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Published online 28 February 2008 | Nature | doi:10.1038/news.2008.632
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'Rain-making' bacteria found around the world
Some microbes are frequent flyers in clouds.
The same bacteria that cause frost damage on plants can help clouds to produce rain and snow. Studies on freshly fallen snow suggest that ‘bio-precipitation’ might be much more common than was suspected.
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Cloud processes are dominated by salt particles, they act as cloud nuclei, but you will not find them in the snow as separate particles, for they get in solution in cloud droplets before these droplets freeze to ice crystals. The more salt particles were involved in cloud formation, the higher the freezing point will be in the water, but this will also depend on the type of rain clouds. I do not expect very significant differences if - from the same type of cloud - more salt nuclei were involved. The role of mineral dust is minimal, only locally (relatively to world scale) it can play a role sometimes. The role of bacteria is neglectable. It is even doubtful if they were ever in the cloud: they may have been trapped by falling snow, so-called scavenging of bigger particles and bacteria are relatively big compared to cloud condensation nuclei. Important is the number of particles with a diameter of around 100 to 200 nanometer, these particles consists of salt particles: ammonium sulphate and ammonium nitrate and sea salt. One litre snow is formed by many many snow crystals each containing a condensation nucleus. If you divide one litre water from melted snow by the number of melted snow crystals that formed this liter, you will find about 5* 10^9 crystals and as much condensation nuclei. This means that between 4 and 120 bacteria cannot play a significant role, even if they really were in the clouds.
I find the entire issue of the ubiquitous presence of bacteria in the world around us fascinating. Many areas that were thought to be bereft of life, such as the deep oceans and deep rock formations have revealed living organisms, as has the antarctic ice. Although in this case, this is not the first time that bacteria have been identified as a source of nucleation for precipitation. Franc & DeMott provided experimental evidence of this and proposed a similar hypothesis back in 1998. Ref: Franc, G.D., and P.J. DeMott, 1998: Cloud Activation Characteristics of Airborne Erwinia carotovora Cells. J. Appl. Meteor., 37, 1293–1300. http://ams.allenpress.com/perlserv/?SESSID=edc638cfc13aabcd6d5336f3a60b7d4a&request=res-loc&uri=urn%3Aap%3Apdf%3Adoi%3A10.1175%2F1520-0450%281998%29037%3C1293%3ACACOAE%3E2.0.CO%3B2
Thoughts about the role of micro-organisms in the formation of atmospheric ice date back to the meteorologist Soulange (1957: Ann. Geophys. 13: 103-134) who observed bacterial-like particles in the center of ice crystals. The discovery of the ice nucleation activity of micro-organisms about 20 years later (1974: Maki et al, Applied Microbiol. 28:456-459; 1976: Arny et al Nature 262:282-284 ) got us thinking again about this idea in more specific terms, especially as the first-identified ice nucleation active micro-organism was the ubiquitous plant epiphyte and pathogen Pseudomonas syringae. In the early 1980's David Sands suspected air-borne sources of inoculum of this pathogen as the origin of a bacterial blight of wheat in fields in Montana. He was aware of the considerable upward fluxes of this bacterium into the atmosphere being measured by other research groups (1982: Lindemann et al. Appl. Environ. Microbiol. 44:1059-1063). The samples he collected in clouds at about 2 km altitude confirmed the presence of ice nucleating strains of this bacterium and led him to propose a "bioprecipitation" cycle whereby P. syringae rides air currents and rainfall to circulate between plant surfaces and clouds, and participates in the formation of rain to assure the cycle (1982: Sands et al J. Hungarian Meteorol. Serv. 86:148-152). The cycle proposed by Franc and DeMott, as described in the comment of Darryl Luscombe on 29 Feb, also offered hypotheses of a similar cycle for another plant pathogen. In this latter case the bacterium is not ice nucleation active but may contribute to cloud formation by aiding the condensation of water rather than its freezing. Although published in 1998, the research by Franc was part of his PhD thesis work from the late 1980's. At the time of Sands' and Franc's work, interest in microbial ice nucleation was focused intensively on processes involved in frost damage to plants, on the molecular characterization of the ice nucleating protein and associated gene, and on industrial and biotech applications of this protein. Today there is a very new and fresh perspective on the possible role that micro-organisms can play in all sorts