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The study of microbial metabolism in
deep petroleum reservoirs is problematic
owing to the technical difficulty of sampling
surface environments provide a likely
analogue for petroleum reservoirs because
microbial processes found in shallow sub-
surface environments are commonly found
in deeper subsurface environments as well3.
We sampled oil-bearing sediments from
an aquifer located in Bemidji, Minnesota,
that had been contaminated with crude
oil. The sediments were incubated under
strict anaerobic conditions at 20 °C as
previously described4. The sediments were
depleted of nitrate (to less than 5 mM) and
sulphate (to less than 10 mM) and more
than 95% of the HCl(0.5 M)-extractable
iron was in the Fe(II) state. These sediments
actively produced methane over time.
When [2-14C]acetate was injected into the
sediments, 14CH4 and
14CO2 were produced
in a ratio of 4:1, which is typical in sul-
phate-depleted sediments where methane
production is the electron-accepting pro-
cess. The concentration of hydrogen was
5–8 nM, characteristic of sulphate-depleted,
When we injected [14C]hexadecane into
the sediments, it was converted to 14CH4
without a lag (Fig. 1). This demonstrated
that the microorganisms were already
adapted for conversion of hexadecane to
methane and suggests that hexadecane was
being degraded in