The next time you sit back and admire the sunset from the shore of a sultry tropical island, pay homage
to the battalions of fluffy cumulus clouds that will probably be arrayed along the horizon. For if Bill Hamilton,
a biologist at Oxford University, and Time Lenton, a biologist at the University of East Anglia, in Britain, are
correct, these clouds are the home-made rafts of bacteria, algae and other microbes voyaging across the
world.
Scientists have known for some time that microbes are important in cloud formation, and hence play
a part in influencing the weather. Cloud samples have often been shown to harbour microbes of many kinds.
Dr. Hamilton and Mr. Lenton think that this is no accident. In a paper about to be published in the journal
Ethnology, Ecology and Evolution, they argue that some microbes have actually evolved to seed
clouds, in order to aid their own dispersal.
The travel-by-cloud hypothesis has three parts. First, if this strategy is to be successful, the organisms
must be able to lift off the globeÕs surface and rise high in the air. Once in the air, they must be able to
seed the formation of clouds and survive the sunÕs fierce rays. Finally, they must be able to come safely
back down to earth.
If you are small and light, getting into the air from land is fairly easy. The wind, after all, is the preferred
vehicle of many a seed or spore. From water, though, flying aloft is more difficult. In order to get away, you
must first escape the surface tension - something with which small, light beings will have particular difficulty.
So you will have to wait until the wind is strong enough (about 20 kph or 12 mph) to cause white caps. Then,
if you are sufficiently small, you may be lucky enough to be hurled aloft in the spray, and swept upwards by
the wind This is effective: samples of the air above breaking water show high concentrations of minute
organisms normally found in the water. But it precludes escape on a day when the water is as flat as a
millpond. Unless, that is, you can somehow create your own wind. And that, proposes Dr. Hamilton, is
precisely what some algae do.
He suggests two mechanisms. First, simply by absorbing sunlight, algae floating on the surface warm
the water. This warmth is quickly absorbed by the air above - creating changes in the local air pressure (i.e.,
wind) and mini-thermals on which small algae might rise upwards.
Second, many species of algae emit a gas known as dimethyl sulphide, or DMS. Some let it go only
when their cells are damaged as a result of being preyed on by zooplankton. But other release it directly.
No one knows why: Theories range from the notion that it might have an antibiotic effect to the idea that it
makes algae less appealing to munching zooplankton. Dr. Hamilton and Mr. Lenton think otherwise. They
reckon that algae emit DMS to help themselves take off.
Interestingly, DMS emitted from algae is the primary natural source of cloud-condensation nuclei.
These are the tiny airborne particles around which water droplets form, thus catalyzing the condensation of
clouds. Without such particles, clouds would be rare: the air would need to be freakishly saturated with
water vapor or strangely cold for condensation to occur.
Through a series of complex chemical reactions, the DMS given off by algae is oxidized into tiny
particles of sulphate, around which the water droplets form. This process releases heat - which in turn draws
more air from below, thereby increasing the local windspeed. Within a few hours, the researchers suggest,
DMS released from a patch of algae on a sunny day may help to create winds, thermals and clouds. Any
algae than can take advantage of this can thus head off in search of better climes.
Another hint in favour of this idea is that algae that emit DMS tend to be smaller (thus facilitating take-off) than those that do not, and seem to be commoner in the tropics. Indeed, some of the strongest
emissions come from agal species found in warm, often windless waters such as the Sargasso Sea. So far,
though, it is not known whether these algae in particular are found in clouds more than algae of other
species.
Finally, algae are not the only organisms that ride around in the sky. Many species of bacteria have
been collected from clouds, along with the spores of a variety of fungi. For these organisms, getting down
from a cloud may be more of an ordeal than getting up: spore-releasing fungi tend to shed spores at the
times of day and year when thermals are most likely, and bacteria are so small and light that the slightest
puff of wind can send them flying.
From a cloud, of course, the obvious route back to earth is via rain. One way to increase the probability
of rain is to seed the formation not of water droplets, but of ice crystals, an important step in precipitation.
Therefore, it is intriguing that the bacteria and fungi best know for their ice-nucleating ability are also famous
for their tendency to frequent clouds.
This may sound like the stuff of cloud-cuckoo land, and none of it has been tested yet. But Dr.
Hamilton says his theory explains too many coincidences to be ignored. Right or wrong, it puts those tropical
summer skies in a new light.
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