Ectomycorrhizal fungi (ECMF) grow on tree roots, providing their host tree with mineral nutrients in exchange for carbohydrates. ECMF are important components of forest ecosystems. Because they are the interface between tree roots and soil, ECMF may protect their host from toxic contaminants such as mercury. Because forest soils are naturally acidic, a high proportion of the total mercury in soil may remain bioavailable for long periods. It is therefore important to understand how mercury affects ECMF, their host plants, and surrounding bacterial communities. In collaboration with Dr. John Dighton, director of the Rutgers Pinelands Field Station, our lab is investigating two major aspects of the effects of mercury on these systems.

A graduate student in the Ecology and Evolution graduate program, Sharron Crane, has shown that mercury decreases fungal growth rate as measured by both colony expansion and biomass increase. Mercury also changes hyphal branching patterns (see figure below). It is well established that fungal biomass passively absorbs metals from solution: by monitoring accumulation of mercury by Suillus decipiens and Laccaria laccata, Sharron has obtained evidence that there may be a biological component to the accumulation of mercury by ECMF. Sharron’s current project involves looking at how mercury added to soil affects both the colonization of Pinus rigida (pitch pine) by ECMF and the growth of the host tree.

The impact of mercury on the morphology of the mycelium of Laccaria laccata as observed under the light microscope. The image on left is of Laccaria laccata grown in presence of 40 µM mercuric chloride (100x magnification) which results in the formation of rhizomorph, a thick, hollow structure usually associated with survival under stress. The image on the right is of Laccaria laccata growing without mercury (200x magnification; a healthy mycelium is apparent.

Another graduate student in our lab, Rachel Ward, is using molecular characterization of microbial communities associated with mycorrhizal tree roots to investigate the effect of mercury on community diversity and composition. Once she has determined which groups of bacteria tend to be associated with mycorrhizal roots in unpolluted soil, she will examine the changes that occur in these communities in the presence of mercury.