Effects of heavy metals in soil on microbial diversity and activity as shown by the sensitivity-resistance index, an ecologically relevant parameter

Abstract

A sensitivity-resistance index was developed, and proved to be a very sensitive biomonitor of soil pollution with heavy metals. The index was developed by a step-by-step approach. Ultimately, the bacterial soil microflora was divided into three groups, senstivive, tolerant, and resistant microflora. Zn and Cd sensitivity was defined as no growth occurring in the presence of 5 and 0.5 mg l^-1 of these metals, respectively, while resistance was defined as distinct growth in the presence of 50 and 16 mg l^-1, respectively. The sensitivity: resistance ratio of a referent clay soil (0.57 mg Cd kg^-1 and 140 mg Zn kg^-1) was 0.53, but for polluted (6 mg Cd kg^-1 + 670 mg Zn kg^-1) clay soil, the ratio was 0.24. For a referent (0.06 mg Cd kg^-1 + 12 mg Zn kg^-1) sandy soil the sensitivity: resistance ratio was 1.50 whereas polluted (2.3 mg Cd kg^-1 + 252 mg Zn kg^-1) sandy soil had a ratio 0.19. The ecological value of the sensitivity-resistance lies in its capacity to reflect potential deradation of aromatic compounds. It has been shown repeatedly that sensitive bacteria grow significantly better on a range of selected aromatic compounds. It has been speculated that resistance fo heavy metals may reduce the bioremediation capacity of soil towards chlorinated aromatics and polyaromatic hydrocarbons.