Nitrogen fixation and transfer in open ocean diatom cyanobacterial symbioses

Nitrogen fixation and transfer in open ocean diatom cyanobacterial symbioses

There are a large proportion of diatoms that occupy ocean waters of a low nutrient level. As they are unable to acquire N from N₂ and the extracellular dissolved fixed inorganic nitrogen pools in the ocean are at such low concentrations, these diatoms are believed to form symbiotic relationships with N₂ fixing cyanobacteria, which transfer N to the associating diatom. There has been much controversy concerning the global N sources and sinks and it is believed the lack of research into such symbiotic relationships between cyanobacteria and diatoms may have resulted in a large underestimation of oceanic N₂ fixing rates.

Despite such symbioses being observed over a wide range of ocean basins there have been few previous studies concerning the N₂ fixation rate for such populations. This study utilised the technological advancements of NanoSIMS, high resolution nanometer scale secondary ion mass spectrometry, which enabled them to provide evidence of the N2 fixation and N transfer between individual cells. They used bulk samples taken from two regions of the Pacific Ocean which they incubated with ¹⁵N₂ in order to trace and quantify N₂ fixation. The observations involved the diatoms Hemiaulus, Chaetoceros and Climacodium, which associate with the cyanobacteria Richelia, Calothrix and Climacodium respectively.

The first experiments used epifluorescence microscopy to calculate the ¹⁵N/¹⁴N ratios and demonstrated that the enrichment pattern of Hemiaulus cells corresponded with the location of the Richelia trichomes. Furthermore, the chloroplasts of the diatoms were shown to be enriched, suggesting transfer of N to the diatoms. Similar results were also observed between the other associated cyanobacteria and diatoms being studied. Although N transfer has been observed via trichome from the cyanobacteria to the diatom, it is still unknown how efficient this exchange is as the symbionts remain outside the cell membrane of the diatom.

Since the diatoms inhabit oligotrophic waters it was believed they would have a slow growth rate and therefore only a small amount of N was expected to be transferred. Nonetheless, the experiments demonstrated enrichment in Hemiaulus cells after just 30 minutes and N saturation after 3 hours, providing further evidence that N₂ fixation rates may be greatly underestimated.

The N₂ fixation rates, transfer of N and growth rates all seemed to be accelerated under symbiotic conditions, therefore it was suggested the symbionts may be fixing a greater amount of N than their growth required. Given that the ¹⁵N/¹⁴N ratios estimated similar growth rates for all the symbionts and their corresponding diatoms it was predicted that Richelia fixed 71-651% more N than needed to support their growth. Considering it is energetically expensive to fix N₂ this suggests the diatoms may be influencing the N metabolism of their associated symbionts.

Even though there have been difficulties in estimating the densities of such symbioses, this study proves that fixed N₂ is directly transferred to associating diatoms, providing evidence that these populations may be an important underestimated source of N and therefore supports the argument that future models of global N should include such diatom symbioses. 

Nitrogen fixation and transfer in open ocean diatom cyanobacterial symbioses

Rachel A Foster^1,2, Marcel M M Kuypers², Tomas Vagner², Ryan W Paerl¹, Niculina Musat² and Jonathan P Zehr¹

The ISME Journal (2011) 5, 1484–1493; doi:10.1038/ismej.2011.26; published online 31 March 2011

http://www.nature.com/ismej/journal/v5/n9/full/ismej201126a.html#tbl1