RESPONSE OF SOIL BACTERIA TO HG(II) IN RELATION TO SOIL CHARACTERISTICS AND CELL LOCATION

The response of indigenous bacterial communities to the addition of in organic mercury (50 mu M of Hg(II)) was compared over a 30 day period in four soils of contrasting properties. Hg(II) impact was estimated b y following population dynamics of viable heterotrophic bacteria (VH) and mercury resistant bacteria (Hg-R) by indirect enumeration in unfra ctionated soil and in the inner and outer parts of soil aggregates obt ained by successive washings. Numbers of VH bacteria in unfractionated soils were not affected by mercury in any of the studied soils, where as an increase in resistant bacteria was observed in all of them. The lag phase before the increase of Hg-R bacteria and the magnitude of th e enrichment could be related to soil organic matter and clay contents suggesting that these factors govern mercury bioavailability. In frac tionated soil, the increase of resistant populations was more pronounc ed in the outer part (rep resenting mainly bacteria in macropores) tha n in the inner part of soil aggregates. This difference in response wa s clearer in soils with well developed aggregates. Qualitative analysi s revealed that exposure to Hg caused a large increase in the proporti on of Gram-negative strains among Hg-R bacteria in both soil parts.