Variability in Bacteria and Virus‐Like Particle Abundances During Purging of Unconfined Aquifers

Abstract

Standard methodologies for sampling the physicochemical conditions of groundwater recommend purging a bore for three bore volumes to avoid sampling the stagnant water within a bore and instead gain samples representative of the aquifer. However, there are currently no methodological standards addressing the amount of purging required to gain representative biological samples to assess groundwater bacterial and viral abundances. The objective of this study was to examine how bacterial and viral abundances change during the purging of bore volumes. Six bores infiltrating into unconfined aquifers were pumped for five or six bore volumes each and bacteria and virus-like particles (VLPs) were enumerated from each bore volume using flow cytometry. In examination of the individual bores trends in bacterial abundances were observed to increase, decrease, or remain constant with each purged bore volume. Furthermore, triplicates taken at each bore volume indicated substantial variations in VLP and bacterial abundances that are often larger than the differences between bore volumes. This indicates a high level of small scale heterogeneity in microbial community abundance in groundwater samples, and we suggest that this may be an intrinsic feature of bore biology. The heterogeneity observed may be driven by bottom up processes (variability in the distribution of organic and inorganic nutrients), top-down processes (grazing and viral lysis), physical heterogeneities in the bore, or technical artifacts associated with the purging process. We suggest that a more detailed understanding of the ecology underpinning this variability is required to adequately describe the microbiological characteristics of groundwater ecosystems.

Putative Effect of Aquifer Recharge on the Abundance and Taxonomic Composition of Endemic Microbial Communities

“…These counts are consistent with commonly reported microbial cell counts of 10 3 −10 8 cells ml -1 in groundwater [25,44–48]. Following the addition of synthetic wastewater, mean bacterial and VLP abundances increased to 10 4 –10 8 cells ml -1 (Fig 5).…”

Deciphering the virus‐to‐prokaryote ratio (VPR): insights into virus–host relationships in a variety of ecosystems

“…The reasons for these low microbial and viral numbers are yet to be elucidated, but may be linked to the oligotrophic nature of groundwater (Wilhartitz et al, 2013). Data from Australian confined and unconfined aquifers suggest, nonetheless, small-scale heterogeneity in microbial communities, both prokaryotic and viral, which in turn suggests active microbial dynamics rather than inactive homogeneous suspensions of cells (Roudnew et al, 2013(Roudnew et al, , 2014.…”

Wells provide a distorted view of life in the aquifer: implications for sampling, monitoring and assessment of groundwater ecosystems

“…Purging wells is necessary to gain representative biological samples20212526272829 because wells represent an artificial environment that is enriched in oxygen and organic matter, may have some light, and contains a large volume of open water compared to that of the aquifer25. As such, wells often contain higher stygofaunal abundances than aquifers252730.…”

“…Previous studies have also shown differences in microbial assemblages between well and aquifer samples using community fingerprinting techniques (e.g. TRFLP)27 and flow cytometry28. Whilst such studies have shown differences in the structure and abundance of microbial communities, the lack of taxonomic resolution in these methods means that the implications of purging (or not) on assessments of biological structure and potential ecosystem function in groundwaters have not yet been adequately explored.…”