NICKEL-HYPERACCUMULATING PLANTS PROVIDE A NICHE FOR NICKEL-RESISTANT BACTERIA

The occurrence and abundance of nickel-resistant bacteria was studied in various soils originating from serpentine and other ultramafic rocks from both temperate and tropical localities. Samples were taken from soil covered with herbs and shrubs not known to accumulate nickel and from areas inhabited by nickel-hyperaccumulating shrubs and trees. In samples taken from the humus and mineral soil layers underneath the canopy of nickel-hyperaccumulating trees, the number of bacteria able to grow at 3, 10 and 20 mM NiCl2, was almost as high as the number of bacteria growing in the absence of nickel ions. In contrast, for soil samples taken outside these areas only moderately nickel-resistant (3 mM NiCl2) bacteria and low numbers were encountered. It is concluded that, in humid climate zones, the predominance of nickel-resistant bacteria in the soil is not due to the nickel ions generated by the weathering of nickel-containing rocks but is due to nickel salts being continuously liberated by the decay of nickel-rich leaf litter. Nickel-hyperaccumulating plants can be considered to drive a nickel cycle resulting in the continuous percolation of the top soil by nickel ions.