Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 7 1 2010 10.5194/bg-7-387-2010 http://www.biogeosciences.net/7/387/2010/ http://www.biogeosciences.net/7/387/2010/bg-7-387-2010.html http://www.biogeosciences.net/7/387/2010/bg-7-387-2010.pdf 387 394 2010-01-29 A possible role of ground-based microorganisms on cloud formation in the atmosphere S. Ekström B. Nozière barbara.noziere@itm.su.se M. Hultberg T. Alsberg J. Magnér E. D. Nilsson P. Artaxo Department of Applied Environmental Science (ITM), Svante Arrhenius väg 8, Stockholm University, 106 91 Stockholm, Sweden Department of Horticulture, Swedish University of Agricultural Sciences, Alnarp, Sweden Department of Physics, University of São Paolo, São Paolo, Brazil The formation of clouds is an important process for the atmosphere, the hydrological cycle, and climate, but some aspects of it are not completely understood. In this work, we show that microorganisms might affect cloud formation without leaving the Earth's surface by releasing biological surfactants (or biosurfactants) in the environment, that make their way into atmospheric aerosols and could significantly enhance their activation into cloud droplets. <br><br> In the first part of this work, the cloud-nucleating efficiency of standard biosurfactants was characterized and found to be better than that of any aerosol material studied so far, including inorganic salts. These results identify molecular structures that give organic compounds exceptional cloud-nucleating properties. In the second part, atmospheric aerosols were sampled at different locations: a temperate coastal site, a marine site, a temperate forest, and a tropical forest. Their surface tension was measured and found to be below 30 mN/m, the lowest reported for aerosols, to our knowledge. This very low surface tension was attributed to the presence of biosurfactants, the only natural substances able to reach to such low values. <br><br> The presence of strong microbial surfactants in aerosols would be consistent with the organic fractions of exceptional cloud-nucleating efficiency recently found in aerosols, and with the correlations between algae bloom and cloud cover reported in the Southern Ocean. The results of this work also suggest that biosurfactants might be common in aerosols and thus of global relevance. If this is confirmed, a new role for microorganisms on the atmosphere and climate could be identified. % vor jede Referenz Ahern, H. E., Walsh, K. A., Hill, T. C. J., and Moffett, B. F.: Fluorescent pseudomonads isolated from Hebridean cloud and rain water produce biosurfactants but do not cause ice nucleation, Biogeosciences, 4, 115–124, 2007. Asa-Awuku, A., Sullivan, A. P., Hennigan, C. J., Weber, R. 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