Depth distribution of active bacteria and bacterial activity in lake sediment.

Abstract 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 Baclight 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.