Bacterial and virus-like particle abundances in purged and unpurged groundwater depth profiles

Bacteria and viruses are ubiquitous in subterranean aquatic habitats. Bacterial abundance is known to vary with depth in aquifers; however, whether viral abundance varies with depth is less well known. Here we use flow cytometry (FCM) to enumerate bacteria and virus-like particles (VLP) from groundwater depth profiles. Groundwater samples were obtained from a set of nested piezometers from depths of 15, 30, 45, 60, 80, and 90m and bacteria and VLP abundances were determined in purged aquifer water and unpurged water at each slot depth. Mean bacterial abundance (cells/mL) was not significantly different in unpurged water (3.2×10⁵) compared to purged water (1.4×10⁵); however, mean VLP abundance (particles/mL) was significantly greater in unpurged water (4.4×10⁵) compared to purged water (2.3×10⁵). Purged water was used to investigate the aquifer depth profile and bacterial and VLP abundances were observed to vary significantly between depths. The virus-bacteria ratio was determined and was observed to steadily increase with depth. Overall, our data indicate the dynamic nature of bacterial and viral abundances in subsurface environments which should be considered when designing groundwater microbial sampling methodologies. Ground Water Monitoring & Remediation.