Characterization of N2-Producing Bacterial Communities from Arctic Sediments

Global climate change is expected to alter biogeochemical cycles in polar sedimentary environments, largely through changes in temperature and organic matter delivery to sediments. Knowledge of the diversity and physiology of nitrogen-transforming microbial communities is crucial to our understanding of benthic-pelagic coupling and how the nitrogen cycle may respond to climate change in polar sediments. The objective of this study was to characterize the microbial communities responsible for N2 production in polar marine sediments using a combination of cultivation-dependent and cultivation-independent techniques. Microbial communities were investigated at three arctic fjord sites (Svalbard, Norway) which varied according to porosity and organic matter content. DNA fingerprinting by Terminal Restriction Fragment Length Polymorphism (TRFLP) profiling was used in conjunction with clone library construction, targeted to either 16S rRNA gene or nosZ gene sequences, to describe community diversity and identify the most abundant taxa. Gene sequences were PCR amplified with three primer sets including general bacterial primers targeted to the 16S rRNA gene (overall bacterial diversity), primers targeted to the 16S rRNA gene that are specific to Planctomycetes (the phylum of anammox bacteria), and primers targeted to the nitrous oxide reductase (nosZ) gene (a marker for denitrifying bacteria). Clone library results indicated that the overall bacterial diversity was dominated by the classes Deltaproteobacteria, Gammaproteobacteria, Flavobacteria, and Sphingobacteria. The dominant 16S rRNA gene TRLP peaks (identified by in silico digest of sequences from clone libraries) included members of the Flavobacteria, Gammaproteobacteria, Deltaproteobacteria, Alphaproteobacteria, and Planctomycetes. Denitrifying bacteria were further characterized by enrichment, isolation, and physiological screening of pure cultures. A total of 17 isolates, comprising 5 genera and 3 phyla, were obtained. Isolates were from the genera Arcobacter, Herminiimonas, Pseudomonas, Psychromonas, and Shewanella. Six representative isolates were grown at 4 temperatures (1.5°C, 5°C, 15°C, 25°C) and grew optimally at 15°C, indicating that they were all psychrophilic.