Abstract
Summary Community structures of active bacterial populations were investigated along a vertical redox profile in coastal Baltic Sea sediments by terminal‐restriction fragment length polymorphism (T‐RFLP) and clone library analysis. According to correspondence analysis of T‐RFLP results and sequencing of cloned 16S rRNA genes, the microbial community structures at three redox depths (179, −64 and −337 mV) differed significantly. The bacterial communities in the community DNA differed from those in bromodeoxyuridine (BrdU)‐labelled DNA, indicating that the growing members of the community that incorporated BrdU were not necessarily the most dominant members. The structures of the actively growing bacterial communities were most strongly correlated to organic carbon followed by total nitrogen and redox potentials. Bacterial identification by sequencing of 16S rRNA genes from clones of BrdU‐labelled DNA and DNA from reverse transcription polymerase chain reaction showed that bacterial taxa involved in nitrogen and sulfur cycling were metabolically active along the redox profiles. Several sequences had low similarities to previously detected sequences, indicating that novel lineages of bacteria are present in Baltic Sea sediments. Also, a high number of different 16S rRNA gene sequences representing different phyla were detected at all sampling depths.