The oxalatecarbonate 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.