Beatriz Baselga-‐Cervera& col.
642
isotopes in ecosystems is well known that biological enzymes sometimes produce
isotopic fractionation (34).
As is well known, uranium found in nature consists largely of two isotopes,
238
U and the less abundant
235
U, which constitutes only approximately 0.7% of
naturally occurring uranium. Most of the commercial nuclear power reactors
require uranium enriched in the
235
U (35).
Biological fractionation of U has been recently tried by an indirect
procedure linking bacterial uranium reduction to the isotopic partitioning (36).
However, one step uranium enrichment by preferential uptake of an isotope
directly using a microorganism not been implemented yet.
Chlamydomonas
isolated from the uranium polluted site of Saelices, was tested for U fractionation.
For this, the uranium isotopic composition in water of Saelices evaporation pond
was compared with the isotopic ratio of U bio-‐accumulated by
Chlamydomonas
(previously the cells were treated with EDTA to eliminate metals on the cell wall).
The analysis through a High Resolution Inductively Coupled Plasma Mass
Spectrometry (HR-‐ICPMS) let to obtain the isotopic distribution of uranium.
The results obtained reveal that
Clamydomonas
is able to U fractionation
(δ
235
U inside the cells with respect to
235
U in Saelices evaporation pond was -‐3.5
%). This spectacular result indicates that microalgae are capable of producing
uranium fractionation effectively. A microalgae plant for uranium enrichment
could be a real possibility.
6. CONCLUSIONS
Once more nature has won the race to the human technology:
Microorganisms were able to adapt to extreme U contamination as well as
accumulate and perform isotopic fractionation of uranium to become the most
feasible creators of the first nuclear reactor in Oklo 1.7 billion years ago. The
extreme environment of the evaporation pond in Saelices uranium mine provides
some ideas of how microorganisms can build a nuclear reactor. Microalgae seem
good candidates for nuclear engineers:
First of all they were able to adapt to the extreme U-‐contaminated
environment of Saelices mining area in less than 40 years. These algae had suffered
a pressure of selection in which pre-‐adaptive mutations that confer resistance
against the toxic effect of uranium can survive.
Second efficient biological accumulation of uranium (115 mg U/g dry mass)
was demonstrated.
Finally, microalgae are able to successfully carry out an isotopic fractioning
of uranium.
