Exploring opportunities for bioprecipitation
By Carolyn King
Results from a flurry of studies over the past decade indicate certain plant-associated bacteria and other biological particles can play a part in ice formation in clouds, leading to precipitation. One possible implication is that in the future, farmers might grow specific crops to produce those particles in order to increase rainfall in drought-affected areas – although many questions would need to be answered before this could become a reality.
By Carolyn King
Land use, including vegetation type, is known to influence cloud formation and precipitation because it affects things like the amount of water vapour released through transpiration. But the idea that biological aerosols – particles so small that they remain suspended in the air – might be an important factor in precipitation is relatively new.
The concept of bioprecipitation was proposed in 1982 by David Sands, who is a professor of plant sciences and plant pathology at Montana State University. At the time, Sands was working on an outbreak of a wheat disease caused by the bacterium Pseudomonas syringae. “He knew this bacterium is seedborne, so they disinfected all the seeds before planting. But within three weeks the whole field was diseased with this pathogen, so he wondered where the disease was coming from,” explains Cindy Morris, a research director in plant pathology at the French National Agricultural Research Institute (INRA) and an affiliate professor at Montana State University.
“He went up in an airplane with a colleague and put his hand out of the plane to collect samples in petri dishes as they flew over the field. He isolated Pseudomonas syringae from samples of ice crystals collected in the clouds. He also tested their ice nucleation activity.”
As a result, Sands proposed a novel idea: a bioprecipitation cycle that starts when the bacteria are picked up off the surface of plants by the wind and carried up into the clouds as aerosols. There, the bacteria facilitate formation of ice crystals, which help in bringing together many tiny water droplets so they are heavy enough to fall. The resulting precipitation carries the bacteria back to the ground and promotes the growth of plants and the bacteria, starting the cycle again.
“For those living micro-organisms that survive this process – because many of them do come down alive – it’s a free trip to new places to colonize, so it’s a way to expand their geographic area,” Morris says.
Initially people dismissed the bioprecipitation concept, but about two decades later, research on the topic really started to take off. “There is one very simple reason for that,” Morris says. “In 2002, David Sands and I met at a conference – we’d known each other but we didn’t work together – and we decided to bring the subject back on the map. Then in 2006, I was able to get money for the first conference, which we called ‘Microbiological Meteorology’.”
“With funding from the European Science Foundation, I had the means to bring in 25 people from different geographic origins and different disciplines. I brought in atmospheric physicists, meteorologists, modellers, microbiologists, etcetera, and we got together for a four-day meeting at the French National Agricultural Research Institute’s centre in Avignon. Since then, interest has snowballed.”
A myriad of interdisciplinary, international meetings have followed that initial meeting. The core group has expanded to include about 140 researchers and they have produced diverse scientific publications.
For example, the researchers have found ice-nucleating microbes in precipitation samples collected in many countries around the world and in airborne sampling. They have simulated how biological ice nucleators would influence cloud processes. They have identified more bacterial species and other biological particles that are able to cause ice nucleation, such as pollen, algae, fungi, lichen and leaf litter fragments.
However, as Morris notes, mineral and biological aerosols are always present in the atmosphere; the types and amounts are constantly changing, so it’s very difficult to determine if and when bioaerosols occur in sufficient numbers to have a significant effect on precipitation.
More research is needed to figure out how common biological ice nucleators are in the atmosphere, what their physical, chemical and biological properties are, and how much of an influence they have on the amount and location of precipitation under different conditions.
For instance, bioaerosols could be important in promoting rainfall in situations where cloud temperatures are relatively warm because biological ice nucleators cause ice to form at much warmer temperatures than most mineral aerosol particles, like dust and soot.
One area Morris and her colleagues are currently researching is a surprising feedback effect in bioprecipitation: it appears rain may sometimes cause more rain.
Field observations and simulations indicate that in some situations a rainstorm can cause a rapid increase in ice-nucleating particles in the air. The increase occurs immediately and can last up to about 20 days after the storm, so those particles could go up in the atmosphere and contribute to rainfall again. The processes involved aren’t known, although the researchers have proposed some hypotheses.
In one of their latest papers, the researchers calculated an index of rainfall feedback using 100 years of daily rainfall data for 1,250 sites in 17 of the western U.S. states, and mapped the results. Their analysis showed the patterns of rainfall feedback were affected by the site’s location and the season.
Morris says, “The type of questions you ask about how land use would affect rainfall in the Central Valley of California are not the same kind of questions you would ask in the Northwest U.S. because they have completely different responses to rainfall. Urban centres are going to behave differently than mountainous areas, are going to behave differently than pine forests, are going to behave differently than flat land with corn fields.”
“So, what we need to do now is to create site-specific hypotheses and think about what is happening in a very local way.”
This type of research might help improve the accuracy of precipitation forecasts and enhance understanding of how land use influences precipitation.
Growing wheat and rain?
Some researchers are exploring questions around the possibility of using crops to strategically influence precipitation. For example, Sands and Morris recently worked with the International Center for Agricultural Research in the Dry Areas (ICARDA) in the Middle East to examine the possibility of selecting wheat lines as sources of ice-nucleating bacteria, in this case Pseudomonas syringae.
Not all strains of Pseudomonas syringae are ice-nucleation active and not all strains cause wheat disease. The researchers compared the ability of 25 different wheat lines to host pathogenic and non-pathogenic strains that were all ice-nucleation active. Twelve of those wheat lines naturally harboured the bacterium in the field and some harboured non-pathogenic strains, so it may be possible to select wheat lines that host non-pathogenic strains of ice-nucleating Pseudomonas syringae.
The researchers also inoculated the wheat seeds with different strains of Pseudomonas syringae to see if the bacteria could be transmitted from the seed to the plant’s leaves and eventually to its seeds. Depending on the wheat line and the bacterial strain, the inoculated seed sometimes transmitted the bacteria to the next-generation seeds. This suggests it might be possible to ensure the wheat plants would have strains of Pseudomonas syringae that are non-pathogenic and also good ice nucleators.
The results suggest wheat crops could potentially be grown for both grain yield and rain yield.
Many other crop production questions would also need to be answered to make it practical for farmers to use crops to “grow rain,” like which crops and which production practices offer the best options – and yield the best results.
Society would also need answers for a tangle of scientific, economic and policy questions. “I think the next think tank we need to have is to work on the question: if we knew the mechanisms of what is going on, what could we actually do with that information? That would help scientists think about the tools we might need if we are going to put policies into place,” Morris says. “For instance, let’s say you could pay farmers to grow specific crops that generated bioaerosols, but then those crops failed because some of these organisms are plant pathogens. What then would be the indicators that the farmers succeeded?”
In that situation, crop yield wouldn’t be a good indicator because the crop failed. Perhaps an indicator related to extra rainfall could be used, but where would the region receiving extra rainfall likely be and how big would it be? And then there are questions like how much would the farmers be paid and who would pay? How much is rainfall worth?