Nov. 20, 2008 Brent Christner, assistant professor of biological sciences at LSU, recently found evidence that bacteria and biological cells are the most efficient ice-forming catalysts in precipitation from locations around the globe. The formation of ice in clouds is important in the processes that lead to snow and rain. Ice-nucleating bacteria – which have been referred to as “rain-making bacteria” – may be significant triggers of freezing in clouds and influence the water cycle.
These findings, which take a big step toward filling the gaps in scientific understanding of ice nuclei in the atmosphere, will be published in the Proceedings of the National Academy of Sciences during the week of Nov. 17.
Christner’s team, which includes Kevin McCarter and Rongman Cai of LSU’s Department of Experimental Statistics, and collaborators at INRA in France and Montana State University, had previously demonstrated the presence of ice nucleating bacteria in precipitation. However, the source remained elusive.
“To address this, we examined the correlations between the presence of biological ice nuclei in precipitation and the composition of aerosols co-deposited in the precipitation,” said Christner. The chemical composition of the aerosols revealed information on their source and the potential environments from which the biological ice nuclei could have originated.
“Our models can accurately predict the concentrations of cells and biological ice nucleators in precipitation using a relatively small number of variables,” he said. “The data provides a first glimpse of the conditions that appear to favor the distribution of biological ice nuclei in the atmosphere and will be useful for predicting their abundance in other contexts.”
The study concludes that vegetation and soils are an important source of biological ice nuclei to the atmosphere at some geographical locations. Though they were detected in snow from places as remote as Antarctica, ice nucleating bacteria may also exist in the ocean, or alternatively, are able to travel large distances in the atmosphere. “The atmosphere provides an efficient conduit for microbial dispersal on a global scale,” said Christner.
Most known ice-nucleating bacteria are plant pathogens, which are basically germs that can cause disease and freezing injury in plants. According to Christner, agricultural losses from ice nucleating bacteria, such as Pseudomonas syringae, often exceed $1 billion dollars per year in the United States, so understanding their mode of dispersal is essential for mitigating their impact on crops. It is possible that dissemination through precipitation is a crucial facet of the life cycle for some plant bacteria, allowing them to colonize new hosts.
The new results provide much territory for further study. For example, many of the variables important for predicting the cell and biological ice nuclei concentration in precipitation are nutrients vital for growth and production of these ice nucleators.
“Previous work has shown that microbes can metabolize and grow in clouds, meaning that the atmosphere may represent an environment for life,” said Christner. “It is possible that cloud-borne microbes could ‘turn on’ their ice nuclear in the atmosphere and subsequently be returned to the ground in snow or rain. This is a very exciting possibility that further research could unearth.”
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