On cloud nine

Algae and bacteria travel on clouds

Published: Friday 15 May 1998

 The silver lining: try and sp algae , bacteria and fungi play an important role in cloud formation, helping in condensation of water droplets, a process known as nucleation, under which, water molecules adhere to small microscopic particles. Bill Hamilton, biologist at Oxford University, uk , and Tim Lenton, biologist at East Anglia University, uk , hypothesise that in order to facilitate their own dispersal, some microbes have evolved methods of seeding clouds ( The Economist , Vol 346, No 8061).

In what they call the travel-by-cloud hypothesis, the biologists attempt to explain one such interesting phenomenon about algae. In order to get a piggy-ride on the clouds, the microbes should be able to leave the ground, facilitate the seeding of clouds while enduring the intense sunlight, and come back to the ground in the rain drops. This process starts with a moderate breeze making them airborne and sweeping them upwards. On a calm day, some algae create their own wind, according to Hamilton.

He suggests two mechanisms through which the microclimate is changed by algae. Firstly, algae floating on the surface of warm water modify the microclimate by absorbing the sunlight, and in doing so, warming the water surface as well. Air absorbs some heat from the warm water, which leads to a change in air pressure and mini thermals (rising columns of warm air), making the algae airborne. Secondly, some algae change the microclimate by releasing a gas known as dimethyl sulphide ( dms ).

dms gets oxidised into sulphate, which is one of the primary sources of cloud condensation nuclei -- tiny airborne particles around which water droplets form. Without these particles, clouds will be very sparse as the air would require to be excessively saturated with water vapour or be very cold to facilitate condensation. During the nucleation process, heat from the water is released into the air, thereby warming it up. The thermal thus created increases the local wind speed necessary to get airborne. Another interesting theory proposed by the biologists is that since dms -emitting algae are smaller, they can "take off" easily with the help of the gas released. dms -emitting algae are common in the tropics, as are some of the strongest emissions of dms in warm, often windless waters.

Some other organisms that frequent the aerial route are bacteria and fungi, many varieties of which have been collected from clouds. In the case of these organisms, coming down is more of an ordeal than getting up. It is particularly difficult for bacteria, as the slightest puff of wind can send them on a spin. Once on the clouds, the route back to the ground is, obviously, rain. For this, they seed ice crystals, and not water droplets, an important step in condensation. Interestingly, bacteria and fungi that are most adept at ice nucleation are found very frequently in clouds.

However, none these theories have been tested yet. But Hamilton argues that his theories explain too many coincidences to be ignored. Even as the biologists try to find more evidence to support their claims, every time you watch a cloud, spare a thought for those tiny creatures struggling in mid-air.

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