The bacterial pathway
Calcium oxalate is present in many biological systems and can play a role in some human diseases (e.g. kidney stones). Despite the low solubility of metal-oxalate complexes and the high degree of oxidation of oxalate, approximately 40 species of bacteria are known to be able to dissolve and use calcium oxalate as sources of carbon and electrons. The aerobic decay of oxalates produce carbonate ions that can then react with the calcium previously released during the consumption of calcium oxalate by bacteria. The transformation of oxalate into carbonate induces an increase in pH that allows the precipitation of calcium carbonate. The oxalate-carbonate pathway has been studied using the production of carbonates by bacteria-decomposing calcium oxalate. Certain strains produce calcite, whereas others preferentially produce vaterite, a polymorph of calcium carbonate. The exopolysaccharides (EPS) and certain types of homo- or hetero-polypetides seem to greatly influence the differential crystallization of calcium carbonate.
For more information:
Khammar N., Martin G., Ferro K., Job D., Aragno M. & Verrecchia E. (2009): Use of the frc gene as a molecular marker to characterize oxalate-oxidizing bacterial abundance and diversity structure in soil. Journal of Microbiological Methods 76, 120-127.