"But clouds present an extra difficulty. Most of the environments that we consider extreme — the icy, the acidic, the salty, the boiling, and so on — are fairly easy for microbes to adapt to, because the conditions remain relatively constant for long periods. The greater challenge comes when the environment contains extreme swings in conditions, as in a tide pool. Creatures that live here must be able to endure changes from wet to dry, cold to hot, as well as rapid fluctuations in how salty it is."
"Mostly, the cloudy residents are bacteria of various kinds. Samples of clouds taken from a meteorological station at the summit of Puy de Dôme, a mountain in central France with an elevation of more than 1,400 meters (almost 5,000 feet), turned up more than 71 strains of bacteria, as well as a variety of fungi; owing to the way the sampling was done, this is a massive underestimate of who’s up there. Of the bacteria detected, many appear to have come from the oceans. More than half have shown themselves capable of growing in cold temperatures, and some are even officially psychrophiles — lovers of cold. Which is to say, they grow when it’s cold, and not when it’s a bit warmer — unlike the bacteria on your food, which slow down when you put them in the fridge. One of the bacteria most often detected was Pseudomonas syringae; intriguingly, this critter has the ability to make ice crystals form around it at relatively warm (-2C, or 28.4F) temperatures.
As to their metabolism — the question of what such microbes “eat” — clouds are, apparently, more nutritious than they look. More nutritious, even, than some freshwater lakes. Cloud water contains a slew of compounds, from different types of organic acids and alcohols, to elements such as nitrogen and sulfur. Laboratory experiments have shown that for a growing bacterium or fungus, cloud water contains plenty of potential food."