From genome streamlining to symbiotic interaction: the curious case of UCYN-A

As discussed in previous posts and the seminar, symbiosis with a nitrogen-fixing bacterium is a smart strategy to deal with the low concentration of fixed nitrogen in oligotrophic waters. Thompson et al. (2012) focus on an uncultivated diazotrophic (“nitrogen-eating”) cyanobacterium (UCYN-A) and establish step by step its involvement in a symbiotic interaction by taking the reader through a series of hypotheses tested using state-of-the-art techniques.

First of all it is noted that UCYN-A has an unusual degree of genomic streamlining (the overall reduction in genome size via a number of mechanisms) and lacks photosystem II, RuBisCo and the TCA cycle. However, UCYN-A possesses all the necessary genes for nitrogen fixation, notably the nitrogenase gene (nifH), which in combination with the extremely reduced genome and the requirement for fixed organic carbon lead to the hypothesis that UCYN-A forms a symbiotic relationship to exchange fixed nitrogen for fixed carbon. 

To test this hypothesis, Thompson et al. first identified possible partners for a symbiotic relationship. Flow-cytometry was used to sort cells from raw seawater samples and UCYN-A-specific quantitative PCR revealed that 94% of the UCYN-A nifH genes were associated with photosynthetic picoeukaryotes (1-3 μm diameter cells). 

The next step was to further narrow down the identity of the photosynthetic picoeukaryote associated with UCYN-A. Firstly they needed to sort the cells from the picoeukaryote sample again in order to isolate only those cells that were positive for UCYN-A nifH. Amplification using PCR of the 18S rRNA gene of the positively sorted single cells revealed that the closest known relative was the calcareous nanoplankton Braarudosphaera bigelowii, which is a free-living photosynthetic prymnesiophyte (e.g. coccolithophorids). 

So a potential partner for symbiosis with the unicellular cyanobacterium had finally been identified. But do they really form a symbiosis and is there exchange of nutrients? Halogenated in situ hybridization nanometer-scale secondary ion mass spectrometry (HISH-SIMS) and nanoSIMS techniques were used to follow and quantify potential exchange of fixed compounds by introducing nitrogen and carbon isotopes. They observed that up to 95% of the fixed nitrogen were transferred from UCYN-A to the partner cell and UCYN-A received 1 to 17% of fixed carbon, which is consistent with the lower carbon requirements of a small slow-growing heterotrophic symbiont. 

Thompson et al. provide very convincing evidence for the symbiosis between a prymnesiophyte and a unicellular cyanobacterium. They hope this symbiosis can be used as model to study the absence of nitrogen fixing plastids, since this interaction is reminiscent of the endosymbiotic events leading to chloroplasts and mitochondria, and briefly mention the implications of their findings for nutrient fluxes.

This paper seems exceptional to me in the way that is unravels the whole story of the symbiosis between two organisms, from the hint in the unusual genome streamlining to quantification of exchanged compounds in the association with another cell. Moreover it reveals yet another element in the nitrogen cycle and further studies are required to fully understand the implications and the spatial and temporal scale of this symbiosis.

Thompson, A., Foster, R. & Krupke, A., 2012. Unicellular Cyanobacterium Symbiotic with a Single-Celled Eukaryotic Alga. Science, 337, pp.1546–1550.