ABSTRACT.    Our aim is to elucidate the role of bacterial-protozoan interactions in response to stress evoked by increased soluble mercury (II) level. Effects of daily mercury (II) supplementation for two months under varied cultivation conditions were examined. The Entodinium caudatum growth depended on the capability of the co-cultured bacterial population to develop resistance to mercury (II) chloride and on cultural conditions. Fermentation activity of E. caudatum microbial community was depressed under mercury stress. Production of fermentation gas was depressed in contrary to methane production and volatile fatty acid patterns were affected. No organic mercury species (methylmercury, ethylmercury, and phenylmercury) were detectable after long-term application of mercury (II) chloride. The major mercury species was inorganic mercury (II) with substantial accumulation in the bacterial fraction, less in black sediment and ciliate fraction. The data indicate that the free-living bacteria protect protozoan cells by eliminating mercury into its insoluble forms. In addition, the accumulation of substantial portion of total culture mercury content into bacterial fraction point to the major role of bacteria in mercury (II) detoxification.
The study was supported by funds from Scientific Grant Agency of Ministry of Education of Slovak Republic and the Slovak Academy of Sciences - VEGA 2/6175/26 and APVT 51-007-6004.