How the rock -inhabiting fungus K. petricola A95 enhances olivine dissolution through attachment

Free-living and mycorrhizal fungi are able to enhance the weathering of rock and other solid substrates. Deciphering the exact mechanisms of these natural processes requires their experimental simulation. Moreover, by performing these simula- tions with genetically amenable rock -weathering fungi, one can knock -out certain fungal traits and consequently identify their weathering -relevant function. Here, the effect of the rock -inhabiting fungus, Knufia petricola A95, on the dissolution kinetics of an Fe -bearing olivine (Mg 1.86 Fe 0.19 SiO 4 ) is investigated at 25 ?C and pH 6 using reproducible batch and mixed flow exper- iments. The availability of a melanin -deficient mutant ( DKppks ) of K. petricola A95, which produces more extracellular poly- meric substances (EPS) than the wild type (WT), enables the comparative study of the role of melanin and EPS in olivine dissolution. In abiotic dissolution experiments, the olivine dissolution rate decreased considerably over time at pH 6 but not at pH 3.5. This inhibition of abiotic olivine dissolution at pH 6 was most likely caused by the in -situ oxidation of ferrous Fe and/or the precipitation of ferric hydroxides at the olivine surface. In corresponding biotic experiments at pH 6, both the wild type K. petricola and its melanin -deficient mutant DKppks solubilised and bound significant amounts of Fe released by olivine dissolution. Fe oxidation and precipitation were thus prevented and olivine dissolution proceeded faster than in the abiotic experiments. By sequestering Fe directly at the olivine surface, the attached wild type K. petricola cells were particu- larly efficient at preventing the oxidation of Fe at the mineral surface: the slowdown of olivine dissolution almost completely disappeared. The attachment capacity of these wild type cells is most likely mediated by wild type -specific EPS. Our presented experimental systems allow the oxidation of mineral -released Fe and include a rock -inhabiting fungus, thus simulating chem- ical, physical and biological conditions that set dissolution rates in a way that is relevant to natural ecosystems. (C) 2020 The Authors. Published by Elsevier Ltd.