GROUNDWATER FILTRATES AS SAMPLES OF THE SUBSURFACE MICROBIAL COMMUNITY

The microbial community in the subsurface exerts a direct control on the chemical composition of groundwater; hence, there is considerable scientific and practical interest in characterizing the community structure there. The community is most readily sampled by filtering planktonic microbes from pumped groundwater, a technique that is rapid and inexpensive. The filtrate may not be representative, however, because perhaps 95 – 99% of the microbes in an aquifer are at any given time found attached to the aquifer matrix. Alternative methods such as aseptic split-spoon drilling and down-well aquifer sampling capture the attached population, but are expensive and time-consuming.

To help gauge the extent to which planktonic populations represent the subsurface community, we compared attached and unattached bacteria from the Mahomet aquifer of east-central Illinois. We suspended mesh bags filled with sterilized Mahomet sand in wells across the regional aquifer and allowed them to be colonized by ambient bacteria. After several months, we retrieved the bags and analyzed the 16S rRNA genes of bacteria attached to the sand, along with bacteria filtered directly from groundwater pumped from the wells. We amplified genes from the samples using PCR and created a genetic profile for each using terminal restriction fragment length polymorphism (T-RFLP). We calculated a Bray-Curtis similarity coefficient for the T RFLP profiles and compared them graphically through non-metric multidimensional scaling.

The comparisons reveal similarities as well as differences between attached and unattached populations. The T-RFLP profiles of each population cluster separately. Many peaks are found in both the attached and unattached populations of a particular well, but important peaks from the attached population are absent in the filtrate from a well, and vice-versa. Notable similarities exist between attached and unattached populations of a particular well, but important strains of bacteria may be missed in studies in which only water, or only sediment has been sampled. These results reinforce a common opinion among aquifer microbiologists that samples of either planktonic or attached microbes reflect the overall community in no more than a qualitative sense.