbon and calcium biogeochemical coupled cycles. As previously mentioned for Milicia excelsa tree ecosystems in Africa, carbonate accumulations observed in the Bolivian tropical forest could be extrapolated to part or the whole Amazon Basin and might constitute an important reservoir that must be taken into account in the global carbon balance of the Tropics.