From: Alejandro Dubrovsky (s328940@student.uq.edu.au)
Date: Thu May 30 2002 - 05:31:35 MDT
this i thought was already established a while ago. the following
article from new scientist 30 May 1998 goes into more detail
(copied without permission)
Send in the clouds 30 May 98
Gaia is still not taken seriously by most respectable scientists. But
what if one of its key theories, that algae help to control climate,
really has a Darwinian explanation, asks Lynn Hunt
MICROBES make the weather. It sounds bizarre, but biologists have known
for years that marine plankton can create clouds. If this were just a
meaningless side effect of their existence the story would end there.
But suppose microbes are in fact orchestrating the elements. Suppose
there is a link between their survival and climate control. What if
these specks of life were using clouds, wind and rain to carry
themselves around the planet like a global taxi service?
It is tempting to dismiss these ideas out of hand, but consider their
pedigree. Bill Hamilton from the University of Oxford is renowned for
his work on social and sexual evolution and has a knack of being ahead
of the field. He was thinking about selfish genes long before Richard
Dawkins popularised the notion. Now working with Tim Lenton, a young
atmospheric chemist from the University of East Anglia, Hamilton has
come up with an astounding idea. By explaining why microbes produce
clouds he has also formulated the first biologically credible mechanism
for Gaia-the theory that Earth acts like a superorganism, with all its
biological and physical systems cooperating to keep it healthy.
It is more than a decade since a group of scientists led by geophysicist
James Lovelock published a paper stating that marine algae are part of a
massive global regulatory system that keeps the climate stable. Most
produce a gas called dimethyl sulphide (DMS), which reacts with oxygen
in air above the sea to form tiny solid particles. These sulphate
aerosols provide a surface on which water vapour can condense to form
clouds. And clouds keep the planet cool by reflecting solar radiation
back into space. Lovelock, who proposed the Gaia hypothesis in 1972,
argues that this process could create a self-regulating global
thermostat. Warmer conditions increase algal activity and DMS output,
seeding more clouds, which block out the Sun. Then, as the climate
cools, algal activity and DMS levels decrease and the cycle continues.
"The DMS paper went down very well," says Lovelock, "but biologists were
not satisfied. They opposed Gaia because they could not see how
organisms had evolved to behave in a way that regulates the planet."
Natural selection works at the level of the individual, weeding out
those that are poorly adapted to their particular lifestyle by dint of
their genes. So possessing the genetic machinery to produce DMS must
benefit individual microbes. But how can individuals working for their
own selfish ends have evolved to influence the global environment?
Good for algae, good for Earth
Lovelock points out that DMS production benefits both the organism and
the planet as a whole. "Without organisms doing their thing the Earth
would be a much warmer place," he says. Peter Liss and Andrew Watson,
environmental scientists from the University of East Anglia, have
recently suggested that algal DMS production cools the planet by 4 °C.
As well as being good for the Earth, cooling is good for algae, because
if the oceans get too hot, the warm upper layers become separated from
the cooler depths and algae at the surface are cut off from sources of
nutrients below. Algae may also benefit from nitrogen raining down from
the clouds they have helped to form.
Such arguments did not convince Hamilton. He became intrigued by Gaia
through chatting to Lovelock. "I was particularly interested in algal
production of DMS, because it is so hard to find an evolutionary
explanation-the effect on the algae themselves is so remote," he says.
Lovelock introduced Hamilton to Lenton, whose research into the control
of nutrient ratios in the ocean had convinced him that natural selection
working on individuals can have large-scale environmental effects. If
Hamilton and Lenton could only discover why algae produce DMS they might
be able to make the jump from local to global.
The immediate biological function of DMS is not clear. Its precursor, a
chemical called dimethylsulphoniopropionate (DMSP), is thought to
protect algal cells from the drying effects of the strong salt solution
in which they live. But the algae carry an enzyme that breaks down DMSP
into DMS and acrylic acid.
One theory is that toxic acrylic acid is released to deter predators
when cells are damaged. In this scenario DMS is just a waste product.
Algae do seem to release greater quantities of DMS when attacked by
other plankton or even by viruses. "The deterrent works well on a
laboratory scale," says Gill Malin, a marine microbiologist at the
University of East Anglia, "but the natural ocean environment is very
different. And the hypothesis does not explain why some algae release
low levels of DMS all the time."
