Page 122 - 79_04

Microalgae:
the
first
nuclear
engineers?

Saelices
evaporation
Zamarra
pond

25.0
b.d.1

Uranium
concentration
(mg
L-­-1)
5.6
<0.1

Pb
(µg
L-­-1)
3.4
6.9

pH
10.8
0.2

Conductivity
(mS
cm-­-1)
4
<0.1

Radioactivity
(µSv
h-­-1)
4
>25

Number
of
microalga
species

(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
LC50
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.

637