ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-11-1191-2011 The ice nucleation ability of one of the most abundant types of fungal spores found in the atmosphere Iannone R. 1 Chernoff D. I. 1 Pringle A. 2 Martin S. T. 3 Bertram A. K. 1 Department of Chemistry, University of British Columbia, Vancouver, British Columbia, V6T 1Z1, Canada Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts, USA School of Engineering and Applied Sciences & Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, USA 11 02 2011 11 3 1191 1201 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/11/1191/2011/acp-11-1191-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/11/1191/2011/acp-11-1191-2011.pdf

Recent atmospheric measurements show that biological particles are a potentially important class of ice nuclei. Types of biological particles that may be good ice nuclei include bacteria, pollen and fungal spores. We studied the ice nucleation properties of water droplets containing fungal spores from the genus <i>Cladosporium</i>, one of the most abundant types of spores found in the atmosphere. For water droplets containing a <i>Cladosporium</i> spore surface area of ~217 μm<sup>2</sup> (equivalent to ~5 spores with average diameters of 3.2 μm ), 1% of the droplets froze by −28.5 °C and 10% froze by –30.1 °C. However, there was a strong dependence on freezing temperature with the spore surface area of <i>Cladosporium</i> within a given droplet. Mean freezing temperatures for droplets containing 1–5 spores are expected to be approximately −35.1 ± 2.3 °C (1σ S. D.). Atmospheric ice nucleation on spores of <i>Cladosporium sp.</i>, or other spores with similar surface properties, thus do not appear to explain recent atmospheric measurements showing that biological particles participate as atmospheric ice nuclei. The poor ice nucleation ability of <i>Cladosporium sp.</i> may be attributed to the surface which is coated with hydrophobins (a class of hydrophobic proteins that appear to be widespread in filamentous fungi). Given the ubiquity of hydrophobins on spore surfaces, the current study may be applicable to many fungal species of atmospheric importance.

 References Abbatt, J. P. D., Benz, S., Cziczo, D. J., Kanji, Z. A., Lohmann, U., and Möhler, O.: Solid ammonium sulfate aerosols as ice nuclei: a pathway for cirrus cloud formation, Science, 313, 1770–1773, 2006. Aimanianda, V., Bayry, J., Bozza, S., Kniemeyer, O., Perruccio, K., Elluru, S. R., Clavaud, C., Paris, S., Brakhage, A. A., Kaveri, S. V., Romani, L., and Latgé, J.-P.: Surface hydrophobin prevents immune recognition of airborne fungal spores, Nature, 460, 1117–1121, 2009. Al-Subai, A. A. T.: Air-borne fungi at Doha, Qatar, Aerobiologia, 18, 175–183, 2002. Andreae, M. O. and Rosenfeld, D.: Aerosol-cloud-precipitation interactions, Part 1. The nature and sources of cloud-active aerosols, Earth-Sci. Rev., 89, 13–41, 2008. Archuleta, C. M., DeMott, P. J., and Kreidenweis, S. M.: Ice nucleation by surrogates for atmospheric mineral dust and mineral dust/sulfate particles at cirrus temperatures, Atmos. Chem. Phys., 5, 