46 | NewScientist | 8 June 2013anoxygenic photosynthesis took place.
The great advantage of using water as the
electron donor instead is that there is an
endless supply of it in the oceans. But there
is a big drawback, too. “Water is incredibly
difficult to oxidise,” says Robert Blankenship
at Washington University in St Louis, Missouri.
We’re still struggling to do it: researchers have
been trying for decades to develop cheap,
energy-efficient ways of splitting water to
produce hydrogen gas for fuel.
So it makes sense that photosynthesising
bacteria first exploited easy-to-oxidise
molecules before switching to water. The
conventional view, supported by Blankenship
and many other researchers, is that oxygenicphotosynthesis gradually evolved from the
anoxygenic version through a series of
intermediate steps. But over the past decade,
John Allen at Queen Mary, University of
London, has devised an alternative scenario
that is almost deliberately implausible. “This
process has to have happened by accident,”
he says. Only that can explain the billion-year
delay, he argues.
Any scenario for how oxygenic
photosynthesis got started has to deal with
four significant facts. Fact one: there are two
related but distinct types of anoxygenic
photosynthesis. Some bacteria have what is
called a type-I reaction centre, which takes
electrons from sources like hydrogen
sulphide and sends them down a one-way
street: each electron is used just once. Other
bacteria carry a type-II reaction centre that
recycles electrons internally, making them
less dependent on an external electron source
(see illustration, above).
Fact two: oxygenic photosynthesis
involves a type-I and a type-II reaction centre
working in tandem. Fact three: even though
cyanobacteria have both reaction centres,
it is only the type-II centre that splits water
and generates oxygen, at a site that contains
four manganese atoms arranged around a
calcium atom. Finally, fact four: anoxygenic
photosynthetic bacteria that have a type-II
reaction centre lack this cluster of manganese
and calcium.
Blankenship thinks it is the final two facts
that are most important and point towards
a simple scenario. The type-I centre evolved
first, he thinks. Then the genes encoding itsmachinery were acquired by another group
of bacteria – gene-swapping was and is rife
among bacteria. In this group, the machinery
gradually became modified, forming the
first type-II reaction centre. Later, the
descendants of these bacteria began to
incorporate metal atoms into it. Eventually
they arrived at a configuration that included
four atoms of manganese and one of calcium.
They could now oxidise water and perform
oxygen-generating photosynthesis using
just a type-II reaction centre.
Only later, claims Blankenship, did this
group’s descendants acquire the type-I
machinery via gene transfer, giving rise to
cyanobacteria. So Blankenship thinks it isjust a coincidence that cyanobacteria have
two different reaction centres.
This scenario makes one clear prediction –
there were once bacteria that generated oxygen
through photosynthesis, but were distinct
from cyanobacteria. They would have been the
missing link between the anoxygenic bacteria
with a type-II reaction centre – including what
are called purple bacteria, alive today – and
the oxygen-generating cyanobacteria, so
let’s call them “indigo” bacteria. No indigo
bacteria have ever been found, though.
Instead, Blankenship and others have tried
to show that they could have existed.
Perhaps most significantly, a team at
Arizona State University in Tempe has tried to
turn a purple bacterium into something like
an indigo bacterium. The researchers
modified the purple one so it could bind a
manganese ion to its reaction centre and use it
to react with molecules containing oxygen
(PNAS, vol 109, p 2314). It’s not oxygenic
photosynthesis, but it’s a step towards it.Marine disaster
Even if biologists do one day engineer an
indigo bacterium in the lab, though, this
wouldn’t prove they could evolve naturally.
And to Allen, the gradual evolution scenario
cannot explain all the facts. Why would such
an apparently simple sequence of events have
taken up to a billion years to occur? Why did
oxygenic photosynthesis evolve only once,
in cyanobacteria, as far as we know? (Plants
acquired the ability to photosynthesise by
allowing cyanobacteria to live inside them –Plants can harvest light
only with the help of
symbiotic cyanobacteriatheir chloroplasts are descended from
cyanobacteria.) And why do all cyanobacteria
have both kinds of reaction centres?
Allen also thinks the type-I centre evolved
first. But from there, his scenario is very
different. Allen thinks that early in their
history, these bacteria experienced some kind
of genetic glitch which duplicated the entire
set of genes for making a type-I reaction
centre. The spare copy was free to take on a
different role, and it evolved the ability to
recycle electrons – the first type II reaction
centre. Having two distinct reaction centres
allowed these “proto-cyanobacteria” to
thrive in a wide range of environments, Allen
proposes. When there was plenty of hydrogen
sulphide, they used their type-I reaction
centre. When hydrogen sulphide ran low,
the bacteria switched to using their type-II
reaction centre, recycling the electrons they
had gathered.
Then one day, disaster struck: some proto-
cyanobacteria drifted into a shallow marine
environment rich in manganese but poor
in hydrogen sulphide. The bacteria duly
switched to a type-II reaction. But when
ultraviolet light hits manganese it strips
off electrons, so there were actually plenty
available – and these electrons quickly
clogged the cyclic type-II reaction centre.
The resulting manganese ions would have
reacted with water to form manganese
oxide, but there was plenty more manganese
around, producing more than enough
electrons to kill the microbes.
Well, almost all of them. One lucky proto-” Bacteria have been photosynthesising for nearly
as long as there has been life on Earth. So why did it
take a billion years for them start making oxygen?”
130608_F_FirstLight.indd 46 30/5/13 14:32:00