Beatriz Baselga-‐Cervera& col.
638
Two Chlorophyta species (
Chlamydomonas fonticola
,
Dictyosphaerium
ehrenbergianum
) and one Bacillariophyta species (
Pleurosigma acuminatum
) were
detected in Saelices evaporation pond. Apparently microalgae from these ponds
are common species that do not differ than those found in non-‐extreme locations.
To check this DNA fragments of
Chlamydomonas
were sequenced and
compared with the N.C.B.I. database revealing as the closest specie discovered
Chl.
fonticola
a current microalgae species, which live in unpolluted environments.
As expected numerous microalgae species proliferates during the yearly
cycle in the Zamarra natural pond without mining activity, which serves as an
indicator of natural pollution levels in that area before mining activity.
Consequently, the key question is: how could these microalgae adapt so
quickly to an environment as extreme?
The first response with the microalgae are facing to environmental stress is
a physiological response (i.e. acclimatization) due to modification of gene
expression (15). Afterwards, when values of environmental stress exceed
physiological capabilities, only a genetic response (i.e. mutations that confer
resistance) can allow adaptation (16-‐19). The simplest genetic response is
achieved by a single mutation in a single gene capable of conferring resistance.
These simple mutations that confer resistance to a contaminant may occur
spontaneously without the selective agent (e.g. uranium) facilitate their
appearance (i.e. pre-‐selective pattern of appearance of mutations). By contrast, the
selective agent may be facilitating mechanisms for the occurrence of these
mutations that confer resistance (i.e. post-‐selective model).
The discussion on how the mutations occur (i.e. post-‐selective model where
mutations occurs in response to selective agent producing a direct and specific
adaptation
versus
pre-‐selective model where mutation occurs spontaneously by
chance prior to selective agent exposure) was one of the great controversies of
biology. Two prominent nuclear physicists Leo Szilard and Max Delbruck were the
key to solve this biological problem employing a complex mixture of laboratory
techniques and statistical analysis (20-‐21). But their complex procedures were not
always understood and among the microbiologists even today the controversy
remains. To determine the mechanism by which mesophilic microalgae might be able
to adapt to the extreme environment of uranium mine, we chose two strains of a
microalgae species (
Chlamydomonas reinhardtii
Dangeard, strains ChlaA from
algae culture collection of UCM) taxonomically similar than those living in Saelices,
but isolated from pristine place that had never been subjected to uranium
contamination. We employ a fluctuation analysis to determine the mechanisms (i.e.
physiological acclimatization, pre-‐selective mutations, post-‐selective mutations…)