of environmental processes. This is bolstered by our ability to trace them and reveal their in situ activities via new and very refined technologies. This is also bolstered by our understanding that micro-organisms are everywhere – yes even in clouds. The pioneering work of Sattler et al (2001: Geophys. Res. Lett. 28:239-242), Bauer et al (2003: J. Geophys. Res. 108:AAC2/1-AAC2/5) and Amato et al (2007: FEMS Microbiol. Ecol. 59:242-254), for example, on the abundance and possible metabolic activities of micro-organisms in clouds leaves no doubt that clouds are not sterile – having as many as 10e5 microbial cells/mL of water. P. syringae was among the bacteria isolated from clouds by Amato and colleagues. The temperatures at which bacteria can be active as ice nuclei are the warmest of all the naturally-occurring ice nuclei in the atmosphere (up to ca. -2°C). This leads to questions about their possible activity when other ice nucleators – although more abundant – might be less efficient. Studies of the physics of ice formation in clouds have, for the most part, ignored the role of biological particles. This is due in part to the fact that current technologies to measure ice nucleation active particles in the atmosphere, prior to the publication of our results, could not distinguish biological particles per se. Theoretical considerations of the possible role of biological ice nucleators in precipitation might rule out their importance. But their role has not been ruled out by experimental evidence. It might turn out that biological ice nucleators have no significant impact on quantities of precipitation but that they might simply exploit freezing processes in the atmosphere to assure the survival of some cells. Technologies are now available to pursue these questions. We have recently called for renewed interest in the role of micro-organisms in rainfall, in the full spectrum of roles of biological particles in atmospheric processes, and in further consideration of the atmosphere as a microbial habitat. This call is fully detailed in: Morris et al (2008: BIogeosciences Discuss. 5:191-212), Deguillaume et al (2008 Biogeosciences Discuss. 5:841–870) and Moehler et al (2007: Biogeosciences 4:1059-1071).
We were surprised to read in the 28 February 2008 naturenews section the statement “The effect of biological ‘ice nucleators’ on precipitation has been a mystery, not least because no one has yet been able to detect them in clouds.†Actually, biological ice nucleators as well as their impact on atmospheric phenomena including cloud-to-ground lightning and sprites, have been discussed by us following identification of biological ice nucleators, including bacteria, spores and fungi in giant hailstones with diameters reaching 3.5 cm. Hailstones of that size are formed in supercell cumulonimbus clouds, in the upper level of the cloud at the top of the anvil near the tropopause, at 8-12 km. Therefore the probability is high that the bioaerosols contained in the hydrometeors were collected mostly in the upper region of the cloud (1). This report must have escaped the author’s notice. The expectation that bioaerosols are good ice nucleators can be easily justified: biological debris, spores, fungi and bacteria are normally hydrophilic, have a rough surface structure and are insolubility in water. Presumably, their capability to bind water vapour is superior to hydrophobic particles of the same size. Even in ambient conditions many surfaces are masked with a crystalline, nanoscopic water layer (2). Such structures are expected to further encourage ice crystal formation. Therefore the role of bioaerosols as ice nucleators could be very significant. Andrei P. Sommer and Dan Zhu, Institute of Micro and Nanomaterials, University of Ulm, 89081 Ulm, Germany References: 1. Sommer, A. P. Electrification vs Crystallization: Principles to Monitor Nanoaerosols in Clouds Cryst. Growth Des. 6, 749-754 (2006). 2. Sommer, A. P., Caron, A. & Fecht, H. J. Tuning Nanoscopic Water Layers on Hydrophobic and Hydrophilic Surfaces with Laser Light. Langmuir, 24, 635-636 (2008).
Actually, Christner et al. in their brevia paper, "Ubiquity of Biological Ice Nucleators in Snowfall" (Science, 29 February 2008, p. 1214), have clearly overlooked the seminal work of A.G. Lochhead in this research area. While Christner et al. have made evident the role of bacteria in the formation of snow, however, it was Lochhead who showed for the first time way back in 1938 (1) that a fresh batch of snow, which fell over Ottawa district, contained Bacillus megatherium as the most abundant species, along with B. vulgatus, B. mesentericus, B. mycoides, B. simplex, B. cereus and Bacillus sp. Sincerely yours, Dr. Upinder Fotadar Research Scientist Basic Sciences Division New York University 345 East 24th, Street New York, NY 10010 Phone: 212-998-9578 E-mail: uf4@nyu.edu References 1) A.G. Lochhead, Science 87: 487 (1938).