THE EFFECT OF LONG-TERM EXPOSURE TO MERCURY ON THE BACTERIAL COMMUNITY IN MARINE SEDIMENT

The aim of this study was to investigate the effect of mercury contami nation on bacterial community structure and function. Bacterial commun ities from two sites, a mercury-contaminated site inside the harbor of Copenhagen, Denmark (CH) and a unpolluted control site, Koge Buge (KB ), were compared with respect to diversity indices, of antibiotic-and heavy metal-resistance patterns, abundance and self transmissibility o f plasmids in resistant isolates (endogenous isolation). Furthermore, the potential for gene transfer between indigenous bacteria was assess ed by the exogenous plasmid isolation approach. It was found that resi stance to all the tested compounds was higher in the mercury-polluted sediment than the control sediment. The abundance of plasmids was high er at the polluted site, where 62% of the isolates contained plasmids, whereas only 29% of the isolates from the control sediment contained plasmids. Furthermore, the frequencies of large plasmids and plasmids per isolates were found to be higher in the contaminated sediment. Exo genous plasmid isolations revealed high occurrence of Hg and tetracycl ine resistance, self-transmissible plasmids in CH sediment (1.8 x 10(- 5) transconjugants per recipients) relative to KB sediment (3.0 x 10(- 8) T/R). Shannon-Weaver diversity indices showed no difference in the diversity of the isolates from the two sites, and Hg-resistant isolate s from CH were found to be as diverse as the CH isolates in total. Thi s may be owing to high level of self-transmissible Hg resistance plasm ids found in CH.