Diazotroph diversity in the Arctic Ocean

Anthropogenic causes of climate change are contributing to enhanced greenhouse gases. As a result of climate change Artic sea ice is melting at alarming rates and this decline in sea ice cover has enhanced phytoplankton primary production due to the water being exposed to higher light intensities.  Nitrogen availability is usually a limiting factor of primary production in these areas, however recent understanding indicate there is an imbalance between the supply, demand and export of nitrogen in the Arctic Ocean, suggesting nitrogen fixation may be involved however little is known about the presence and role of diazotrophs in these regions.

Fernández-Méndez et al. aimed to assess the significance of nitrogen fixing organisms and their contribution to the nitrogen budget in the Arctic Ocean. They used PCR to carry out targeted analysis of nifH gene, which is responsible for the coding of the nitrogenase enzyme involved in nitrogen fixation, in 26 samples of melt ponds, sea ice and surface waters of the Central Arctic Ocean in summer 2012. Bacteria community was then identified through the analysis of the 16s rRNA region.. The effect of temperature and nutrient availability on the role and occurrence of diazotrophs was also investigated.

Results showed that of the total of 529 sequences retrieved the sequences were clustered into 43 clusters at 92% amino acid similarity. Oragnisms detected and identified within samples included both cyanobacterial and non-cyanobacterial phylotypes.  Sequences identified were associated with Proteobacteria, Firmicutes, Cyanobacteria, members of the Archaea, putative anaerobes including sulfate reducing genera of the Deltaproteobacteria, genera such as Clostridium and several uncultivated microorganisms. nifH paralogs that are thought to function in metabolic processes other than nitrogen fixation were also identified from sea ice and melt pond samples. Vast majority of sequences retrieved across the different environments of the Central Arctic Ocean belonged to non-cyanobacterial diazotrophs. Temperature gradients between sample sites did seem to have an effect on the diazotroph communities due to the optimum temperatures of different organisms. Nutrient availability also influenced the assemblage and occurrence of diazotrophs as cyanobacterial organisms were associated with high phosphate environments, therefore affecting their abundance and distribution.

Overall I believe this to be an important paper because it highlights the potential for nitrogen fixation in Arctic environments where they have not been previously detected. Understanding these diazotrophs role and distribution in the environment is vital to dictate the implications and relevance to climate change and the influence it has on the Arctic’s primary production. I believe this paper to be a key stone paper for future research of nitrogen fixers in polar environments and to assess their importance and contribution to the ecosystem.

Fernández-Méndez, M., Turk-Kubo, K., Buttigieg, P., Rapp, J., Krumpen, T., Zehr, J. and Boetius, A. (2016). Diazotroph Diversity in the Sea Ice, Melt Ponds, and Surface Waters of the Eurasian Basin of the Central Arctic Ocean. Frontiers in Microbiology, 7.