Environmental Engineering Reference
In-Depth Information
In a recent study from our laboratories, Bacillus-like and Herbaspirillum-like
bacteria were abundant during denitrification in acetate biostimulated micro-
cosms prepared from sediments representative of the Sellafield nuclear site in
North West England and supplemented with 10mM nitrate and a 5
m
M techne-
tium spike (Law et al.
2008
). Their abundance was demonstrated by 16S rRNA
and functional narG (alpha subunit of the membrane bound nitrate reductase)
gene analyses of samples taken after 30-day incubation when nitrate depletion
was most pronounced but before any metal reduction or Tc(VII) had been
observed (Law et al.
2008
). Similar narG gene sequences of Bacillus spp. were
abundant in sodium nitrate treated samples from uranium mining waste piles
in Haberland that had been incubated for 4 weeks under anaerobic conditions
(Geissler
2007
). The dominance of Bacillus spp. in this sample was also demon-
strated by 16S rRNA gene analysis (Selenska-Pobell et al.
2008
). This suggests
that in radionuclide contaminated sediments Bacillus spp. are also involved in
the reduction of nitrate, in agreement with the detection of distinct Bacillus
spp. in a broad range of heavy-metal and radionuclide contaminated samples
(Selenska-Pobell et al.
1999
; Martinez et al.
2006
). This is of interest, given
their ability to sorb large amounts of uranium and other heavy metals
(Selenska-Pobell et al.
1999
).
Conclusions
It is important that we improve our understanding of the mechanisms under-
pinning the biogeochemistry and mobility of radionuclides in the environ-
ment, to underpin safety case assessments and bioremediation efforts. The
studies presented in this chapter are examples where microbiological and
geochemical analyses have been combined in the laboratory to obtain a better
understanding of the biogeochemical cycle of priority radionuclides. These
approaches, applied to field-scale investigations, and operating at environmen-
tally relevant concentrations of radionuclides, are potentially challenging
but crucial to drive this area forward and realise the potential of in situ
bioremediation at nuclear facilities in Europe.
There are also specific questions raised by the studies described in this
review. For example, the impact of competing 'direct' enzymatic and 'indirect'
(Fe(II)/sulphide-mediated) reductive processes needs clarifying for some radio-
nuclides, most notably Tc(VII). In the case of uranium, more research is clearly
needed to understand both the precise mechanisms of U(VI) reduction (and
long-term stability of U(IV) phases under field conditions) and also the potential
roles that bacteria can play in the in situ precipitation of other insoluble
phases such as U(VI) phosphate. The tools of molecular ecology can play a role
in these investigations, but it is necessary to expand our studies not only to look
at the microbial community by targeting long-lived DNA, but also to target
more transient mRNA to track the metabolism of the 'active' components of
