A review of: Foster, R.A. et al. (2011) Nitrogen fixation and transfer in open ocean diatom-cyanobacterial symbioses, The ISME Journal, 5, 1484-1493.
Marine diatoms contribute significantly to primary production. This paper focuses on the diatoms found within low-nutrient waters which often exist symbiotically with nitrogen fixing cyanobacteria. Without actual evidence, it was assumed that the diatoms receive fixed nitrogen from the cyanobacterial symbionts, but little was known about the rates of fixation and supply within these symbiotic populations, and previous studies have included measurements from other unicellular populations. The paper took advantage of newer techniques which enabled the authors to measure stable nitrogen isotopes within individual cells and so gain a more accurate picture of rates of nitrogen fixation.
The seawater sampled in the experiment was collected from the Gulf of California and the subtropical North Pacific. A number of experimental designs were used, but all samples were amended with 15N2. A combination of epi-fluorescence microscopy and nano-SIMS analysis (nanometer scale secondary ion mass spectrometry) were used to identify symbionts and measure ratios of 15N/14N. Cell dimensions were measured and numerous calculations performed to assess nitrogen fixation rates and transfer rates.
The results focused on three pairings; the diatoms Hemiaulus, Chaetoceros and Climacodium which are associated with the cyanobacterial symbionts Richelia, Calothrix, and Crocosphaera respectively. In all cases it was shown that the cyanobacteria were supplying fixed nitrogen to their diatom partners, and also fulfilling the diatoms requirements, as demonstrated by the equal or higher nitrogen enrichment observed within the diatoms, compared to within the vegetative cells of the cyanobacteria. Nitrogen transfer was very rapid, often within 30 minutes.
Growth and nitrogen fixation rates of the symbiotic Richelia and Calothrix cells were compared to those of free-living Richelia and Calothrix cells, and it was found that the symbiotic cells had accelerated rates. The authors suggest that the diatoms may co-ordinate changes to the symbionts growth and metabolism in order to ensure sufficient nitrogen is being fixed for both organisms. Further evidence for this was the discovery that Richelia fix much more nitrogen than needed for their own growth, with up to 97.3% being transferred to the diatom.
Diatoms,require dissolved fixed inorganic nitrogen pools (nitrate and ammonium) as they cannot obtain nitrogen from N2, and this paper gives an important insight into how organisms can gain a significant advantage by exploiting other organisms in a low-nutrient environment. It would be interesting to further investigate the authors’ theory that the diatoms influence growth and metabolism of the symbionts. The authors were unable to determine how much nitrogen is released by the symbioses, but conclude by suggesting that the symbioses should be included in global N models.
2 comments:
This is a great paper using this exciting new technique. The observation that cyanobacteria are in close association with diatoms and transferring nitrogen directly to them completely alters our perception of the nitrogen cycle. Organisms are obviously in intimate partnerships as suggested by Malfatti and Azam's EM pictures.
Really enjoyed this paper. It gives a nice example of how significant symbionts can be in the marine environment. The role of global N models appear to be a topic of relative controversy. With so many factors acting upon the N cycle and gaps in our knowledge, it seems that none of these avenues of enquiry should be excluded from global N models. - as soon as one idea is formed on this thread, another remarkable discovery is made or possibility outlined.
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