EVALUATION OF MICROSCOPIC TECHNIQUES TO OBSERVE IRON PRECIPITATION INA NATURAL MICROBIAL BIOFILM

Iron biomineralization in a microbial biofilm consortium from Canadian Shield groundwaters has been investigated with different microscopic techniques. The advantages and disadvantages of the different methods of observing a biofilm growing on an opaque mineral surface are discus sed. Scanning electron microscopy was able to show the initial attachm ent and dispersion of bacteria on the mineral surfaces, whereas transm ission electron microscopy gave greater detail and revealed the precis e location of the iron precipitation on cell surfaces, including S-lay ers, and also throughout the extrapolymeric slime of the biofilm. Epis copic Nomarski differential interference contrast microscopy allowed d irect observation of biofilm dynamics and confirmed the precipitation of iron directly onto certain bacteria, which were then specifically i ngested by protozoa. This novel ingestion of iron-coated bacteria by p rotozoans essentially eliminated iron from solution and trapped it wit hin the biofilm. Over time in the natural environment, this iron, enme shed within a biofilm, may become incorporated into iron-rich sediment s. (C) 1998 Federation of European Microbiological Societies. Publishe d by Elsevier Science B.V. All rights reserved.