(Top) DGGE fingerprints (in triplicate) of PCR-amplified 16S rRNA gene fragments obtained from NA and control biofilms before (4 months) and after (4 days) exchange using bacterial primers GM5F and 907RC. Arrows indicate the visible dominant bands. (Bottom) 16S rRNA gene cloning showing differences in the bacterial community composition in NA and control biofilms before and after exchange to opposite NO3− conditions. The relative abundance (percent) of sequences in each clone library (i.e., treatment) with the closest relative and the taxonomic affiliation is given. α, Alphaproteobacteria; γ, Gammaproteobacteria; δ, Deltabateria; ɛ, Epsilonbacteria; Alterom, Alteromonas sp.; Arcob, Arcobacter sp.; ARK, environmental Artic pack ice clones; CFB, Cytophaga/Flavobacteria/Bacteriodes; Colwell, Colwellia sp.; Methyloph, Methylophaga sp.; Roseob, Roseobacter sp.; Oleispir, Oleispira sp.; uncult, uncultured; mar, marine.