Microalgae: the first nuclear engineers?
637
Saelices evaporation
Zamarra pond
Uranium concentration
(mg L
-‐1
)
25.0
b.d.1
Pb (µg L
-‐1
)
5.6
<0.1
pH
3.4
6.9
Conductivity (mS cm-‐1)
10.8
0.2
Radioactivity (
μSv h
-‐1
)
4
<0.1
Number of microalga species
4
>25
(b.d.l. = below detection limit).
3. MICROALGAE ARE ABLE TO SURVIVE IN AN ENVIRONMENT CONTAMINATED
BY URANIUM
Microalgae are very successful organisms, dominating a large number of
habitats including extreme environments. Its biological characteristics (i.e.
haploids, short generation times, enormous population size) allow them to adapt
very quickly under environmental stress.
Extreme environments characterized by extreme values of stress factors
(i.e. toxics, pH…) often support communities of phytoplancton living at the extreme
limits of their tolerance (9-‐11). U is very hazardous for microalgae as a heavy
metal besides its radioactivity. The high U concentration in Saelices evaporation
pond (25 mg L
-‐1
, around 700 times higher that LC
50
for microalgae species
according US EPA Ecotox Database) indicates that these waters constitute extreme
environments. Even in areas influenced by mining activities, the usual U
concentration is less of 3.5 mg L
-‐1
(12).
It has generally been accepted that adaptation to extreme environments is
achieved gradually by selection of several mutations with minor effects following a
slowly process involving long temporal scales according to Charles Darwin’s axiom
‘
natura non facit saltum
’ (reviewed by Gould (13)). The early work of Goldschmidt
suggested that unique mutations with large effect could have evolutionary
importance, but evolutionary biologists overwhelmingly rejected Goldschmidt’s
work arguing that the multiple mutations of small effect are the pacemaker of
adaptation (reviewed in Gould (13-‐14)). According to this conception, the
organisms that live in these extreme environments are extremophiles that after
getting their adaptation over millions of years differ very much from their
mesophilic ancestors. However, adaptation of microalgae to extreme environment
of uranium mining ponds Saelices could change this preconception.
Amazingly, some microalgae species were able to colonize this extremely U-‐
contaminated pond very rapidly since mining activities started in 1960. There are
few known cases where microalgae are able to colonize as quickly an extremely
polluted environment, which make available a fascinating laboratory to study
rapid adaptation of current microorganisms to extreme contamination.
