Until now, aerosol transformation was assumed to be via chemical or physical processes. Here we present evidence that an important class of organic aerosols -- dicarboxylic acids (DCA) -- can be efficiently transformed by existing airborne microbes (bacteria and fungi) in the boundary layer. Isotopic studies indicate that microbiological entities transform and use DCA as nutrients. Several observed products are toxicants or pathogens. Identified volatile products indicate that DCA can be recycled back to the atmosphere via microbiological processes. Thus, biodegradation could be an important atmospheric transformation pathway for organic compounds.