2617–2634, http://dx.doi.org/10.5194/acp-5-2617-2005doi:10.5194/acp-5-2617-2005, 2005. Ariya, P. A., Sun, J., Eltouny, N., Hudson, E., Hayes, C. T., and Kos, G.: Physical and chemical characterization of bioaerosols Ð implications for nucleation processes, Int. Rev. Phys. Chem., 28, 1–32, http://dx.doi.org/10.1080/01442350802597438doi:10.1080/01442350802597438, 2009. Ashworth, E. N. and Kieft, T. L.: Measurement of ice nucleation in lichens using thermal analysis, Cryobiology, 29, 400–406, 1992. Baker, M. B. and Peter, T.: Small-scale cloud processes and climate, Nature, 451, 299–300, 2008. Bertram, A. K., Koop, T., Molina, L. T., and Molina, M. J.: Ice formation in (NH$_4)_2$SO<sub>4</sub>-H<sub>2</sub>O particles, J. Phys. Chem. A, 104, 584–588, http://dx.doi.org/10.1021/jp9931197doi:10.1021/jp9931197, 2000. Carlile, M. J., Watkinson, S. C., and Gooday, G. W.: The Fungi, 2nd ed., Academic Press, London, UK, 608~pp., 2001. Christner, B. C., Morris, C. E., Foreman, C. M., Cai, R., and Sands, D. C.: Ubiquity of biological ice nucleators in snowfall, Science, 319, 1214–1214, http://dx.doi.org/10.1126/science.1149757doi:10.1126/science.1149757, 2008. Davies, R. R.: A study of air-borne \textitCladosporium, T. Brit. Mycol. Soc., 40, 409–414, 1957. DeMott, P. J., Cziczo, D. J., Prenni, A. J., Murphy, D. M., Kreidenweis, S. M., Thomson, D. S., Borys, R., and Rogers, D. C.: Measurements of the concentration and composition of nuclei for cirrus formation, P. Natl. Acad. Sci. USA, 100, 14655–14660, http://dx.doi.org/10.1073/pnas.2532677100doi:10.1073/pnas.2532677100, 2003. DeMott, P. J. and Prenni, A. J.: New directions: need for defining the numbers and sources of biological aerosols acting as ice nuclei, Atmos. Environ., 44, 1944–1945, 2010. DeMott, P. J., Prenni, A. J., Liu, X., Kreidenweis, S. M., Petters, M. D., Twohy, C. H., Richardson, M. S., Eidhammer, T., and Rogers, D. C.: Predicting global atmospheric ice nuclei distributions and their impacts on climate, P. Natl. Acad. Sci. USA, 107, 11217–11222, 2010. Diehl, K., Quick, C., Matthias-Maser, S., Mitra, S., and Jaenicke, R.: The ice nucleating ability of pollen – Part I: Laboratory studies in deposition and condensation freezing modes, Atmos. Res., 58, 75–87, http://dx.doi.org/10.1016/S0169-8095(01)00091-6doi:10.1016/S0169-8095(01)00091-6, 2001. Diehl, K., Matthias-Maser, S., Jaenicke, R., and Mitra, S.: The ice nucleating ability of pollen – Part II: Laboratory studies in immersion and contact freezing modes, Atmos. Res., 61, 125Ð133, http://dx.doi.org/10.1016/S0169-8095(01)00132-6doi:10.1016/S0169-8095(01)00132-6, 2002. Diehl, K., Simmel, M., and Wurzler, S.: Numerical sensitivity studies on the impact of aerosol properties and drop freezing modes on the glaciation, microphysics, and dynamics of clouds, J. Geophys. Res., 111, D07202, http://dx.doi.org/10.1029/2005JD005884doi:10.1029/2005JD005884, 2006. Dymarska, M., Murray, B. J., Sun, L., Eastwood, M. L., Knopf, D. A., and Bertram, A. K.: Deposition ice nucleation on soot at temperatures relevant for the lower troposphere, J. Geophys. Res., 111, D04204, http://dx.doi.org/10.1029/2005JD006627doi:10.1029/2005JD006627, 