Kinetic Mechanism For The Fungal Weathering Of Lizardite

The release of anthropogenic greenhouse gases (e.g., CO2) into the atmosphere has been linked to global climate change. Therefore, scientists believe that reducing the concentration of CO2 in the atmosphere is an important breakthrough to remit the global warming. Among the proposed schemes, mineral carbonation has unique advantages, but it is limted by the Mg acquisition. The weathering of serpentine results in serious environmental problems as the released nickel could accumulate in the environment. Researchers put foreward the method of phytoremediation. But it suffers from the low bioavailability of nickel. Since reseachers find out that microorganisms are effective in mineral weathering, more and more scientists pay close attention to the biotechnological potential of bioweathering. Till now, the kinetics and efficient of the fungus-mediated weathering of serpentine mineral is now well understood. And the mechanism during the course is poorly constrained quantitatively. Thus, the present study will combine field and laboratory work to investigate the dissolution mechanism of lizardite in the involvement of indeginous fungus.Sets of work were carried out in the field of a serpentine site (Donghai, China, 34°37'N,118°30'E) to sample soils, water, and unweathered lizardite mineral. After culturing, isolating, identificating and screening to obtain the model fungus, bioweathering of lizardite by the model fungus were conducted in shaking and static settings. The process were probed by using inductively coupled plasma-atomic emission spectroscopy (ICP-OES) to measure the concentrations of dissolved elements, pH meter to monitor the pH of the bulk solution, ELISA kit and microplate reader to quantify glucose and siderophores, high performance liquid chromatography (HPLC) to analyze the contents of organic acids, scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX) to abserve and analyze the morphology and chemical compositions of mineral surface, confocal laser scanning microscopy (CLSM) to detect local pH near single cells, atomic force microscopy (AFM) to characterize cell attachment and growth at surfaces and the topography of CMIs, focused ion beam (FIB) to prepare samples for high resolution transmission electron microscopy-energy dispersive X-ray spectroscopy (HRTEM-EDX) analysis of chemical compositions and crystal structure at the CMIs.Experimental results show that (1) lizardite in the root of plant undergone intersive weathering, mineral phases were thransfered and the chemical compositions were leached. Fungal cells were widespreded in the soils and a large number of secondary Mg carbonate mineral were formed on lizardite surface; (2) the number of culturable fungal isolates showed a positive coorelation with the weathering extent of the soil samples and the isolated species belonged to a range of common genera including Penicillium, Talaromyces, Trichoderma, Paecilomyces, Botryotinia, Fusarium, Cladosporium, Alternaria, Clavicipitaceae, Cosmospora, microdochium, Monascus and Plectosphaerella. Only Talaromyces flavus can keep high activity in high Mg2+(1 mol·L-1) or Ni2+(10 mmol·L-1) supplemented Czapek medium; (3) T. flavus can significantly accelerate the dissolution efficiency of lizardite. The bioweathering rates were greater as the temperature increased from 18 to 38?, approximately 48 wt% of Mg was released during the fungus-mediated dissolution of lizardite of 45-58 ?m at 38? in a period of 30 days. And the effect is 40%-50% higher as the mineral and fungal cells were mixed than that was separated; (4) various organic acids (oxalic acid, gluconic acid, formic acid, tartaric acid, malic acid, acetic acid, fumaric acid and citric acid) and siderophores with differnent concnetrations were excreted during fungal weating of lizardite, the pH of the bulk solution was acidified. Much more metabolites were biosynthesized and excreted by the attached fungal cells; (5) in weak acidic to neutral solution, the release rates of Mg and Si decerased with increasing pH, but the release rates of Fe and Ni did not decrease in solution with siderophores and oxalic acid, respectively; (6) local acidification was detected of the attached fungal cells (lower than the bulk solution by> 1.1 units); (7) channels with depth of?1000 nm and pits with depth of ?50 nm were produced by the hyphae and spores, respectively. The two dissolution features accounted for 9.17% and 34.45% of lizardite surface area. The altered layer, selective extraction of Mg and Fe, of hypha-mineral interface and spore mineral interface were ?8.0 ?m and ?4.5 ?m, respectively. Fe below solution-lizardite interface was undisturbed. A loss of crystallinity was found beneath the hypha-mineral interface, but not for that below spore-mineral interface. Estimated mineral loss at the interface suggests cellular dissolution can ultimately account for ?40%?50% of the overall bio-weathering.Theese findings suggest that (?) fungi play important roles in the weathering of lizardite and the biogeochemical cycling of elements in serpentine sites; (?) Mg and Si release was proton-promoted, while Fe and Ni release via ligand-promoted, and the biosynthesized siderophores and oxalic acid are responsible for the mobilization of Fe and Ni; (?) the activity of the attached cells were higher than those suspended in solution, and the attached ones increase lizardite dissolution in a much higher rates; (?) hypha enhances lizardite dissolution through the combination of biomechanical and biochemical forces, while only chemical forces for spore. Upon protonation leading to the weakening of the H bonds connecting individual TH-OH units and the initial release of M, the hyphal mechanical forces further separate the OH from the TH layer and ultimately breach the long-range order in lizardite; (?) intention for Fe from lizardite may be the motivation for the interfacial dissolution.In this study, scientific and standardized work was performed, from field to the labrotary, from bulk solution to fungal cell-mineral interface, revealing the kinetic mechanism for the fungal weathering of lizardite. The findings have potential to significantly advance our understanding of fungus-mineral interactions. It has implications for the global climate change and the bioremediation of heavy metal contaminated soils.