Page 125 - 79_04

Beatriz
Baselga-­-Cervera&
col.

cultures
fluctuates
from
more
than
105
resistant
cells
ml-­-1
to
0
resistant
cells
ml-­-1,

whereas
all
the
set
2
cultures
showed
around
102
resistant
cells
ml-­-1).

Consequently,
the
U-­-resistant
cells
arose
randomly
by
rare,
pre-­-selective,

spontaneous
mutations
and
not
as
a
response
to
the
effect
of
residual
waters
from

U
mining
(i.e.
by
specific
physiological
acclimatization,
adaptive
mutants
or
post-­-
selective
mutations).
These
U-­-resistant
mutants
were
isolated
and
maintained
in

BG-­-11
medium
in
the
absence
of
the
selective
agent
(i.e.
residual
waters
from
U

mining),
then
confirming
that
the
culture
was
able
to
retain
resistance
to
residual

waters
of
U-­-mining
throughout
successive
generations.

The
rapid
colonization
by
microalgae
Saelices
evaporation
pond
can
be

explained
following
an
Occam’s
razor
line
of
reasoning:
i)
new
U-­-resistant
mutants

arise
prior
to
the
uranium
exposure,
because
mutation
is
recurrent;
ii)
prior
to
the

uranium
exposure
many
of
these
U-­-resistant
mutants
are
eliminated
by
selection

or
by
chance;
iii)
The
balance
between
new
U-­-resistant
genotypes
appeared
by

mutation
and
the
U-­-resistant
genotypes
eliminated
by
selection
will
determine
that

a
small
number
of
U-­-resistant
mutants
are
presents
within
microalgae
populations

at
a
given
time.
When
uranium
contamination
occurs,
U-­-sensitive
wild
type
cells

die
and
only
the
U-­-resistant
cells
are
able
to
colonize
the
new
extreme

environment
(Figure
3).

Figure
3.-­-Graphic
work
representation
of
the
number
of
genotypes
able
to
survive
under
an

increasing
environmental
stress
present
among
the
studied
ponds
due
to
the
uranium

concentration
and
the
adaption
strategy
depending
in
the
uranium
concentration.

Recent
evidences
suggest
that
mutations
of
single-­-genes
that
confer
a
large

adaptive
value
do
happen
and,
when
competing
with
multiple
small-­-effect

mutations,
they
usually
win
(reviewed
by
Chouard
(22)).
In
our
experience
over

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