Distribution patterns of attached and suspended bacteria in pristine and contaminated shallow aquifers studied with an in situ sediment exposure microcosm

We used specially designed microcosms filled with natural substrate to study microbial colonization in a shallow aquifer. Sterilized sediments were exposed to 3 types of groundwater varying in physical, chemical and biological characteristics: (1) pristine groundwater (site PI 92); (2) groundwater in an observation well at a pristine site (OMV 11); and (3) contaminated groundwater at a landfill site (OMV 5). The number of suspended bacteria was always highest at the landfill site (4.0 +/- 4,2 [standard deviation, SD] x 10(6) cells cm(-3)), i.e. on average 16 times higher than in the well water (2.5 +/- 3.0 x 10(5) cells cm(-3)) and 96 times higher than in the pristine groundwater (4.1 +/- 1.3 x 10(4) cells cm(-3)). Sediments in the microcosms were rapidly colonized and the total number of attached bacteria after 10 mo of exposure was highest at the landfill site (1.8 +/- 0.4 x 10(8) cells cm(-3)) followed by the sediment incubated in well water (1.5 +/- 0.5 x 10(8) cells cm(-3)) and in pristine groundwater (5.0 +/- 1.5 x 10(7) cells cm(-3)). As estimated from image analysis, attached cells from the landfill site were on average characterized by higher cell carbon contents (28 +/- 36 fgC cell(-1)) than at the well water (24 +/- 23 fgC cell(-1)) and the pristine groundwater site (21 +/- 23 fgC cell(-1)). The ratio of attached to suspended bacteria after 10 mo of exposure was highest in the microcosm incubated in pristine groundwater (1657:1) and lowest at the contaminated site (59:1). On the basis of our results we emphasize the importance of attached microbial communities in porous subsurface systems and underline the need for groundwater as well as sediment samples for a serious microbiological characterization of the subsurface. Furthermore, the ratio of attached to suspended bacteria in shallow aquifer systems is suggested to be an indicator of prevailing nutrient concentrations.