Project

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Importance of fungal highways in soil functioning: the “oxalate-carbonate pathway” as a model system of microbial interactions

Applicant Bindschedler Saskia
Number 147742
Funding scheme Advanced Postdoc.Mobility
Research institution Institut de Minéralogie et de Physique des Milieux Condensés Université Pierre et Marie Curie
Institution of higher education Institution abroad - IACH
Main discipline Other disciplines of Environmental Sciences
Start/End 01.10.2013 - 31.07.2014
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All Disciplines (2)

Discipline
Other disciplines of Environmental Sciences
Environmental Research

Keywords (9)

oxalogenesis; soils; oxalotrophy; pedogenic carbonates; mineral-microbe interactions; Ca and C cycling; geomicrobiology; fungal highways; fungi-bacteria interactions

Lay Summary (French)

Lead
Importance des autoroutes fongiques dans le fonctionnement du sol: la "voie oxalate-carbonate" en tant que système modèle d'interactions microbiennes
Lay summary

Lead

La voie oxalate-carbonate (VOC) est un processus naturel décrit en milieu tropical, dont l’effet est d’alcaliniser les sols. La VOC a un impact sur le cycle du C et sur la fertilité des sols. Ses acteurs sont les plantes, champignons et bactéries. L’oxalate produit par les plantes et les champignons forme dans les sols un stock d’oxalate de calcium (CaOx). Malgré une très faible solubilité, le CaOx ne s’accumule pas dans l’environnement. Ceci est le fait de bactéries oxalotrophes, qui utilisent l’oxalate pour leur croissance. Cela mène à une alcalinisation et si le pH augmente suffisamment, du CaCO3 se forme. Le CaOx est transformé par les bactéries, mais il a démontré que dans le sol, la présence des bactéries et des champignons est essentielle pour observer une VOC active.

 

Contenu et objectifs du travail de recherche

Pour expliquer l’effet bénéfique des champignons sur les bactéries oxalotrophes un mécanisme appelé « autoroutes fongiques » a été proposé : les bactéries utilisent les hyphes de champignons comme support physique pour se déplacer vers les patchs de CaOx dans des milieux hétérogènes comme les sols. Dans ce projet il a donc été choisi de s’intéresser au fonctionnement des « autoroutes fongiques » en utilisant la VOC comme système modèle. En particulier, deux aspects seront traités : 1) comment l’interaction entre champignons et bactéries sous forme d’ « autoroutes fongiques » affecte la mobilité dans les sols d’un cation essentiel à la VOC, le calcium ; 2) qui sont les organismes impliqués dans des associations efficaces d’autoroutes fongique dans un écosystème VOC naturel.

 

Contexte scientifique et social du projet de recherche

D’un point de vue fondamental les résultats permettront de mieux comprendre certaines règles qui régissent le mécanisme d’autoroute fongique. D’un point de vue plus pratique, cela mènera à une meilleure compréhension de certains aspects liés aux facteurs biologiques qui permettent à la VOC d’être effective et efficace.

Direct link to Lay Summary Last update: 18.07.2013

Responsible applicant and co-applicants

Collaboration

Group / person Country
Types of collaboration
UFZ Leipzig, Environmental Microbiology Germany (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication

Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
Conferences Jacques-Monod CNRS “Bacterial-fungal interactions: a federative field for fundamental and applied microbiology.” Talk given at a conference Impact of fungal mycelia on the dispersal and activity of oxalotrophic bacteria in soils. 07.12.2013 Roscoff, France Bindschedler Saskia;


Abstract

The oxalate-carbonate pathway (OCP) is a process described in tropical environments and occurring at the ecosystem scale, which has two main impacts on soils. First, it is able to store in otherwise carbonate-free soils substantial amounts of calcium carbonate (CaCO3). When calcium (Ca) originates from a carbonate-free source, this process can lead to the formation of a long term carbon (C) sink, as CaCO3 has a residence time in average 100 times higher than organic C. Second, the OCP is assumed to have a positive effect on soil fertility, an aspect which still needs to be more thoroughly assessed. The main biological players of this pathway are plants, fungi and, bacteria. The role of plants and fungi consists in supplying oxalate to bacteria. Oxalate is common in terrestrial ecosystems, mainly as calcium-oxalate (Ca-ox). Despite its low solubility, this salt does not accumulate in soils. This is largely due to oxalotrophic bacteria, which use oxalate as C and energy sources, leading to its oxidation into CO2. The oxidation of Ca-ox leads to a pH increase, allowing subsequent CaCO3 precipitation. While in Petri dishes with Ca-ox as the only C source the sole presence of bacteria is required to complete this process, recent studies performed in microcosms amended with Ca-ox demonstrate that the concomitant presence of fungi and bacteria is essential to lead to the pH increase. A proposed mechanism to explain this fact is the use of fungal hyphae as access-routes by bacteria, in order to reach Ca-ox patches. The migration of flagellated bacteria along fungal hyphae is known under the term of “fungal highways”. This mechanism has been proposed to explain bacterial dispersion in unsaturated porous media, such as soils. Indeed, in soils, resources and water are distributed in a patchy manner. As a result, diffusion of substrates is limited and access to it is restricted to organisms that do not have the capacity to cross air-filled pores, such as bacteria. However, the filamentous lifestyle of fungi allows them to cross these gaps. Therefore, the fungal highways hypothesis proposes that bacteria can benefit from the thin water film surrounding the hyphae in order to use the fungal network to spread into the heterogeneous soil matrix. However, several aspects regarding this interaction between bacteria and fungi remain enigmatic. In my current research project (“Influence of fungal highways on the efficiency of the OCP in unsaturated environments”; n° PBLAP2-140105) I could demonstrate that fungal networks are, under some conditions, a positive factor for oxalotrophic bacteria, allowing a faster dispersion and consequently an increased Ca-ox turnover. The outcome of the fungus-bacteria interaction (either association into fungal highways or exclusion between both partners) is dependant on factors such as C source, water content, and heterogeneity of the environment. Finally, a soil microcosm experiment performed with natural fungal and bacterial communities from an OCP ecosystem, suggests that fungal highways are involved in enhancing the overall soil oxalotrophic activity. In this project, which is a continuation of my previous SNF fellowship for prospective researchers, it is proposed to get deeper insights into the importance of fungal highways in soil functioning by focusing on the interaction between fungi and bacteria in the context of the OCP. Based on the results obtained so far in, two points are addressed in this proposal. First, it aims at getting evidences on how is Ca mobilisation and/or release affected by the mode of interaction between fungi and bacteria (collaboration into fungal highways or exclusion between both partners). Second, it intends to complement the soil microcosm experiment with molecular ecology analyses in order to characterize the microbial communities which are important in OCP microbial ecosystems and involved in fungal highways. The results obtained in this project will be two-fold: i) they will increase the understanding of the fungal highway mechanism and ii) they will lead to a better understanding of some aspects related to the biological factor leading to an efficient functioning of the OCP.
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