In vitro weathering of phlogopite by ectomycorrhizal fungi

Oxalate accumulation in external medium under hyphal mats of two ectomycorrhizal species is strongly stimulated (1.7 to 35 fold) by a simultaneous depletion of available K⁺ and Mg²⁺. Pisolithus tinctorius strain 441 accumulates oxalate both on NH₄–N and on NO₃–N whereas Paxillus involutus strain COU only accumulates oxalate on NO₃–N. On NO₃–N, under a simultaneous K⁺ and Mg²⁺ deficiency, P. involutus COU is a very active oxalate producer compared to P. tinctorius 441. The present results could explain the various mineralogical evolutions of a phlogopite mica previously recorded under P. involutus COU or P. tinctorius 441 and suggest a key role for fungal oxalic acid during mineral weathering in response to nutrient deficiency.