2006. Eastwood, M. L., Cremel, S., Gehrke, C., Girard, E., and Bertram, A. K.: Ice nucleation on mineral dust particles: onset conditions, nucleation rates and contact angles, J. Geophys. Res., 113, D22203, http://dx.doi.org/10.1029/2008JD010639doi:10.1029/2008JD010639, 2008. Elbert, W., Taylor, P. E., Andreae, M. O., and Pöschl, U.: Contribution of fungi to primary biogenic aerosols in the atmosphere: wet and dry discharged spores, carbohydrates, and inorganic ions, Atmos. Chem. Phys., 7, 4569–4588, http://dx.doi.org/10.5194/acp-7-4569-2007doi:10.5194/acp-7-4569-2007, 2007. Fröhlich-Nowoisky, J., Pickersgill, D. A., Després, V. R., and Pöschl, U.: High diversity of fungi in air particulate matter, P. Natl. Acad. Sci. USA, 106, 12814–12819, http://dx.doi.org/10.1073/pnas.0811003106doi:10.1073/pnas.0811003106, 2009. Fulton, J. D.: Microorganisms of the upper atmosphere. III. Relationship between altitude and micropopulation, Appl. Microbiol., 14, 237–240, 1996. Fulton, J. D. and Mitchell, R. B.: Microorganisms of the upper atmosphere. II. Microorganisms in two types of air masses at 690 meters over a city, Appl. Microbiol., 14, 232–236, 1966. Gilbert, G. S. and Reynolds, D. R.: Nocturnal fungi: Airborne spores in the canopy and understory of a tropical rain forest, Biotropica, 37, 462–464, 2005. Green, R. L. and Warren, G. J.: Physical and functional repetition in a bacterial ice nucleation gene, Nature, 317, 645–648, 1985. Gregory, P. H.: Distribution of airborne pollen and spores and their long distance transport, Pure Appl. Geophys., 116, 309–315, 1978. Gurian-Sherman, D. and Lindow, S. E.: Bacterial ice nucleation: Significance and molecular basis, FASEB J., 7, 1338–1343, 1993. Hameed, A. A. A., and Khodr, M. I.: Suspended particulates and bioaerosols emitted from an agricultural non-point source, J. Environ. Monit., 3, 206–209, 2001. Harvey, R.; Air-spora studies at Cardiff: I. \textitCladosporium, Trans. Brit. Mycol. Soc., 50, 479–495, 1967. Hegg, D. A. and Baker, M. B.: Nucleation in the atmosphere, Rep. Prog. Phys., 72, 056801, http://dx.doi.org/10.1088/0034-4885/72/5/056801doi:10.1088/0034-4885/72/5/056801, 2009. Heise, H. A. and Heise, E. R.: The distribution of ragweed pollen and alternaria spores in the upper atmosphere, J. Allergy, 19, 403–407, 1948. Herrero, A. D., Ruiz, S. S., Bustillo, M. G., and Morales, P. C.: Study of airborne fungal spores in Madrid, Spain, Aerobiologia, 22, 135–142, 2006. Hirst, J. M.: Changes in atmospheric spore content: diurnal periodicity and the effects of weather, Trans. Brit. Mycol. Soc., 36, 375–393, 1953. Hirst, J. M., Stedman, O. J., and Hogg, W. H.: Long-distance spore transport: methods of measurement, vertical spore profiles and the detection of immigrant spores, J. Gen. Microbiol., 48, 329–355, 1967a. Hirst, J. M., Stedman, O. J., and Hurst, G. W.: Long-distance spore transport: vertical sections of spore clouds over the sea, J. Gen. Microbiol., 48, 357–377, 1967b. Hoose, C., Kristjánsson, J. E., and Burrows, S. M.: How important is biological ice nucleation in clouds on a global scale?, Environ. Res. Lett., 5, 024009, http://dx.doi.org/10.1088/1748-9326/5/2/024009doi:10.1088/1748-9326/5/2/024009, 2010. Hung, H.