Functional Diversity of Arthrobacter Strains Across the Dynamic Capillary Fringe and Adjacent Sediment Zones

A sediment zone at the interface of the water-saturated aquifer and the unsaturated vadose zone often referred to as the capillary fringe undergoes steep gradients in hydraulic activity, geochemical conditions, and biological activity. Structure and function of microbial communities are known to be influenced by these environmental factors. However, little is known about the functional adaptation of the microorganisms in and adjacent to the highly dynamic capillary fringe at a single geographic coordinate, in response to changing geochemistry. To begin to delve into this complex unknown, we isolated eight distinct bacterial strains belonging to the genus Arthrobacter from different depths of a single sediment core and adjacent groundwater well at the Oak Ridge Reservation Field Research Center in Oak Ridge, TN. We present the laboratory characterization of the eight strains as well as an in-depth genomic comparison to demonstrate the plasticity of metabolic function in the vadose, capillary fringe, and saturated zones as well as groundwater. We also utilized geochemical data to correlate metabolic and genomic traits with corresponding environmental parameters.

Our results indicate that spatial adaptation of Arthrobacter have occurred, correlating with the environmental conditions and carbon sources associated with different sediment depths. For example, isolates from the vadose zone and groundwater samples possessed more genes related to osmoprotectant synthesis and transport as well as a greater number of antibiotic synthesis and resistance genes. Isolates from the capillary fringe and saturated zones exhibited genes for contaminant-degrading metabolic pathways. The results provide insight into selective pressures acting in the various sediment zones as well as potential differences in transience of sediment versus groundwater microorganisms.