The idea that algae might produce DMS to get themselves into the air
occurred to Hamilton first. "Tim had mentioned that DMSP has a possible
function as an antifreeze," he recalls. "Now why would a cell in a
tropical ocean need antifreeze? Perhaps they sometimes end up high in
the air, shot up there by a waterspout. Or maybe there are other ways
they could go. Convective energy created by cloud formation would help
them." Flying high, the algae would be exposed to very low temperatures.
Idle speculation rapidly led to the formation of a theory that
beautifully explains why algae produce DMS. "Seldom have I had a run of
reading where so many papers were relevant or connected and nothing
contradicted my ideas," says Hamilton. "I felt certain that there was
something interesting here."
If algae were using the atmosphere as a route for global dispersal then
any adaptation that helped them on the way would have strong
evolutionary benefits. "Dispersal is the third priority for an organism,
after survival and reproduction," says Lenton. Indeed, it is more than
two decades since Hamilton and biologist Robert May devised a model
highlighting the importance of dispersal. The benefits of successfully
colonising a new area are so great, they concluded, that organisms will
evolve to send their progeny away from home even if they never end up in
a better place. "The crucial thing," Hamilton points out, "is that this
mechanism only has to confer a very tiny advantage to make it likely to
have evolved. Some of these marine algae have been evolving for at least
600 million years."
Hitching a ride on thermals and air currents is a highly efficient way
of getting around the planet. So it is hardly surprising that air is
teeming with microbes. Ten thousand particles per cubic metre of air is
quite normal near the ground, and live bacteria and fungal spores have
even been found 50 kilometres up in the atmosphere. In 1993, William
Marshall, an aerobiologist working for the British Antarctic Survey,
cultured organisms, including algae, arriving at the Antarctic, in an
air mass that had travelled 1500 kilometres from South America. "This
was the first time intercontinental aerial dispersal of anything had
actually been recorded," says Marshall. "They must have gone up very
high in the atmosphere."
"Everybody knows that fungal spores, bacteria and pollen are dispersing
in the air," says Hamilton, "but it was always assumed to be passive. We
suggest that algae have evolved a specific means of actively getting
themselves into the air."
The first hurdle for marine algae intent on going up is breaking the
ocean's high surface tension. "Getting off the sea surface is no great
problem for small unicellular algae," says Hamilton. He believes they
probably use a mechanism similar to one documented for bacteria.
"Bubbles whipped up by sea-spray concentrate microorganisms in their
surface layer, and as they burst, the organisms are propelled into the
air." Marine algae may even have a hand in it themselves. When they
gather together as algal blooms at the ocean surface, the high
concentration of photosynthesising cells in the water, all absorbing
sunlight, tends to heat up the water surface and air just above it. The
warm air rises, creating a convection current which can cause a breeze,
ruffling the water surface and creating bubbles.
A much greater problem for the algal cells is reaching the required
height for effective dispersal. This is where DMS comes in. As water
condenses around the sulphate aerosols it releases energy in the form of
heat. This warms the surrounding air, which starts to rise. Air
underneath is sucked upwards, creating an updraft that lifts clouds as
they form. The individual algal cells benefit in two ways. First, any
cells already in the air will rise with the air current. Secondly, the
increased air movements create further breezes at the sea surface so
that cells remaining in the bloom can escape from the water in the
bubbles of breaking wavelets.
"What you have is a direct benefit to the algae of metabolising DMSP
into DMS and releasing it," says Lenton. "Our theory works better if
algal blooms are clonal, and all the cells carry essentially the same
genes." This eliminates the risk of freeloaders hitching a lift.
Instead, the payback for releasing DMS will be conferred on the genes
that produced it, whether they are in the same cell or in one of its
replicas. So those genes will be successfully dispersed and preserved by
evolution because of that advantage. "Algologists already assume quite a
lot of clonality in these blooms," says Malin.
One problem with this idea is the amount of time it takes for DMS to
create uplift. "Conventional ideas about the oxidation of DMS have
suggested that it takes a few days for the aerosols to form," says
Lenton, "so the air mass could be hundreds of kilometres away from the
bloom before the uplifting effects happen." This posed a distinct
challenge to the hypothesis because algal cells that produce DMS might
not benefit from it. "But there was one research cruise in 1991 which
showed a remarkable correlation between DMS concentrations at the ocean
surface and the density of clouds forming above, so there must be a
faster chemical pathway from DMS to aerosol," he says.
