Experimental calcium-oxalate crystal production and dissolution by selected wood-rot fungi

Twenty-six species of white-rotting <i>Agaricomycotina fungi</i> (<i>Basidiomycota</i>) were screened for their ability to produce calcium-oxalate (CaOx) crystals in vitro. Most were able to produce CaOx crystals in malt agar medium in the absence of additional calcium. In the same medium enriched with Ca²⁺, all the species produced CaOx crystals (weddellite or whewellite). Hyphae of four species (<i>Ganoderma lucidum</i>, <i>Polyporus ciliatus</i>, <i>Pycnoporus cinnabarinus</i>, and <i>Trametes versicolor</i>) were found coated with crystals (weddellite/whewellite). The production of CaOx crystals during the growth phase was confirmed by an investigation of the production kinetics for six of the species considered in the initial screening (<i>Pleurotus citrinopileatus</i>, <i>Pleurotus eryngii</i>, <i>Pleurotus ostreatus</i>, <i>P. cinnabarinus</i>, <i>Trametes suaveolens</i>, and <i>T. versicolor</i>). However, the crystals produced during the growth phase disappeared from the medium over time in four of the six species (<i>P. citrinopileatus</i>, <i>P. eryngii</i>, <i>P. cinnabarinus</i>, and <i>T. suaveolens</i>). For <i>P. cinnabarinus</i>, the disappearance of the crystals was correlated with a decrease in the total oxalate concentration measured in the medium from 0.65 μg mm^-2 (at the maximum accumulation rate) to 0.30 μg mm^-2. The decrease in the CaOx concentration was correlated with a change in mycelia morphology. The oxalate dissolution capability of all the species was also tested in a medium containing calcium oxalate as the sole source of carbon (modified Schlegel medium). Three species (<i>Agaricus blazei</i>, <i>Pleurotus tuberregium</i>, and <i>P. ciliatus</i>) presented a dissolution halo around the growth zone. This study shows that CaOx crystal production is a widespread phenomenon in white-rot fungi, and that an excess of Ca²⁺ can enhance CaOx crystal production. In addition, it shows that some white-rot fungal species are capable of dissolving CaOx crystals after growth has ceased. These results highlight a diversity of responses around the production or dissolution of calcium oxalate in white-rot fungi and reveal an unexpected potential importance of fungi on the oxalate cycle in the environment.