MINERAL ALTERATIONS BY ECTOMYCORRHIZAL FUNGI IN RESPONSE TO WATER AND NUTRIENT AVAILABILITY

Mineral weathering by mycorrhizal fungi is one of the key suppliers of non-nitrogen nutrients for plants, but the interplay between water and nutrient acquisition by fungi in water-limited ecosystems is less understood. Since climate models predict a drying trend in Ponderosa pine ecosystems, we investigated the response of fungi to water and nutrient limitations and the impact of water availability on fungus-driven mineral weathering. In a laboratory experiment, the available moisture was regulated through the relative humidity of the growth environment. Three environments were maintained throughout the experiment with 40, 60, and 80% relative humidity. Pure cultures of Suillus kaibabensis (two strains) and Hebeloma crustilinforme were isolated from Ponderosa pine in Flagstaff, AZ; then the cultures were grown on agar media for 10 weeks. The experiment was set up at two nutrient levels. One with sufficient nutrients supplied in the agar media and one where Ca, Mg, K, P, and Fe were limited and only supplied in mineral plugs of apatite, anorthite, and biotite. Pure quartz plugs served as mineral controls, and fungus without minerals served as fungal controls. Each treatment was replicated five times. Each dish was assembled under sterile conditions and checked for contamination once a week. The growth of fungi was monitored and documented using digital imaging followed by fungal colony diameter measurements. After 10 weeks, fungi were harvested for analysis, mineral plugs were preserved for scanning electron microscopy and fungal colonization analysis. Preliminary results indicate that fungal colony growth was the greatest under the lowest (40%) relative humidity conditions. Biotite mineral plugs were colonized the most, followed by apatite, anorthite, and then quartz. Mineral alteration by fungal weathering is expected to increase in the nutrient poor treatments with decreasing moisture availability, which may also be linked to increased fungal aquaporin (water transport protein) activity. The results of this experiment guide the implementation of a field study, where mineral or rock-containing mesh bags are deployed to test the interactions between water and nutrient acquisition in increasingly water-limited Ponderosa pine ecosystems.