Microorganisms as metal sorbents: comparison with other soil constituents in multi-compartment systems

A multi-compartment experimental system (PIGS) was used to study the importance of microorganisms for metal distribution in simulated soil systems. The influence of pH, solute composition (NaCl and CaCl2) and presence of fulvic acid on the distribution of zinc, cadmium and mercury among different phases in the multi-compartment system was studied, i.e., among bacteria, fungi and five other solid soil components and a solution phase. Using the multi-compartment system combined with a factorial design made it possible to study several soil factors as well as solid soil components simultaneously as well as to estimate interaction effects between soil factors. The microorganisms accumulated a considerable part (up to 38%) of the metal, despite the fact that they constituted only a minor fraction (0.4 or 1.7%) of the total solid mass. In contrast, quartz and feldspar, which together constituted 80% of the solid mass, accumulated less than 10%. The fraction associated with peat was generally large (11–57%) and the other solid components accumulated intermediate amounts of metal. Solution pH was the single factor that had the largest effect on the metal distribution. The effect of pH was less pronounced on fungi than on most other solid components, which indicates that the relative importance of fungi as metal sorbents increases as pH decreases. Changing solution composition from NaCl to CaCl2 decreased metal sorption to most sorbents except for the microorganisms where an increased accumulation was observed. The combined effects of pH and fulvic acid were considerable in some cases. This study stresses that the microbial sorbents can respond to changes in the soil solution chemistry in other ways than other solid soil components do. Thus, the microorganism fraction of the solid phase should not be neglected in metal distribution studies of soil.