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Effects of heavy metals on microbial activityof water and sediment communitiesM. Cristina Romero 1 *, E. Mercedes Gatti 2 and D. Estela Bruno 2The effects of Cd 2+ , Cr 3+ and Zn 2+ on the microbial activity of water and sediment samples from a contaminatedstream were studied. The maximum [ 14 C]glucose uptake (V max ) and the mineralization ( 14 CO 2 ) rates were deter-mined. A 10% reduction in V max was obtained at lower metal concentrations in water samples than in sedimentones. Moreover, a 10% decrease in14 CO 2was observed at significantly minor metal levels, so14 CO 2was moresensitive to evaluated heavy metal pollution. On the basis of MICs obtained for both communities, they were moresensitive to Cd 2+ than to Cr 3+ and Zn 2+ . Zinc was less inhibitory to V max and14 CO 2rates; Cr 3+ showed an inter-mediate toxicity, and Cd 2+ was 10–100 times more inhibitory than the other metals.Key words:14 CO 2 , heavy metals, MICs, sediment communities, V max , water communities.Many metals are essential components of the cells at lowlevels, but they exert toxic effects at high concentrationssuch as those encountered in polluted environments. Inresponse to toxic concentrations of heavy metals, micro-organisms are able to develop tolerance (Kawai et al.1990; Hasnain et al. 1993). Bacteria, in particular, areknown to employ a variety of mechanisms for adaptationto the presence of toxic metal ions (Silver et al. 1993;Silver & Ji 1994). Among them, metal sorption, mineral-ization, uptake and accumulation, extracellular precipi-tation and enzymatic oxidation or reduction to a lesstoxic form, have been reported (Mergeay 1991; Hughes &Poole 1991; Nies 1992; Urrutia & Beveridge 1993; Joshi-Tope & Francis 1995; Anderson & Appanna 1993).Microorganisms also mediate the biogeochemical cy-cles of the nutrients in the environment, in as much as thedevelopment of metal tolerance by the natural microbialcommunities would allow these important functions tobe maintained despite metal inputs to the environment(Gadd & White 1993).Most previous toxicological research has estimatedthe numbers, biomass and the susceptibility of isolatedstrains to toxic metals in pure culture (Abbas & Edwards1989; Chander & Brookes 1993). Studies at the bacterialcommunity level are not so common and they have beenoften performed on soil habitats (Diaz-Ravin˜a et al. 1994;Diaz-Ravin˜a & Ba?a?th 1996; Knight et al. 1997).The aim of the present study was to examine the re-sponse of a sediment microbial community to Cd 2+ , Cr 3+and Zn 2+ pollution, to compare them with the responseof a water bacterial community and to determine theinhibitory concentrations that affect the activity andmineralization rates of both communities. These threeheavy metals were chosen for this research, because theyare listed as priority pollutants for the U.S. Environ-mental Protection Agency (U.S. EPA, 1979); and previousstudies on sediments in the area showed high levels ofthem: as 75 ppm Cd 2+ , 50 ppm Cr 3+ and 550 ppm Zn 2+(Ronco et al. 1992; Porta & Ronco 1993).Materials and MethodsSamples collection and preparationTwenty soil and water samples were taken from a contaminatedstreamnearanoilrefinery,LaPlata,Argentina.Soilsampleswereobtained by collecting the top 2–3 mm of superficial sludges witha sterile spatula and diluted 1:100 with physiological solution(v/v). Water samples were collected submerging 1.5 l sterileglass bottles, and they were assayed without dilution.Soil and water samples were divided into aliquots: (i) auto-claved sample; (ii) viable microbial sample to which nothing wasWorld Journal of Microbiology & Biotechnology, Vol 15, 1999 179World Journal of Microbiology & Biotechnology 15, 179–184ª 1999 Kluwer Academic Publishers1‘‘Instituto Spegazzini’’, Fac. Ciencias Naturales;2Fac. Ciencias Veterinarias-Univ. Nac. de La Plata, Argentina. The author M.C. Romero is now with the calle528 bis n? 1632, 1900 La Plata, Argentina. Fax: 0054-21-22 2904. *Corre-sponding author.