-M., Malinowski, A., and Martin, S. T.: Kinetics of heterogeneous ice nucleation on the surfaces of mineral dust cores inserted into aqueous ammonium sulfate particles, J. Phys. Chem. A, 107, 1296–1306, 2003. Hyde, H. A. and Williams, D. A.: The incidence of \textitCladosporium herbarum in the outdoor air at Cardiff, 1949–1950, T. Brit. Mycol. Soc., 36, 260–266, 1953. Imshenetsky, A. A., Lysenko, S. V., and Kazakov, G. A.: Upper boundary of the biosphere, Appl. Environ. Microb., 35, 1–5, 1978. Jung, J. H., Lee, C. H., Lee, J. E., Lee, J. H., Kim, S. S., and Lee, B. U.: Design and characterization of a fungal bioaerosol generator using multi-orifice air jets and a rotating substrate, J. Aerosol Sci., 40, 72–80, http://dx.doi.org/10.1016/j.jaerosci.2008.09.002doi:10.1016/j.jaerosci.2008.09.002, 2009a. Jung, J. H., Lee, J. E., Lee, C. H., Kim, S. S., and Lee, B. U.: Treatment of fungal bioaerosols by a high-temperature, short-time process in a continuous-flow system, Appl. Environ. Microb., 75, 2742–Ð2749, http://dx.doi.org/10.1128/AEM.01790-08doi:10.1128/AEM.01790-08, 2009b. Kajava, A. V. and Lindow, S. E.: A model of the three-dimensional structure of ice nucleation proteins, J. Mol. Biol., 232, 709–717, 1993. Kanji, Z. A., Florea, O., and Abbatt, J. P. D.: Ice formation via deposition nucleation on mineral dust and organics: dependence of onset relative humidity on total particulate surface area, Environ. Res. Lett., 3, 025004, http://dx.doi.org/10.1088/1748-9326/3/2/025004doi:10.1088/1748-9326/3/2/025004, 2008. Kärcher, B., Möhler, O., DeMott, P. J., Pechtl, S., and Yu, F.: Insights into the role of soot aerosols in cirrus cloud formation, Atmos. Chem. Phys., 7, 4203–4227, http://dx.doi.org/10.5194/acp-7-4203-2007doi:10.5194/acp-7-4203-2007, 2007. Kelly, C. D. and Pady, S. M.: Microbiological studies of air over some nonarctic regions of Canada, Can. J. Botany, 31, 90–106, 1953. Kieft, T. L.: Ice nucleation activity in lichens, Appl. Environ. Microb., 54, 1678–1681, 1988. Kieft, T. L. and Ahmadjian, V.: Biological ice nucleation activity in lichen mycobionts and photobionts, Lichenologist, 21, 355–362, 1989. Kieft, T. L. and Ruscetti, T.: Characterization of biological ice nuclei from a lichen, J. Bacteriol., 172, 3519–3523, 1990. Koop, T., Ng, H., Molina, L. T., and Molina, M. J.: A new optical technique to study aerosol phase transitions: The nucleation of ice from H<sub>2</sub>SO<sub>4</sub> aerosols, J. Phys. Chem. A, 102, 8924–8931, 1998. Kurkela, T.: The number of \textitCladosporium conidia in the air in different weather conditions, Grana, 36, 54–61, 1997. Latgé, J.-P., Bouziane, H., and Diaquin, M.: Ultrastructure and composition of the conidial wall of \textitCladosporium cladosporioides, Can. J. Microbiol., 34, 1325–1329, 1988. Lee, R. E., Warren, G. J., and Gusta, L. V. (eds.): Biological ice nucleation and its applications, American Phytopathological Society, St. Paul, MN, ISBN 978-0-89054-172-2, 370~pp., 1995. Li, D.