do bac­te­ria make it rain?

Popular Science - - CHARTED -

MI­CROBES ARE EV­ERY­WHERE—IN YOUR GUT, ON YOUR SKIN, EVEN IN THE sky. Along with wa­ter va­por and par­ti­cles, clouds also con­tain liv­ing bac­te­ria. And cer­tain air­borne species have a su­per­power: They can cause wa­ter to freeze into ice, which in turn prompts the heav­ens to open. Once frozen gran­ules form around bits of dust, min­er­als, or tiny or­gan­isms, they fall, melt­ing on the way down to be­come rain­drops or form­ing flakes to make snow. This means the lit­tle life-forms might con­trib­ute to rain­fall—but how much? “If you loaded up a cloud with these mi­crobes, it would cre­ate freez­ing and the pro­cesses that lead to pre­cip­i­ta­tion,” con­firms Univer­sity of Florida’s Brent Christ­ner, who stud­ies at­mo­spheric bac­te­ria. “The un­known fac­tor is, are there enough of them up there to mat­ter?” Christ­ner and other re­searchers are send­ing weather bal­loons and planes on sam­pling mis­sions to try to an­swer the ques­tion—and shed light on what these cit­i­zens of the sky get out of the deal. They might use clouds to travel long dis­tances be­fore rain­ing down on fresh new habi­tats.

1. Cloud Stor­age

The frigid va­por a cou­ple of miles above Earth’s sur­face seems pris­tine, but it’s filthy with par­ti­cles, rang­ing from clumps of sul­fates and min­eral dust to bac­te­ria. Each cu­bic foot of air can con­tain from 300 to 30,000 mi­crobes.

4. In For­ma­tion

Sci­en­tists think that as wa­ter mol­e­cules in the cloud get near the pro­teins, chem­i­cal forces cause the H2O to line up along the bac­terium’s sur­face in an or­derly lat­tice. That po­si­tion­ing en­cour­ages the mol­e­cules to form ice.

2. Zoom In

At least one of those mi­crobe species, Pseu­domonas sy­ringae, can spur ice for­ma­tion at rel­a­tively warm tem­per­a­tures. First iden­ti­fied decades ago on plants, the easy-togrow bac­te­ria now helps ski re­sorts make snow for their slopes.

3. Ice Shap­ing

P. sy­ringae pro­duces pro­teins that ar­range them­selves on the bac­terium’s sur­face. Sci­en­tists know that the pat­tern mat­ters: When a mi­crobe dies, or sci­en­tists de­lib­er­ately mod­ify its pro­tein ar­range­ment, it doesn’t form ice as quickly or eas­ily.

5. Cold Fu­sion

The frozen ker­nel at­tracts more and more H2O, grad­u­ally grow­ing larger and heav­ier. When it gets too heavy to re­main air­borne, it plum­mets to Earth as snow or rain, de­pend­ing on the tem­per­a­tures it en­coun­ters on the way down.

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