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CURRENT RESEARCH

Biogeography of microbial communities

Despite their ubiquity, the biogeographical patterns exhibited by soil microorganisms have received little attention. While many studies have examined microbial community structure at local scales, we do not adequately understand the extrinsic and intrinsic factors that regulate the structure of soil microbial communities across regional and continental scales. I am presently involved in a number of research projects examining this topic. In a recently completed project, we compared bacterial communities from approximately 100 soil samples collected from across North and South America. In addition, we have done more focused work comparing archaeal, fungal, bacterial, and viral diversity across a handful of soils. Ongoing work examines the biogeographical patterns exhibited by stream bacteria, soil ammonia-oxidizers, soil acidobacteria, and bacteria that inhabit leaf surfaces. We are currently employing a novel barcoded pyrosequencing technique that allows us to rapidly survey hundreds of bacterial communities at an unprecedented level of detail.

Airborne microorganisms:

Microorganisms are surprisingly abundant in the lower layers of the Earth's atmosphere with numerous disease-causing and beneficial microorganisms capable of being transported long distances in the atmosphere. The nature of these airborne microorganisms is likely to be highly relevant to human health (particularly with regards to allergies and asthma) and the health of agricultural crops. We are working on a series of projects examining the spatial and temporal variability in airborne microbial populations across the Colorado Front Range. We are particularly interested in examining how land-use type influences the diversity and abundance of microorganisms found in the atmosphere. A related study is examining the role of airborne bacteria in atmospheric ice nucleation events.

Linking microbial communities and processes across a land-use gradient:

The role that soils play in mediating global biogeochemical processes is a significant area of uncertainty in ecosystem ecology. One of the main reasons for this uncertainty is that we have a limited understanding of belowground microbial community structure and how this structure is linked to soil processes. Building upon established theory in soil microbial ecology and ecosystem ecology, we predict that the structure of belowground microbial communities will be a key driver of carbon and nutrient dynamics in terrestrial ecosystems. We are combining DNA-based techniques of microbial community analysis with stable isotope tracer techniques to track the flow and fate of plant-derived carbon through the soil food webs of a series of well-characterized land-use types found in the Calhoun Experimental Forest, SC. By doing so, we expect to advance our conceptual and practical understanding of the fundamental linkages between soil microbial community structure and ecosystem-level carbon and nutrient dynamics.

Although only a handful of studies have directly examined the full extent of microbial diversity on leaf surfaces, these studies have consistently shown that leaves provide a habitat for many hundreds of bacterial and fungal species, the majority of which are previously undescribed. At present, the functional role of these microorganisms is not well understood. It is likely that leaf-surface inhabiting microorganisms can affect their host plants both negatively (by damaging leaves and altering leaf physiology) and positively (by preventing infections from pathogenic microorganisms and fixing nitrogen). We are examining inter-species and intra-species (temporal and spatial) variability in the microbial communities inhabiting trees and shrubs in the Colorado Front Range and across an elevation gradient in eastern Peru.

The flux of VOCs from terrestrial sources to the atmosphere has an important impact on atmospheric chemistry at local, regional, and global scales. For this reason, many studies have focused on the production of VOCs by plants, assuming that plants are the largest terrestrial source of biogenic VOCs. Surprisingly, only a handful of studies have looked at the production of VOCs by soil microbes despite a number lines of evidence suggesting that, like plants, microbes may also be an important terrestrial source. We are currently examining soil VOC production across a wide range of soil and litter types with the goal of understanding how these VOCs may regulate soil microbial communities and processes. In addition, we are assessing VOC production during litter decomposition and the influence of N fertilization on the quantities and types of VOCs released.