-W. and Kendrick, B.: A year-round outdoor aeromycological study in Waterloo, Ontario, Canada, Grana, 34, 199–207, 1995. Lim, S. H., Chew, F. T., Dali, S. D. B. M., Tan, H. T. W., Lee, B. W., and Tan, T. K.: Outdoor airborne fungal spores in Singapore, Grana, 37, 246–252, 1998. Linder, M. B.: Hydrophobins: proteins that self assemble at interfaces, Curr. Opin. Colloid In., 14, 356–363, http://dx.doi.org/10.1016/j.cocis.2009.04.001doi:10.1016/j.cocis.2009.04.001, 2009. Linder, M. B., Szilvay, G. R., Nakari-Setälä, T., and Penttilä, M. E.: Hydrophobins: the protein-amphiphiles of filamentous fungi, FEMS Microbiol. Rev., 29, 877–896, 2005. Lindow, S. E., Lahue, E., Govindarajan, A. G., Panopoulos, N. J., and Gies, D.: Localization of ice nucleation activity and the iceC gene product in \textitPseudomonas syringae and \textitEscherichia coli, Mol. Plant Microbe In., 2, 262–272, 1989. Lohmann, U. and Hoose, C.: Sensitivity studies of different aerosol indirect effects in mixed-phase clouds, Atmos. Chem. Phys., 9, 8917–8934, http://dx.doi.org/10.5194/acp-9-8917-2009doi:10.5194/acp-9-8917-2009, 2009. Lüönd, F., Stetzer, O., Welti, A., and Lohmann, U.: Experimental study on the ice nucleation ability of size-selected kaolinite particles in the immersion mode, J. Geophys. Res., 115, D14201, http://dx.doi.org/10.1029/2009JD01295doi:10.1029/2009JD01295, 2010. Maki, L. R., Galyan, E. L., Chang-Chien, M.-M., and Caldwell, D. R.: Ice nucleation induced by \textitPseudomonas syringae, Appl. Microbiol., 28, 456–459, 1974. Mallo, A., Nitiu, D., and Gardella Sambeth, M.: Airborne fungal spore content in the atmosphere of the city of La Plata, Argentina, Aerobiologia, 27(1), 77–84 http://dx.doi.org/10.1007/s10453-10010-19172-10450doi:10.1007/s10453-10010-19172-10450, 2011. Marcolli, C., Gedamke, S., Peter, T., and Zobrist, B.: Efficiency of immersion mode ice nucleation on surrogates of mineral dust, Atmos. Chem. Phys., 7, 5081–5091, http://dx.doi.org/10.5194/acp-7-5081-2007doi:10.5194/acp-7-5081-2007, 2007. Martin, S. T.: Phase transformations of the ternary system (NH$_4)_2$SO<sub>4</sub>-H<sub>2</sub>SO<sub>4</sub>-H<sub>2</sub>O and the implications for cirrus cloud formation, Geophys. Res. Lett., 25, 1657–1660, 1998. Martin, S. T., Andreae, M. O., Artaxo, P., Baumgardner, D., Chen, Q., Goldstein, A. H., Guenther, A., Heald, C. L., Mayol-Bracero, O. L., McMurry, P. H., Pauliquevis, T., Pöschl, U., Prather, K. A., Roberts, G. C., Saleska, S. R., Silva Dias, M. A., Spracklen, D. V., Swietlicki, E., and Trebs, I.: Sources and properties of Amazonian aerosol particles, Rev. Geophys., 48, RG2002, http://dx.doi.org/10.1029/2008RG000280doi:10.1029/2008RG000280, 2010. Mitakakis, T. Z. and Guest, D. I.: A fungal spore calendar for the atmosphere of Melbourne, Australia, for the year 1993, Aerobiologia, 17, 171–176, 2001. Möhler, O., DeMott, P. J., Vali, G., and Levin, Z.: Microbiology and atmospheric processes: the role of biological particles in cloud physics, Biogeosciences, 4, 1059–1071, http://dx.doi.org/10.5194/bg-4-1059-2007doi:10.5194/bg-4-1059-2007, 2007. Morris, C. E., Georgakopoulos, D., and Sands, D.: Ice nucleation active bacteria and their potential role in precipitation, J. Phys IV, 121, 