Marine aerosols are formed primarily by the eruption of rising bubbles through the sea-surface microlayer (SML), and aerosol formation is the main vector for transport of bacteria and viruses across the air-sea interface. The processes by which materials are transported to and through the SML to the atmosphere results in an enrichment in the SML of microbial and other organisms. We evaluated concentrations of marine bacteria and viruses in natural aerosols and in those simulated by bubbling sea sprays and compared them to the concentrations in SML (200-400 mu m thick) and in subsurface water. Association of microorganisms with transparent gel-like organic particles and physiological status of bacteria were also assessed. We found a 15-25-fold enrichment in bacteria and viruses during transport from subsurface waters to the SML, and then into the atmosphere. The majority of microorganisms in aerosols were found embedded in the organic particles. Large portions of microorganisms in the SML were also associated with the particles, while in subsurface waters most of them were free-living. A larger percentage of damaged and less active bacterial cells occur in the microlayer than in subsurface waters, and preliminary data suggest that an even greater percentage occurs in aerosols. Our data support the idea that the SML in a major source of microorganisms entering the atmosphere from water bodies. Aerosolization is potentially an important long-distance dispersal mechanism and may account for observed cosmopolitan distributions of some bacteria. (c) 2004 Elsevier Ltd. All rights reserved.