Page 123 - 79_04

Beatriz
Baselga-­-Cervera&
col.

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…)

638