87–103, http://dx.doi.org/10.1051/jp4:2004121004doi:10.1051/jp4:2004121004, 2004. Murray, B. J., Wilson, T. W., Broadley, S. L., and Wills, R. H.: Heterogeneous freezing of water droplets containing kaolinite and montmorillonite particles, Atmos. Chem. Phys. Discuss., 10, 9695–9729, 2010. Nash III, T. H.: Lichen Biology, 2nd ed., Cambridge University Press, New York, 496~pp., 2008. Pady, S. M. and Kelly, C. D.: Numbers of fungi and bacteria in transatlantic air, Science, 117, 607–609, 1953. Parsons, M. T., Mak, J., Lipetz, S. R., and Bertram, A. K.: Deliquescence of malonic, succinic, glutaric, and adipic acid particles, J. Geophys. Res., 109, D06212, http://dx.doi.org/10.1029/2003JD004075doi:10.1029/2003JD004075, 2004. Phillips, V. T. J., DeMott, P. J., and Andronache, C.: An empirical parameterization of heterogeneous ice nucleation for multiple chemical species of aerosol, J. Atmos. Sci., 65, 2757–2783, 2008. Phillips, V. T. J., Andronache, C., Christner, B., Morris, C. E., Sands, D. C., Bansemer, A., Lauer, A., McNaughton, C., and Seman, C.: Potential impacts from biological aerosols on ensembles of continental clouds simulated numerically, Biogeosciences, 6, 987–1014, http://dx.doi.org/10.5194/bg-6-987-2009doi:10.5194/bg-6-987-2009, 2009. Pouleur, S., Richard, C., Martin, J.-G., and Antouin, H.: Ice nucleation activity in \textitFusarium acuminatum and \textitFusarium avenaceum, Appl. Environ. Microb., 58, 2960–2964, 1992. Pratt, K. A., DeMott, P. J., French, J. R., Wang, Z., Westphal, D. L., Heymsfield, A. J., Twohy, C. H., Prenni, A. J., and Prather, K. A.: In situ detection of biological particles in cloud ice-crystals, Nat. Geosci., 2, 397–400, http://dx.doi.org/10.1038/NGEO521doi:10.1038/NGEO521, 2009. Prenni, A. J., Petters, M. D., Kreidenweis, S. M., Heald, C. L., Martin, S. T., Artaxo, P., Garland, R. M., Wollny, A. G., and Pöschl, U.: Relative roles of biogenic emissions and Saharan dust as ice nuclei in the Amazon basin, Nat. Geosci., 2, 401–404, http://dx.doi.org/10.1038/NGEO517doi:10.1038/NGEO517, 2009. Proctor, B. E.: The microbiology of the upper air. II, J. Bacteriol., 30, 363–375, 1935. Proctor, B. E. and Parker, B. W.: Microbiology of the upper air. III. An Improved apparatus and technique for upper air investigations, J. Bacteriol., 36, 175–185, 1938. Pruppacher, H. R.: A new look at homogeneous ice nucleation in supercooled water drops, J. Atmos. Sci., 52, 1924–1933, 1995. Pruppacher, H. R. and Klett, J. D.: Microphysics of Clouds and Precipitation, Kluwer Academic Publishers, Dordrecht, 954~pp., 1997. Pyrri, I. and Kapsanaki-Gotsi, E.: A comparative study on the airborne fungi in Athens, Greece, by viable and non-viable sampling methods, Aerobiologia, 23, 3–15, 2007. Reponen, T., Willeke, K., Ulevicius, V., Grinshpun, S. A., and Donnelly, J.: Techniques for dispersion of microorganisms into air, Aerosol Sci. Tech., 27, 405–421, 1997. Reponen, T., Grinshpun, S. A., Conwell, K. L., Wiest, J., and Anderson, M.: Aerodynamic versus physical size of spores: measurement and implication for respiratory deposition, Grana, 40, 119–125, 2001. Sands, D. C., Langhans, V. E., Scharen, A. L., and