Functional Diversity of the Litter-Associated Fungi from an Oxalate-Carbonate Pathway Ecosystem in Madagascar

Abstract

The oxalate–carbonate pathway (OCP) is a biogeochemical process linking oxalate oxidation and carbonate precipitation. Currently, this pathway is described as a tripartite association involving oxalogenic plants, oxalogenic fungi, and oxalotrophic bacteria. While the OCP has recently received increasing interest given its potential for capturing carbon in soils, there are still many unknowns, especially regarding the taxonomic and functional diversity of the fungi involved in this pathway. To fill this gap, we described an active OCP site in Madagascar, under the influence of the oxalogenic tree Tamarindus indica, and isolated, identified, and characterized 50 fungal strains from the leaf litter. The fungal diversity encompassed three phyla, namely Mucoromycota, Ascomycota, and Basidiomycota, and 23 genera. Using various media, we further investigated their functional potential. Most of the fungal strains produced siderophores and presented proteolytic activities. The majority were also able to decompose cellulose and xylan, but only a few were able to solubilize inorganic phosphate. Regarding oxalate metabolism, several strains were able to produce calcium oxalate crystals while others decomposed calcium oxalate. These results challenge the current view of the OCP by indicating that fungi are both oxalate producers and degraders. Moreover, they strengthen the importance of the role of fungi in C, N, Ca, and Fe cycles.

Soil Microbes from Saline–Alkali Farmland Can Form Carbonate Precipitates

“…Many studies have investigated the bacterial ability forming carbonate precipitates [12,[24][25][26][27][28][29][30][31][32][33]. As far as we know, the results firstly exhibit that fungal strains, belonging to Aspergillus, Cladosporium, and Trichoderma, isolated from saline-alkali farmland soil, contain the capacity of forming carbonate minerals (Figure 2), which extends the understanding of the carbon metabolism process of fungi in saline-alkali soils [34][35][36]. The capacity of carbonate precipitate formation may be a passive protection mechanism for fungi to adapt to the saline-alkali surroundings [34,37].…”

“…It has also been found that crops growing in saline-alkali soil can produce a large amount of calcium oxalate in their bodies to resist stress [38]. Such substances could be metabolized and utilized by some fungi to generate carbonate precipitates [34][35][36][37], which proves that soil fungi in saline-alkali soil may play a very important role in the formation of carbonate precipitates. In the present study, 10 strains were isolated on the basis of specific culture media, 7 of which contain the capacity of forming carbonate precipitates.…”

Calcium Oxalates in Soils within Disturbed Landscapes and Rock on the Territory of Yakutia (Russia), Formation Conditions in a Sharply Continental Cryoarid Climate

“…Sample 1663 was found in the form of a film on the surface of plant residues; according to [62,63], it was probably formed by micromycetes in the process of primitive soil formation of alluvial soil. A distinctive characteristic of the alluvial soils is periodic inundation by thawing waters, which is accompanied by the penetration and deposition of new mineral material on the soil surface as well as the burial of the horizon and formation of a new one, sometimes with a completely different composition.…”

Termite constructions as patches of soil fertility in Cambodian paddy fields

“…These peaks are not observed in CTRL and suggest that high MinC value in TN could correspond to biogenic carbonate, which could be derived from the oxalotrophic activity of termite-associated fungi and bacteria feeding on decaying oxalogenic plant tissues (Cailleau et al 2011;Mujinya et al 2011;Suryavanshi et al 2016;Francis and Poch 2019;Jouquet et al 2022). Additionally, these oxalate crystals could also have a bacterial (Hervé et al 2016) or fungal origin (Hervé et al 2021). Since termite constructions harbour microbial communities distinct from those present in the surrounding soil (Baker et al 2020), oxalogenic microorganisms could be more abundant in termite constructions than in soil.…”