Groundwater samples do not reflect bacterial densities and activity in subsurface systems

Total cell numbers, abundance of respiring bacteria and [3H]thymidine and [14C]leucine incorporation rates were investigated in four groundwater wells of low nutrient content. Total cell numbers in the pumped groundwater were low (14 × 103 to 279 × 103 cells mL−1), and [3Hthymidine and [14C]leucine incorporation rates were, with one exception, below the detection limit. Therefore we exposed sediments in situ for 2 months which allowed us to determine bacterial numbers and incorporation rates of labeled substrates by bacteria attached to sediment particles. The two habitats differed considerably in all bacterial parameters both in magnitude and seasonal trends. Total bacterial numbers of sandy sediments (52.1 ± 21.3 × 106 cells cm−3) corresponded in average to 663 cm3 of pumped groundwater (78.5 ± 61.5 × 106 cells L−1). For the fraction of respiring bacteria this ratio was on average 3032 cm3 (sediments: 10.3 ± 5.4 × 106 respiring cells cm−3; groundwater: 3.39 ± 6.01 × 106 respiring cells L−1). The percentage of respiring bacteria in sandy sediments was between 6.0 and 41.4% (average 19.8) compared to 1.0 to 24.9% (average 5.23) in the pumped groundwater. Our results stress the importance of studying the microbial communities attached to sediment, as pumped groundwater samples may not be representative for the real structure and dynamics of microbial assemblages in subsurface environments.