Prey-predator dynamics in communities of culturable soil bacteria and protozoa: differential effects of mercury Maria S. Holtze a,b, * , Flemming Ekelund a , Lasse D. Rasmussen c , Carsten S. Jacobsen b , Kaare Johnsen b,d a Zoological Institute, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen Ø, Denmark b Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark c Department of General Microbiology, University of Copenhagen, Sølvgade 83H, DK-1307 Copenhagen K, Denmark d Danish Veterinary Institute, Bu¨lowsvej 27, DK-1790 Copenhagen V, Denmark Received 11 June 2002; received in revised form 26 March 2003; accepted 15 April 2003 Abstract We investigated whether the prey-predator dynamics of bacteria and protozoa were affected by inorganic mercury at concentrations of 0, 3.5 and 15 mg Hg(II) kg soil 21 . The amount of bioavailable Hg was estimated using a biosensor-assay based on the mer – lux gene fusion. The numbers of bacterial CFUs on the general medium 1/100 tryptic soy agar (TSA) were significantly decreased when the soil had been amended with Hg. In contrast, no effect was seen on the number of CFUs on the Pseudomonas-specific medium Gould’s S1 agar. Protozoan numbers estimated by the most probable number (MPN) method with 1/100 TSB as growth medium were also negatively affected by Hg. The different fractions of protozoa were affected to different degrees suggesting that amoebae were less sensitive than slow-growing flagellates, which again were less sensitive than the fast-growing flagellates. In contrast, Hg did not induce any detectable changes in the diversity of flagellate morphotypes. In the treatment with 15 mg Hg kg 21 a transiently increased number of bacteria was seen at day 6 probably concomitant with a decrease in the numbers of protozoa. This might indicate that Hg affected the prey-predator dynamics in communities of culturable bacteria and protozoa in soil. Furthermore, we showed that the number of Pseudomonas spp. was not affected by Hg whereas the number of bacteria growing on a general medium was. q 2003 Elsevier Ltd. All rights reserved. Keywords: Bioavailable mercury; Microcosms; Prey-predator dynamics; Protozoa; Pseudomonas 1. Introduction Mercury is widespread in the environment since it originates from natural sources, the burning of fossil fuels, industry and the use of fungicides (Dhawale et al., 1996; Boening, 2000). Heavy metals are often found as co- contaminants in soils heavily polluted with organic compounds (Baldrian et al., 2000; Roane et al., 2001). Since Hg is highly toxic to all living cells (Foster, 1987; Giller et al., 1998), bioremediation of organic pollutants in soils co-contaminated with Hg can be impaired. However, some Hg-resistant bacteria are capable of transforming Hg(II) to the less toxic Hg(0) (e.g. Summers and Silver, 1987; Foster, 1987; Summers and Barkay, 1989). As an element Hg cannot be mineralised and bioremediation of Hg per se is therefore not an option. The bioremediation of co- contaminated soils could however be optimised by using a microbial consortium consisting of bacteria capable of transforming Hg(II) to the less toxic Hg(0) thereby permitting the activity of degraders specific to the organic contaminant. A successful bioaugmentation (bioremediation with addition of microorganisms) requires that the bacterial inoculum can be established in the soil. Establishment can be prevented by the indigenous populations of protozoa grazing on the bacteria (Heynen et al., 1988; Recorbet et al., 1992). On the contrary, some bacteria when inoculated into soil are not grazed upon by the protozoa (Rønn et al., 2001). Furthermore, the protozoan Heteromita globosa stimulated the degradation of toluene when grazing upon the degraders 0038-0717/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0038-0717(03)00178-0 Soil Biology & Biochemistry 35 (2003) 1175–1181 www.elsevier.com/locate/soilbio * Corresponding author. Present address: Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. Tel.: þ 45-3814-2000; fax: þ45-3814-2050. E-mail address: msh@geus.dk (M.S. Holtze).