Group Post: The importance of symbiotic relationships in the open oceans

It has been documented that there are associations between cyanobacteria and diatoms in all oceans. The assumption is, that cyanobacteria fix nitrogen for the diatoms, yet there has been no evidence. New advances in stable-isotope tracer techniques, on a tiny scale, have allowed the authors to see the cell-to-cell transfer of nitrogen occurring in this symbiosis.

Foster et al uses epifluorescence and NanoSIMS to visualise and quantify N₂ assimilation and shows how nitrogen fixation rates might be widely underestimated. NanoSIMS uses a primary ion beam directed at a region of interest (ROI), detectors then measure the ratio of ¹⁵N:¹⁴N that is present, by picking up ion signals from the secondary beam.

Water samples were collected at 10m and incubated with ¹⁵N for a range of times: 30 minutes – 72hours. The samples were filtered, fixed and analysed with epi-fluorescence to select ROI. The ROI were around cell structures, such as the trichome of the cyanobacteria. Free-living cyanobacteria were also selected for comparison. Two symbionts were found for the diatom Hemiaulus: Richelia and Calothrix. The Richelia was positioned inside the frustule, whereas Calothrix was located on the outside.

The NanoSIM and epi-fluorescence images were overlaid to visualise the transfer of N₂ in context with the ROI. The results clearly showed the transfer of nitrogen from the cyanobacteria to the diatom.

The time taken for the nitrogen to be transferred was a lot faster than originally expected. After 30 minutes the assimilated ¹⁵N was equally as enriched as after three hours. The presumption that it would be a slow process was because of the position of the Calothrix. However the images from the NanoSIM revealed that not only could the nitrogen be passed by the trichomes but also across the membrane.

They further went on to assess how much N₂ is fixed for the diatom, compared to how much the cyanobacteria need themselves. 71-650 % more N₂ is fixed by the cyanobacteria when living in symbiosis and 97 % of that is transferred. The cyanobacterium is fully supporting the diatoms need for nitrogen. Estimates derived from these results suggest that diatom symbioses with diazotrophic cyanobacteria could provide a substantial nitrogen source in the open ocean previously unaccounted for.

Given that nitrogen fixation is a costly process; what benefit is this to the cyanobacteria? 

Written: Kat, Elyssa and Dean

Reference: Foster, R. A., Kuypers, M. M., Vagner, T., Paerl, R. W., Musat, N., & Zehr, J. P. (2011). Nitrogen fixation and transfer in open ocean diatom–cyanobacterial symbioses. The ISME journal, 5(9), 1484-1493.

http://web.b.ebscohost.com/ehost/pdfviewer/pdfviewer?sid=8436bd02-edc9-4dbc-a2dc-57030b3c5251%40sessionmgr113&vid=0&hid=118