Soil Microorganism-mineral-organic Matter Interactions and the Impact on Metal Mobility

Soils are huge reservoirs of micro-organisms. They contain all the main groups of microorganisms with a very large diversity of bacteria, fungi, algae, protozoa. In the upper layers of soils, bacteria can have a biomass of 1,000 to 4,000 kg·ha

−1

and an average number of 106 to 109 g

−1

of dry soil. Fungi have approximately the same to slightly higher biomass. Both bacteria and fungi are now well recognized as key agents of biogeochemical cycling of major and trace elements and as main actors of soil functioning. Other microorganisms as algae (photosynthetic organisms), present at the soil surface, are not so well known in soils and play a minor role, but are considered as endolithic pioner organisms involved in rock weathering particularly in cold and warm deserts. Lichens, a symbiotic association between algae and fungi, are also interesting as colonizing organisms and weathering agents.