de Smet, G.: The association between bacteria and rain and possible resultant meteorological implications, J. Hungarian Meteorol. Serv., 86, 148–152, 1982. Sassen, K., DeMott, P. J., Prospero, J. M., and Poellot, M. R.: Saharan dust storms and indirect aerosol effects on clouds: CRYSTAL-FACE results, Geophys. Res. Lett., 30, 1633, http://dx.doi.org/10.1029/2003GL017371doi:10.1029/2003GL017371, 2003. Schubert, K., Groenewald, J. Z., Braun, U., Dijksterhuis, J., Starink, M., Hill, C. F., Zalar, P., de Hoog, G. S., and Crous, P. W.: Biodiversity in the \textitCladosporium herbarium complex (\textitDavidiellaceae, \textitCapnodiales), with standardisation of methods for \textitCladosporium taxonomy and diagnostics, Stud. Mycol., 58, 105–156, 2007. Shilling, J. E., Fortin, T. J., and Tolbert, M. A.: Depositional ice nucleation on crystalline organic and inorganic solids, J. Geophys. Res., 111, D12204, http://dx.doi.org/10.1029/2005JD006664doi:10.1029/2005JD006664, 2006. Stennett, P. J. and Beggs, P. J.: \textitAlternaria spores in the atmosphere of Sydney, Australia, andrelationships with meteorological factors, Int. J. Biometeorol., 49, 98–105, 2004. St\cepalska, D., and WoÅ‚ek, J.: Intradiurnal periodicity of fungal spore concentrations (\textitAlternaria, \textitBotrytis, \textitCladosporium, \textitDidymella, \textitGanoderma) in Cracow, Poland, Aerobiologia, 25, 333–340, 2009. Szyrmer, W. and Zawadzki, I.: Biogenic and anthropogenic sources of ice-forming nuclei: a review, B. Am. Meteorol. Soc., 78, 209–228, 1997. Vali, G., Christensen, M., Fresh, R. W., Galyan, E. L., Maki, L. R., and Schnell, R. C.: Biogenic ice nuclei. Part II: Bacterial sources, J. Atmos. Sci., 33, 1565–1570, 1976. von Blohn, N., Mitra, S., Diehl, K., and Borrmann, S.: The ice nucleating ability of pollen. Part III: New laboratory studies in immersion and contact freezing modes including more pollen types, Atmos. Res., 78, 182–189, http://dx.doi.org/10.1016/j.atmosres.2005.03.008doi:10.1016/j.atmosres.2005.03.008, 2005. Wessels, J. G. H.: Hydrophobins: proteins that change the nature of the fungal surface, Adv. Microb. Physiol., 38, 1–45, 1997. Whiteford, J. R. and Spanu, P. D.: Hydrophobins and the interactions between fungi and plants, Mol. Plant Pathol., 3, 391–400, 2002. Wise, M. E., Baustian, K. J., and Tolbert, M. A.: Internally mixed sulfate and organic particles as potential ice nuclei in the tropical tropopause region, P. Natl. Acad. Sci. USA, 107, 6693–6698, 2010. Wood, S., Baker, M., and Swanson, B.: Instrument for studies of homogeneous and heterogeneous ice nucleation in free-falling supercooled water droplets, Rev. Sci. Instrum., 73, 3988–3996, http://dx.doi.org/10.1063/1.1511796doi:10.1063/1.1511796, 2002. Wösten, H. A. B.: Hydrophobins: Multipurpose proteins, Ann. Rev. Microbiol., 55, 625–646, 2001. Zuberi, B., Bertram, A. K., Cassa, C. A., Molina, L. T., and Molina, M. J.: Heterogeneous nucleation of ice in (NH$_4)_2$SO<sub>4</sub>-H<sub>2</sub>O particles with mineral dust immersions, Geophys. Res. Lett., 29, 1504, http://dx.doi.org/10.1029/2001GL014289doi:10.1029/2001GL014289, 2002.