Understanding the nature, sources, and geographic and seasonal distribution of atmospheric ice nucleating particles (INPs), i.e., particles that promote freezing of cloud droplets, is critical to improving regional and global models of precipitation formation in the atmosphere. Recent work has shown that the presence of microbiota and organic matter in both soil and sea spray aerosol particles is frequently associated with a stronger propensity to initiate freezing. However, the relationship between these biogenic INPs in the atmosphere and in source materials such as soil, freshwater and seawater is not well-understood; in particular, the assumption that both the microbial community composition and the INP concentration are the same in both the source material and the aerosol, has not been tested.
We propose to use a suite of physical, chemical and biological capabilities to characterize the biological aerosol particles, and other associated aerosols, in ambient air samples, collected during different seasons and in different locations (continental and marine). We will also attempt to relate the microbial communities in these samples to those in the source material, and their associated properties. The proposed work will utilize EMSL's unique capabilities to characterize the particles' physical and chemical properties, including their ice nucleation activity, using a suite of EMSL capabilities: Ice Nucleation Chamber, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), high-resolution transmission electron microscopy (HR-TEM), and Raman spectroscopy, and Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). The chemical and physical data will be combined with biological information from metagenomic and metatranscriptomic sequencing at JGI from simultaneously collected samples.
This set of experiments will generate a unique, rich dataset that we will probe to answer several science questions. First, we will use metagenomics data to determine whether the microbial communities in aerosol differ significantly in their species composition and relative abundances from the microbial communities in the associated source material. We will then examine whether any observed differences in the microbial community composition of the aerosol and the source material can be associated with physical or chemical properties of the source material, such as soil grain minerology or size, which will be observed through SEM/EDS, HR-TEM and Raman measurements at EMSL. We will also investigate whether any such differences can be associated with biological functionality of those taxa, such as the production of adhesive compounds, which will be observed by applying transcriptomics to the source material (soil or seawater samples) at JGI. We will use FTICR MS to characterize the soil organic matter (Tfaily et al., 2015), and explore whether the molecular composition of the soil organic matter can be associated with adhesive compounds, and/or with the biological functionality of the microbiome as observed by metatranscriptomics. We will also measure the concentration and characteristics of INPs in both the source material and aerosol samples, and from metagenomic data we will determine the prevalence of known and possibly also putative microbial taxa that express proteins or other macromolecules that enhance the freezing of cloud droplets. We will determine whether they are proportionally more or less likely to be represented in the aerosol, compared to the source material.
The results of this work will inform understanding of the sources of biological aerosol particles that may play a role in cloud and precipitation processes in the atmosphere, and lead to improvements in models that represent the geographic and seasonal distribution of ice-nucleating particles.