METAL TOLERANCE OF ISOLATES OF RHIZOBIUM-LEGUMINOSARUM BIOVAR TRIFOLII FROM SOIL CONTAMINATED BY PAST APPLICATIONS OF SEWAGE-SLUDGE

We tested the tolerance to Cu, Ni, Cd and Zn of two isolates of Rhizobium leguminosarum biovar trifolii isolated from root nodules of clover plants grown in metal-contaminated sewage sludge treated plots (S-isolates) at Woburn Experimental Farm, and two isolates from root nodules of clover plants grown in uncontaminated control plots treated with farmyard manure (F-isolates). Survival of the isolates was compared in solutions containing different concentrations of Cu, Ni, Cd or Zn. The S-isolates from metal-contaminated soil were tolerant to larger concentrations of Cu, Ni, Cd and Zn compared to the F-isolates from uncontaminated control soil. The F-isolates were killed at concentrations of 0.002-mu-g ml-1 Cu, 0.2-mu-g ml-1 Cd 0.8-mu-g ml-1 Ni and 0.8-mu-g ml-1 Zn within 72 h, whilst the S-isolates survived, albeit in reduced numbers, at concentrations of 0.01-mu-g ml-1 Cu and 1.0-mu-g ml-1 Zn, but were killed by 1.0-mu-g ml-1 Ni and 0.8-mu-g ml-1 Cd within 72 h. A particularly strong increase in tolerance to Zn was shown by the S-isolates compared to other metals at the same concentrations. Thus, the order of decreasing toxicity in solution to the two F-isolates was Cu > Cd > Zn = Ni, but for the two S-isolates it was Cu > Cd > Ni > Zn. The S-isolates have multiple metal tolerance that enables them to survive in metal-contaminated soil, but they have lost their ability to fix nitrogen with Trifolium repens L. The F-isolates can fix nitrogen, but these do not survive in metal-contaminated soil because they lack tolerance to these metals.