New Scientist - USA (2013-06-08)

(Antfer) #1
8 June 2013 | NewScientist | 47

cyanobacterium survived, Allen suggests,
because a mutation wrecked the switch that
turned on only one kind of reaction centre at
any time. With both kinds in action together,
electrons from the manganese could flow
through the type-II centre before being
siphoned off by the type-I centre, preventing
a blockage. In other words, the two reaction
centres would have been working together,
just as they do in cyanobacteria today (FEBS
Letters, vol 579, p 963).
But how did the descendants of this
bacterium go from getting electrons from

manganese to getting them from water?
Well, in a way they didn’t. To this day
manganese provides the electrons needed
for photosynthesis in all plants. However,
the electrons now come from a cluster of
manganese atoms within the type-II reaction
centre, and this cluster has a remarkable
ability – after it has given up electrons, it
steals others from water molecules, splitting
them apart and liberating oxygen.
Once early cyanobacteria had evolved this
kind of type-II centre, they needed only trace
amounts of manganese. They could then
spread from manganese-rich environments
and start exploiting the abundant CO 2
available at the time, with the help of an
unlimited supply of water and sunshine.
Soon immense numbers of cyanobacteria
were spewing out enough oxygen to transform
the atmosphere.
If Allen’s hypothesis is correct, proto-
cyanobacteria had to stumble into a highly
unusual manganese-rich environment and
lose control of a key genetic switch at the same
time. Allen agrees this is improbable, but this
could be why oxygenic photosynthesis took
a billion years to appear. “The way I look at it,
it was only a matter of time until one of these
bacteria had two accidents at once,” he says.
Remarkably, there is now hard evidence to
back Allen’s idea: we’ve found one of those
rare manganese-rich environments.
Woodward Fischer at the California Institute
of Technology in Pasadena and his colleagues
have been studying rocks laid down in what is
now South Africa just before levels of oxygen
began to rise. In one spot they found a

superabundance of manganese oxide in rocks
that formed, significantly, in the absence of
oxygen. Not even ultraviolet light could have
generated manganese oxide on the scale
found in the rocks. This leaves photosynthesis
as it existed in Allen’s proto-cyanobacteria as
the only plausible scenario, the team told a
meeting in December.
“It is big news, hugely exciting – and spot
on for John’s hypothesis,” says William Martin
at Heinrich Heine University in Düsseldorf,
Germany, who studies early evolution. Martin
is a supporter of Allen’s scenario, and has
been working with him to gather supporting
evidence. But Blankenship is sticking to his
guns. He describes his many discussions with
Allen and Martin on the origin of oxygenic
photosynthesis as “very spirited, yet friendly”.
What would settle the debate once and for
all is the discovery of living representatives of
one of the proposed intermediate forms –
either indigo bacteria or proto-cyanobacteria.
Surprisingly, Blankenship and Allen are both
confident that their respective organisms
still exist somewhere in the world. “You find
specialist environments today that correspond
to typical conditions 2.4 billion years ago,”
says Allen. “It’s not absurd to think that these
microorganisms are still out there.”
Whatever the ancestor of cyanobacteria
turns out to be like, we have reason to be very
grateful to it. “This organism – maybe by
accident – was hugely important,” says Allen.
“Quite simply, it changed the world forever.” n

Colin Barras is a freelancer writer based near Ann
Arbor in Michigan

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Three ways to harvest light
There are several forms of photosynthesis, but only one makes oxygen as a by-product

Done by
Advantage

Disadvantage

Green sulphur bacteria

Type I
reaction
centre

Type II
reaction
centre

Take electrons from
hydrogen sulphide (H 2 S)

Recycle
electrons

Take electrons
from water

Purple bacteria Plants, algae, cyanobacteria
H 2 S is easy to split

H 2 S

CO 2 Sugars CO 2 Sugars

Manganese cluster

Light

Light

Light

Energy

Energy

S

e–

H 2 O

O 2

e– e–

No electron source required Unlimited supply of water

Supply of H 2 S is limited Provides energy only. Other reactions
needed to turn CO 2 into food

Water is hard to split

I I
II II

130608_F_FirstLight.indd 47 30/5/13 14:32:10

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