You’re The Only One For Me: UCYN-A and its exclusive prymnesiophyte host.

Previously the work by Thomson et al, 2012 has been discussed during a class seminar. The cyanobacteria UCYN-A work was concluded to be in a symbiotic relationship with a host photosynthesising prymnesiophyte. With the symbiotic relationship believed to be of an obligate nature due to the provision of nitrogen by the UCYN-A to the prymnesiophyte and the provision of carbon to the UCYN-A by the prymnesiopyte. This latest study by Thomson et al, 2014 takes the research a step further by examining the phylogeny of UCYN-A cyanobacteria (with its numerous clades) and the host prymnesiophyte.  

Through phylogenetic analysis of UCYN-A samples possessing the nitrogenase fixation gene (nifH) three distinct clades were identified, all of which did not possess the Photosystem II (carbon fixation pathway) and TCA cycle. These clades were appropriately denoted as UCYN-A1, UCYN-A2 and UCYN-A3. The Thomson et al, 2014 study focused on the UCYN-A2 cyanobacteria from a coastal site, examining the cell’s morphology and genetic identification and comparing the findings with the original UCYN-A1 cyanobacteria. The nifH gene expression of UCYN-A2 was measured over 3 diel cycles and was found to increase during daylight hours much like the UCYN-A1 clade, reflecting the fixation of nitrogen during the day time. This coincides with the most appropriate time for photosynthesis to occur by photosynthesising organisms (including its symbiotic host). As the UCYN-A clades do not possess carbon fixation pathways the production of N₂ does not have to be separated into different cells, allowing the fixation of N₂ at any time, day or night.

Analysis of the rRNA sequences revealed the two cyanobacteria UCYN-A1 and UCYN-A2 to be of two distinct clades, this led to the conclusion that each clade co-evolved separately with a specific prymnesipohyte host. The analysis identified that there were differences between the clades at nucleotide level, reflected by only 86% similarity at amino acid level yet more than 96% similarity in genes over all. The differences between the clades may be due to habitation within different marine environments, leading to the UCYN-A nifH sequences being further examined to see if environmental habitats of each clade were separate or overlapped. UCYN-A 2 nifH sequences were found in the same geographical locations as previously recorded UCYN-A1 locations, confirming the overlap of clades geographic locations, this was also the case with the UCYN-A3 clade. In addition to the three known clades of UCYN-A, further unidentified sequences were also found using flow cytometry, indicating there may be additional UCYN-A clades in the global oceans. Additional clades within the oceans could lead to previous levels of N₂ input by nitrogenising cyanobacteria being under estimated, meaning their ecological importance could be greater than initially thought.

To identify whether each clade was in partnership with a specific host prymnesiophyte cell sorting, microscopy and PCR techniques were undertaken. From the samples of UCYN-A2 positive cells 10 phytoplankton cells were exposed to nested PCR using 18s rRNA primers to identify the host. Seven cells possessed the prymensiophyte sequences showing a great similarity to the UCYN-A1 host, however, there was a 2% variation between the nucleotide sequences. This supports the theory that the different UCYN-A clades 1 and 2 possess genetically different prymnesiophyte hosts from each other. The presence of diverse UCYN-A clades with genetically different pymnesiophytes may also support the theory of co-evolution. During the sequencing of the UCYN-A2 host a 100% genetic match was made to Braarudosphaera bigelowii. As B.bigelowii is a calcareous plate forming cell it is likely that the UCYN-A2 host is also a calcareous plate forming cell as discussed as a possibility in Thomson et al, 2012.  

The research conducted in Thomson et al 2014 answered questions from the Thomason et al, 2012 study by identifying further clades of the UCYN-A cyanobacteria, and their close symbiotic relationship with genetically specific B.bigelowii hosts. This could be investigated further by completing investigations upon the UCYN-A3 and its associated symbiotic partner to see if the same conclusions can be ascertained through genetic sequencing. Further investigations into the unidentified UCYN-A clades may also provide additional insight into the symbiotic relationships between nitrogen fixing cyanobacteria and their carbon fixing hosts.

Thompson, A.,Carter, B.J., Turk-Kubo, K., Malfatti, F., Azam, F and Zehr, J.P. (2014) Genetic diversity of the unicellular nitrogen-fixing cyanobacteria UCYN-A and its prymensiophyte host. Environmental Microbiology 16 3238-3249

http://onlinelibrary.wiley.com/doi/10.1111/1462-2920.12490