Too cold to fix?

Biological N₂ fixation is an important source of nitrogen (N) in tropical and subtropical oceans relatively poor in nutrients. Previous research on N₂ fixation has focused on the well-known cyanobacterium Trichodesmium and the diatom symbiont Richelia, which are found in warm waters (>20 °c) with low concentrations of fixed N. However, not a lot of research has been done in waters with lower temperatures and higher concentrations of fixed inorganic nitrogen. This paper by Harding et al., (2018) looked at the cyanobacterial symbiont UCYN-A and how it fixed N₂ in these conditions.

The nifH gene, which is a widely-used proxy for N2-fixing microorganisms was sequenced to identify UCYN-A haptophytes. CARD-FISH and nanoscale secondary ion mass spectrometry was used to measure 13N2 uptake into unicellular cyanobacterial symbionts. It showed that the UCYN-A symbiosis was actively fixing N2. Results showed that low temperatures in polar waters does not limit the distribution or activity of N2-fixing cyanobacteria. 

This paper is interesting as it challenges commonly held ideas about global marine N2 fixation. This research will be useful in understanding and predicting global patterns of N2 fixation. One limitation of this study is that more research is needed to determine the quantitative significance of N₂ fixation by UCYN-A.

Harding, K., Turk-Kubo, K., Sipler, R., Mills, M., Bronk, D. and Zehr, J. (2018). Symbiotic unicellular cyanobacteria fix nitrogen in the Arctic Ocean. Proceedings of the National Academy of Sciences, 115(52), pp.13371-13375.