麻豆传媒

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鈥攖he 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.

鈥淭he DMS paper went down very well,鈥� says Lovelock, 鈥渂ut 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.鈥� 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. 鈥淲ithout organisms doing their thing the Earth would be a
much warmer place,鈥� 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 掳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. 鈥淚 was particularly interested in algal production of DMS,
because it is so hard to find an evolutionary explanation鈥攖he effect on
the algae themselves is so remote,鈥� 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. 鈥淭he deterrent works well on a laboratory scale,鈥� says Gill Malin, a
marine microbiologist at the University of East Anglia, 鈥渂ut the natural ocean
environment is very different. And the hypothesis does not explain why some
algae release low levels of DMS all the time.鈥�

The idea that algae might produce DMS to get themselves into the air occurred
to Hamilton first. 鈥淭im had mentioned that DMSP has a possible function as an
antifreeze,鈥� he recalls. 鈥淣ow 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.鈥� 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. 鈥淪eldom have I had
a run of reading where so many papers were relevant or connected and nothing
contradicted my ideas,鈥� says Hamilton. 鈥淚 felt certain that there was something
interesting here.鈥�

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.
鈥淒ispersal is the third priority for an organism, after survival and
reproduction,鈥� 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. 鈥淭he crucial thing,鈥� Hamilton
points out, 鈥渋s 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.鈥�

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. 鈥淭his was the first time intercontinental aerial dispersal of anything
had actually been recorded,鈥� says Marshall. 鈥淭hey must have gone up very high in
the atmosphere.鈥�

鈥淓verybody knows that fungal spores, bacteria and pollen are dispersing in
the air,鈥� says Hamilton, 鈥渂ut it was always assumed to be passive. We suggest
that algae have evolved a specific means of actively getting themselves into the
补颈谤.鈥�

The first hurdle for marine algae intent on going up is breaking the ocean鈥檚
high surface tension. 鈥淕etting off the sea surface is no great problem for small
unicellular algae,鈥� says Hamilton. He believes they probably use a mechanism
similar to one documented for bacteria. 鈥淏ubbles whipped up by sea-spray
concentrate microorganisms in their surface layer, and as they burst, the
organisms are propelled into the 补颈谤.鈥� 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.

鈥淲hat you have is a direct benefit to the algae of metabolising DMSP into DMS
and releasing it,鈥� says Lenton. 鈥淥ur theory works better if algal blooms are
clonal, and all the cells carry essentially the same genes.鈥� 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. 鈥淎lgologists already assume
quite a lot of clonality in these blooms,鈥� says Malin.

One problem with this idea is the amount of time it takes for DMS to create
uplift. 鈥淐onventional ideas about the oxidation of DMS have suggested that it
takes a few days for the aerosols to form,鈥� says Lenton, 鈥渟o the air mass could
be hundreds of kilometres away from the bloom before the uplifting effects
happen.鈥� This posed a distinct challenge to the hypothesis because algal cells
that produce DMS might not benefit from it. 鈥淏ut 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,鈥� he says.

Liss may have the answer. His team has been looking at trace gases in the
atmosphere above the sea. 鈥淲e have discovered that algae sometimes release
ammonia, as well as DMS,鈥� 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. 鈥淭his makes the nucleation much more rapid,鈥�
says Liss.

Getting airborne

Where is the evidence to support this algal aerial dispersal scenario? 鈥淢any
features of marine micro-algae seem consistent with getting into the air,鈥� 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. 鈥淭he best DMS producers are
the smallest algae and the most abundant bloom formers,鈥� 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鈥檚 harder to get airborne.

The fact that many algal blooms are red is a further clue. 鈥淭he red colouring
might provide the cells with protection against ultraviolet in the atmosphere,鈥�
says Hamilton. 鈥淐ertain 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
濒别惫别濒.鈥�

Hamilton and Lenton have very clear ideas about the research needed to
support their theory. 鈥淭he most important thing is to show that the dinophytes
and haptophytes are common in the aerial spora,鈥� says Hamilton. Algae tend to
have been overlooked, because nobody imagined they meant to be there. 鈥淥ften
people record particulate organic matter, but they don鈥檛 identify it because it
is too small and difficult to classify.鈥� 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鈥攎any of them common plant pathogens鈥攖hat 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. 鈥淭hese are tiny organisms with a global distribution,鈥� 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. 鈥淟arge 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鈥檚 like an explosion.鈥� 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? 鈥淲e are expecting a very defensive
response from professional meteorologists, algologists and others working in
this area.鈥� And he anticipates the response with some excitement. 鈥淭his is a new
view of the properties of microbes, a new explanation for why they do what they
诲辞.鈥�

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. 鈥淭his marks a turning point in the Gaia theory,鈥� says Lovelock.
鈥淏iologists are beginning to take the ideas seriously.鈥� What biologists should
also be doing, according to Liss, is working in atmospheric science. 鈥淗ow many
meteorology departments have a biologist?鈥� he asks. If microbes are controlling
cloud formation and rainfall, then weather is no longer just the domain of
physical scientists.