Aerobic mercury-resistant bacteria were isolated from the sediments of two highly mercury-polluted
areas of the Tagus Estuary (Barreiro and Cala do Norte) and one natural reserve area (Alcochete) in order
to test their capacity to transform mercury. Bacterial species were identified using 16S rRNA amplification
and sequencing techniques and the results indicate the prevalence of Bacillus sp. Resistance patterns
to mercurial compounds were established by the determination of minimal inhibitory concentrations.
Representative Hg-resistant bacteria were further tested for transformation pathways (reduction, volatilization
and methylation) in cultures containing mercury chloride. Bacterial Hg-methylation was carried
out by Vibrio fluvialis, Bacillus megaterium and Serratia marcescens that transformed 2–8% of total mercury
into methylmercury in 48 h. In addition, most of the HgR bacterial isolates showed Hg2þ-reduction
and Hg0
-volatilization resulting 6–50% mercury loss from the culture media.
In summary, the results obtained under controlled laboratory conditions indicate that aerobic Hgresistant
bacteria from the Tagus Estuary significantly affect both the methylation and reduction of
mercury and may have a dual face by providing a pathway for pollution dispersion while forming methylmercury,
which is highly toxic for living organisms.
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