Evolution and adaption of UCYN-A symbionts towards lineage specific N-fixation 'organelles'

Symbiotic relationships play a significant role in biological nutrient cycling, the role of cyanobacteria in nitrogen fixing in soil systems has been highly studied and understood. Now the role of cyanobacteria in marine Nitrogen cycling is being explored with some interesting results. Small cyanobacteria (3-10µm) are increasingly being considered as major players in Nitrogen cycling in the seas. Most recently the symbiosis between UCYN-A and microalgae cells is becoming increasingly noted and studied.

Cornejo-Castillo and his team investigated the lineage-specific roles that UCYN-A1 and UCYN-A2 play in Nitrogen cycling, using metagenomics retrieved from 68 TARA ocean expedition sites and CARD-FISH (Catalysed reporter deposition fluorescence in situ hybridization), as well as cDNA synthesis and sequencing the team were able to detect and distinguish UCYN-A1 and UCYN-A2 lineages in symbiosis with two distinct prymnesiophyte partners, Braarudosphaera bigelowii and an uncultured prymnesiophyte.

Both linages of UCYN-A are highly adapted for efficient Nitrogen fixation, with streamlined genomes which dedicate around 25% of the genes to Nitrogen fixation. The Ka/Ks ratio (number of Non-synonymous/ number of synonymous nucleotide substitutions ratio) was used as a determination of the selection pressure these symbionts experienced, the Ka/Ks ratio of 887 shared protein coding genes was calculated, the ratios indicated that 873 of the genes went under purifying selection, with no evidence of positive selection or any strong adaptions to niches in the UCYN-A lineages suggesting that the prymnesiophyte partners experienced the evolutionary forces for niche adaptation, which was followed by adaptation in the symbionts.

The genome studies also revealed that both UCYN-A1 and UCYN-A2 linages lacked the same major pathways and proteins, from this we can hypothesise that these symbionts genetically adapted to the hosts before they were separated by speciation. Phylogenomic and Bayesian relaxed molecular clock analyses were used to determine that UCYN-A1 and UCYN-A1 lineages diverged from a common ancestor in the late cretaceous (around 91Myr ago), the symbiotic relationship between the UCYN-A1/A2 common ancestor and Braarudosphaera- related species, was established to deal with the extremely low nutrient availability in the oceans, during the Jurassic period when the nutrient avalibiltiy was its lowest in the last 550 Myr.

Symbionts can be utilized by the host in many ways, some hosts may us multiple symbionts, such as corals and zooxanthella with each coral hosting many symbionts to increase productivity, where as some hosts live one-on-one with their symbiont, in the case we can see both examples with the UCYN-A linages. Only 1 or 2 UCYN-A1 cells live in each prymnesiophyte cell, yet each B. bigelowii cells host numerous UCYN-A2 cells which are enclosed in a symbiosome-like compartment, previous studies have also showed UCYN-A2 cells enclosed in an envelope within B. bigelowii consisting of 3 layers, possibly an outer membrane, peptidoglycan wall and plasma membrane (Hagino, Onuma, Kawachi, & Horiguchi, 2013), interestingly this structure can also be observed in the free state, indicating that the UCYN-A2 cells must be expressing the genes to make the cell wall.

The B. bigelowii cells represent a generally larger size class (7-10µm) of prymnesiophyte then the other non-cultured prymnesiophyte (1-3µm) partner considered in this study. This information along with the consideration that the expressed nifH transcripts in UCYN-A2 were almost double that then in UCYN-A1 even when UCYN-A1 was more abundant for example in the >0.8µm size class, gives us some indication in to the specific niches that each lineage takes on, with the UCYN-A2 lineage partnering a larger more energy demanding prymnesiophyte, requiring multiple individuals to meet its Nitrogen demands, this can be further suggested with the compartmentalization of the UCYN-A2 cells from the rest of the cell, which maybe a protective mechanism by the UCYN-A2 cells in order to separate itself from any oxygen in the prymnesiophyte cell produced by photosynthesis.

This study gives us an interesting insight into the evolution of the UCYN-A lineages and how they are used to exploit the oceans Nitrogen supplies. This paper hints at a possible coupling between the prymnesiophyte cell division and the number of symbiont cells, especially for UCYN-A1, as well as a possible evolution towards the UCYN-A symbionts becoming organelles, with the immense genome streamlining by purifying selection, and in UCYN-A2 the cells being enclosed in an envelope separating them from the prymnesiophyte host cytoplasm.

Reviewed paper- Cornejo-Castillo, F. M., Cabello, A. M., Salazar, G., Sánchez-Baracaldo, P., Lima-Mendez, G., Hingamp, P., … Acinas, S. G. (2016). Cyanobacterial symbionts diverged in the late Cretaceous towards lineage-specific nitrogen fixation factories in single-celled phytoplankton. Nature Communications, 7, 11071. doi:10.1038/ncomms11071, http://www.nature.com/articles/ncomms11071

Hagino, K., Onuma, R., Kawachi, M., & Horiguchi, T. (2013). Discovery of an Endosymbiotic nitrogen-fixing Cyanobacterium UCYN-A in Braarudosphaera bigelowii (Prymnesiophyceae). PLoS ONE, 8(12), e81749. doi:10.1371/journal.pone.0081749