Depth distribution of active bacteria and bacterial activity in lake sediment

The bacterial activity in sediments is often low considering the generally high bacterial abundance. Still, a large fraction of bacteria have been found active even in deep sediments. These findings suggest that sediment bacteria have comparatively low cell‐specific production. We studied bacterial activity and the active fraction of bacteria in a lake sediment profile. Bacterial production and metabolism were measured by thymidine and leucine incorporation and by microcalorimetry. In addition to counts of total bacteria, we estimated the nucleoid‐containing fraction of the bacteria by adding a destaining step to the DAPI staining method, and the live fraction using the Live/Dead Bac light bacterial viability kit. The bacterial activity and abundance decreased with sediment depth, while the proportion of active bacteria remained similar at all depths. Between 57 and 63% of the bacteria were scored viable, and 13–52% were scored as nucleoid‐containing cells. Consequently, there was no accumulation of dead bacterial cells in deeper sediments. Cell‐specific production of sediment bacteria may be severely underestimated if the active fraction of the sediment bacterial community is not considered during enumeration.