Cooperative Institute for Research in Environmental Sciences

A winning proposal for the Innovative Research Program, 2008:

Do bacteria influence the weather?: Exploring the role of bacteria in atmospheric ice formation

Investigators: Margaret Tolbert, Noah Fierer, Ray Fall (note: these 3 co-investigators will contribute equally to the proposed work)

Objectives: Bacteria are abundant throughout the atmosphere and many bacteria are potent ice nucleators capable of catalyzing ice formation at temperatures far warmer than those required for spontaneous ice formation. This may have important implications for local and regional climate patterns as ice nucleation plays a key role in regulating precipitation events. However, this topic is remarkably understudied despite anecdotal evidence suggesting that bacteria may be important nucleators in the troposphere. The proposed work addresses two key research questions: 1) How does the abundance and composition of airborne bacterial communities shift in response to changes in the atmospheric environment?, and 2) What is the contribution of bacteria to tropospheric ice formation? Addressing these research questions requires the adoption of a novel set of approaches and an interdisciplinary team (a microbiologist, an atmospheric chemist, and a biochemist) that can effectively bridge the disciplines of microbiology and atmospheric science.

Background and Importance: There is a long history of research on airborne bacteria, but we still know surprisingly little about airborne bacteria as traditional microbiological techniques only identify a small fraction of the bacterial diversity present in the atmosphere and most studies only focus on the small minority of bacteria that are known pathogens. As a result, we have a very limited understanding of the types of bacteria present in the atmosphere, how bacterial abundance and diversity are influenced by atmospheric conditions, and what role (if any) bacteria play in atmospheric processes.

We know that bacteria are abundant and ubiquitous in the atmosphere, with 105-1010 bacterial cells per m3 of air, and that bacterial diversity can vary dramatically across time and space due to changes in atmospheric conditions. Although it is commonly assumed that bacteria are simply passive participants in the atmospheric environment, having no direct influence on atmospheric conditions, recent evidence suggests that this assumption is not valid. Bacteria may not only respond to atmospheric conditions, they may also directly alter atmospheric conditions and influence local climatic conditions by functioning as ice nucleators in the atmosphere.

Ice nucleators are often essential for the freezing processes that induce cloud formation and precipitation events in the troposphere where temperatures may not be low enough to cause spontaneous freezing of supercooled liquid water. Although a wide range of organic and inorganic materials can function as ice nucleators, bacteria appear to be particularly effective ice nucleators as they are capable of initiating ice formation at very warm temperatures. Such ice-nucleating bacteria are commonly found in the troposphere and a large downward flux of ice-nucleating bacteria has been observed during precipitation events leading to some speculation that ice-nucleating bacteria can modify weather by enhancing precipitation and cloud formation under certain conditions. Not suprisingly, much of the evidence in support of this proposed “bioprecipitation” cycle is anecdotal. We do not know how ice-nucleating bacteria compare to other ice-nucleating agents present in the atmosphere with respect to their weather-altering capabilities, nor do we know how the abundances of ice-nucleating bacteria in the atmosphere vary across time and space or if this variability has any influence on weather dynamics. Although this “bioprecipitation” cycle remains to be verified, it is an intriguing avenue of research and a potentially important type of biosphere-atmosphere interaction.

Research Plan: We will conduct the air sampling at the Storm Peak Laboratory (http://stormpeak.dri.edu), a mountain-top facility in northern CO that provides an ideal location for the proposed research. It is readily accessible, aerosol and meteorological conditions in the atmosphere are measured continuously, and the lab is uniquely situated for conducting studies of the free troposphere and aerosol-cloud interactions given that it is frequently above cloud base and has a clear upwind fetch. We will visit the site at regular intervals throughout the year, sampling airborne bacteria at multiple times during each visit to allow for comparisons of free tropospheric and boundary layer air as well as comparisons of in-cloud and cloudfree air. During the course of this project, we expect to perform at least 40 sampling events, characterizing the ice-nucleating agents, measuring bacterial abundances, and examining the airborne bacterial communities present in each volume of air sampled.

During each sampling event, airborne bacteria will be collected onto sterile polycarbonate membranes by vacuum filtration with bacterial abundances measured by epifluorescence microscopy. Bacterial DNA will be extracted directly from the membranes and the composition of the airborne bacterial communities determined using molecular techniques developed in Fierer’s laboratory. Unlike conventional microbiological techniques, we can use these molecular approaches to identify potential ice-nucleating bacteria and describe the full extent of bacterial diversity present in a given volume of air without introducing the biases associated with cultivation that have severely limited previous studies on airborne bacterial diversity.

We will use two novel approaches to determine the relative importance of bacteria in promoting atmospheric ice nucleation. First we will use a series of unique experiments developed in the Tolbert laboratory to investigate the chemical properties governing ice nucleation. The planned experiments will combine optical microscopy and Raman spectroscopy to examine ice nucleation, water uptake, and the chemical properties associated with bacteria and aerosol particles collected directly from the atmosphere. Aerosol samples for the microscopy studies will be collected alongside the bacterial samples and these samples will be subjected to simulated tropospheric conditions using a Linkam heating and freezing cell. The cell will be humidified and cooled until ice nucleation is observed both visually and based on Raman spectroscopy. At the onset of the phase change, ice saturation ratios will be calculated using direct measurements of temperature and vapor pressure. Individual nucleating particles will be singled out visually and surveyed using Raman spectroscopy to determine the chemical properties of the particle responsible for ice nucleation. In addition, we will also use a modified ‘drop freeze’ assay developed in the Fall laboratory to measure the relative importance of biotic versus abiotic aerosols in regulating atmospheric ice-nucleation. Briefly, all of the particles on the polycarbonate filters will be eluted into a buffered solution and the freezing profile of this solution determined. Aliquots of the solution will be boiled and the freezing profile determined in a similar manner. Boiling should render ice-nucleating bacteria ineffective allowing us to quantify the importance of bacteria versus inorganic substances in mediating atmospheric ice-nucleation.

Why is this work innovative? This work will represent the first comprehensive study examining bacteria in the troposphere and their role as ice nucleators. In other words, this work will not only demonstrate how bacterial communities respond to atmospheric conditions, it will also allow us to determine how bacteria may be able to directly alter atmospheric conditions by functioning as ice nucleators. If bacteria do indeed have the capacity to alter precipitation patterns it would be an astounding finding as it would represent a novel mechanism by which micron-sized organisms can influence atmospheric conditions evident at kilometer-scales. Our results are likely to be of interest to researchers in a range of scientific disciplines as we will directly link the fields of atmospheric science and microbiology, two fields that are usually considered to be non-overlapping entities. This year-long project will not provide all the answers, but it will significantly improve our understanding of a unique biosphere-atmosphere interaction and will yield a solid set of results that can be leveraged to obtain additional funding.