BAC­TE­RIA CON­TROLS THE WEATHER

Could we end the threats of droughts and famine, by forc­ing rain to fall on de­mand?

Focus-Science and Technology - - COVER FEATURE - WORDS: TOM IRE­LAND Tom Ire­land is a sci­ence jour­nal­ist and manag­ing ed­i­tor at the Royal So­ci­ety of Bi­ol­ogy.

Bac­te­ria do many things for us – from mak­ing our food and fuels to break­ing down waste. But re­search sug­gests they could also de­ter­mine whether or not it rains.

The micro­organ­isms that af­fect weather are known as ice-nu­cle­ation ac­tive bac­te­ria. They are of­ten found on crops and can cause plant dis­ease. They have spe­cial pro­teins on their sur­face that help wa­ter in the air turn to ice at slightly warmer tem­per­a­tures – around -3°C in­stead of -8°C. On the ground, these pro­teins can cause frost dam­age to crops. But when the bac­te­ria are blown into the sky, they can cause ice crys­tals to form in the at­mos­phere – a cru­cial first step in the cre­ation of rain or snow. The process, called ‘bio pre­cip­i­ta­tion’, was once thought to play only a small part in caus­ing snow or rain. But the im­pact of mi­crobes on rain­fall is be­ing re­con­sid­ered, ac­cord­ing to Dr Cindy Mor­ris, an ex­pert in ice-nu­cle­at­ing bac­te­ria from France’s In­sti­tute for Agri­cul­tural Re­search. “It’s much more im­por­tant than we first thought,” says Mor­ris. “We now know that when ice forms at warmer tem­per­a­tures, like be­tween -3°C and -8°C, it ex­plodes into lots more ice crys­tals – so there is a mul­ti­ply­ing ef­fect.”

There is even tan­ta­lis­ing ev­i­dence that bac­te­ria blown off plants could cause rain on the other side of the world, says Mor­ris. “We’ve found bac­te­ria in a creek in the New Zealand wilder­ness that was ge­net­i­cally iden­ti­cal to bac­te­ria caus­ing can­taloupe blight in France.”

Mi­crobes with rain-caus­ing prop­er­ties are thought to have evolved long be­fore ter­res­trial plants ex­isted. “If you are a tiny or­gan­ism like a bac­terium and you

get wafted up into the air and into the tur­bu­lence, you’re done for – grav­ity has no ef­fect and you just keep go­ing up,” says Mor­ris. “The only way down is in a rain­drop. The chances of be­ing hit by a rain­drop are very small, so you need to form one your­self.”

So given how ef­fec­tive these bac­te­ria are at form­ing rain, could we use them to seed clouds and cre­ate rain­fall where it is needed? Ac­cord­ing to Mor­ris, we are prob­a­bly al­ready do­ing it to some de­gree. She says that large ex­panses of crops can cause huge num­bers of these bac­te­ria to be blown into the air. She be­lieves that by colonis­ing moun­tain slopes with plants, di­rectly be­low cur­rents of wet air high in the sky, we could help cre­ate rain­fall in ar­eas in des­per­ate need of wa­ter, such as Cal­i­for­nia. “But it’s not sim­ple,” she ex­plains. “Do the farm­ers there want yield or rain? You’d need to work out sys­tems to pay peo­ple grow­ing those crops. These bac­te­ria also cause dis­ease, so it’s about a bal­ance be­tween driv­ing rain and caus­ing dis­ease.”

What’s more, ‘seed­ing clouds’ is a fine art – too many ice-nu­cle­at­ing par­ti­cles can ac­tu­ally ‘con­sti­pate’ a cloud, pre­vent­ing rain­fall. And po­lit­i­cal dis­putes be­tween na­tions ‘tak­ing’ oth­ers’ rain have sti­fled pre­vi­ous projects to ma­nip­u­late weather. So it could be a while be­fore we can truly make it rain – but our un­der­stand­ing of pre­cip­i­ta­tion is chang­ing. Just think, next time you feel a drop of rain on your head, it could be a mi­crobe re­turn­ing from an epic jour­ney.

“When the bac­te­ria are blown into the sky, they can cause ice crys­tals to form – a first step in the cre­ation of rain or snow”

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