Structure and species composition of mercury-reducing biofilms

Mercury-reducing biofilms from packed-bed bioreactors treating nonsterile i ndustrial effluents were shown to consist of a monolayer of bacteria by sca nning electron microscopy. Droplets of several micrometers in diameter whic h accumulated outside of the bacterial cells were identified as elemental m ercury by electron-dispersive X-ray analysis. The monospecies biofilms of P seudomonas putida Spi3 initially present were invaded by additional strains , which were identified to the species level by thermogradient gel electrop horesis (TGGE) and 16S rDNA sequencing. TGGE community fingerprints of the biofilms showed that they were composed of the effluent bacteria and did no t contain uncultivable microorganisms. Of the 13 effluent bacterial strains , 2 were not mercury resistant, while all the others had resistance levels similar to or higher than the inoculant strain.