Liss may have the answer. His team has been looking at trace gases in
the atmosphere above the sea. "We have discovered that algae sometimes
release ammonia, as well as DMS," he says. The ammonia neutralises the
sulphate, forming a salt, which is much more likely than a single ion to
collide with another sulphate ion and form a particle. "This makes the
nucleation much more rapid," says Liss.
Getting airborne
Where is the evidence to support this algal aerial dispersal scenario?
"Many features of marine micro-algae seem consistent with getting into
the air," says Hamilton. The algae best known for producing DMS are the
dinophytes and haptophytes. Both include very small species, with cells
between 2 and 20 micrometres across that would easily float in air. "The
best DMS producers are the smallest algae and the most abundant bloom
formers," says Lenton. Algal blooms are concentrated near the surface,
and often form foams and slime that also make it easier to get airborne.
Algologists are also perplexed by the rate of DMS production, which
varies independently of the rates of photosynthesis and other metabolic
processes, and can even be different in two blooms of the same species.
Emiliana huxleyi, for example, releases lots of DMS in the tropics and
not so much in temperate areas. Hamilton and Lenton believe that this is
because the air is generally stiller in tropical areas and so it's
harder to get airborne.
The fact that many algal blooms are red is a further clue. "The red
colouring might provide the cells with protection against ultraviolet in
the atmosphere," says Hamilton. "Certain carotenoids, such as
astaxanthin which is found in some algae, do provide protection. One of
the things we could test is whether the protection provided by these
pigments is more appropriate to the spectrum of radiation in the
atmosphere than to what algae would experience at sea level."
Hamilton and Lenton have very clear ideas about the research needed to
support their theory. "The most important thing is to show that the
dinophytes and haptophytes are common in the aerial spora," says
Hamilton. Algae tend to have been overlooked, because nobody imagined
they meant to be there. "Often people record particulate organic matter,
but they don't identify it because it is too small and difficult to
classify." Also, most of the small dinoflagellates are naked cells. They
do not have a protective silica coating, and so they are likely to
shrivel when they are collected. Hamilton has commissioned Marshall to
go out on the Atlantic this summer and take samples from the air above
algal blooms.
If algae really use wind and clouds to travel the globe, do other
organisms share this ability? Hamilton and Lenton believe their theory
extends to a group of bacteria and fungi-many of them common plant
pathogens-that have a talent known as ice-nucleating ability. These
organisms grow ice crystals around their bodies by exuding certain
chemicals in conditions where the air is below freezing but still
contains water. Microbiologists believe these pathogens evolved this
ability to promote frost damage on leaves. Hamilton has a different
idea. "These are tiny organisms with a global distribution," he says.
Why should they not also be using the atmosphere for dispersal?
As they are minute, they have less of a problem getting aloft than in
dropping back down again. Hamilton and Lenton suggest that these
creatures use their ice-nucleating ability to seed clouds with ice
crystals, which eventually fall as rain, bringing their living nuclei
down with them. The groups of bacteria and fungi which tend to have
ice-nucleating ability are also commonly found in the air.
An engine for clouds
While working on this theory, Hamilton has been struck by the way clouds
move. "Large cumulus clouds often have several new towers, rising behind
the main bulk of the cloud, as if something inside them is generating
new sources of latent heat. It's like an explosion." Is this because
microbes within the clouds are actively creating them, and in the
process, driving themselves around the globe?
Hamilton is the first to admit that it all seems a little extraordinary.
These ideas have convinced him that Gaia may be a real biological
phenomenon, but how will they fare among his peers? "We are expecting a
very defensive response from professional meteorologists, algologists
and others working in this area." And he anticipates the response with
some excitement. "This is a new view of the properties of microbes, a
new explanation for why they do what they do."
And perhaps it is a new perspective on the climate system. We can begin
to see how life on Earth is involved in regulating climate, not just
passively, but actively. "This marks a turning point in the Gaia
theory," says Lovelock. "Biologists are beginning to take the ideas
seriously." What biologists should also be doing, according to Liss, is
working in atmospheric science. "How many meteorology departments have a
biologist?" he asks. If microbes are controlling cloud formation and
rainfall, then weather is no longer just the domain of physical
scientists.
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