Before sunrise in central Pennsylvania, fog can settle low and still over the landscape, quiet enough to seem almost empty. But inside those suspended droplets, Arizona State University researchers found something far less passive: bacteria that are alive, dividing, and feeding on airborne pollutants. That finding pushes fog out of the category of simple weather and into something more biologically active. In a study published in mBio, the team reports that radiation fog, the kind that forms close to the ground in calm air, acts as a temporary water habitat for microbes. Some of the most common bacteria in those droplets appear able to consume formaldehyde, a toxic air pollutant that contributes to ozone smog and can harm human health. “There’s very limited knowledge about what kinds of bacteria are present in fogs, which are like clouds at the ground level,” said lead researcher Thi Thuong Thuong Cao, who worked on the project as a PhD student in ASU’s School of Molecular Sciences and is now a postdoctoral researcher at Virginia Tech. The question that …