Isolation and characterization of oxalotrophic bacteria from tropical soils

The oxalate–carbonate pathway (OCP) is a biogeochemical set of reactions that involves the conversion of atmospheric CO₂ fixed by plants into biomass and, after the biological recycling of calcium oxalate by fungi and bacteria, into calcium carbonate in terrestrial environments. Oxalotrophic bacteria are a key element of this process because of their ability to oxidize calcium oxalate. However, the diversity and alternative carbon sources of oxalotrophs participating to this pathway are unknown. Therefore, the aim of this study was to characterize oxalotrophic bacteria in tropical OCP systems from Bolivia, India, and Cameroon. Ninety-five oxalotrophic strains were isolated and identified by sequencing of the 16S rRNA gene. Four genera corresponded to newly reported oxalotrophs (<i>Afipia</i>, <i>Polaromonas</i>, <i>Humihabitans</i>, and <i>Psychrobacillus</i>). Ten strains were selected to perform a more detailed characterization. Kinetic curves and microcalorimetry analyses showed that <i>Variovorax soli</i> C18 has the highest oxalate consumption rate with 0.240 μM h^-1. Moreover, <i>Streptomyces achromogenes</i> A9 displays the highest metabolic plasticity. This study highlights the phylogenetic and physiological diversity of oxalotrophic bacteria in tropical soils under the influence of the oxalate–carbonate pathway.