Group post: The relationship between a cyanobacterial cell and a prymnesiophyte

Nitrogen-fixing symbioses between bacteria or cyanobacteria and eukaryotes and the associated interactions in the marine environment have been poorly documented compared to those on land. A study by Thompson et al. (2012) focused on a unicellular cyanobacterium with a proposed name of Candidatus Atelocyanobacterium thalassa but referred to as UCYN-A throughout the paper. The authors reported a mutualistic symbiosis with a unicellular eukaryotic algae known as a prymnesiophyte, of which some members are characterised by calcareous plates covering the surface of the cell.

UCYN-A showed evidence of a reduced genome, containing only 1.44 million base pairs. Analysis of this streamlined genome showed that the cyanobacteria did not possess photosystem II, the tricarboxylic acid (TCA) cycle and possible indications that this organism cannot fix CO₂ due to a lack of RuBisCo, a key enzyme in the Calvin cycle. This suggested that this cyanobacteria could not be free living as it lacked crucial biosynthetic pathways in order to be self-sufficient, thus it lived in obligate symbiosis with a prymnesiophyte. In this relationship, UCYN-A provided the fixed nitrogen through nitrogenase genes present in the genome and the prymnesiophyte supplied the fixed carbon from its own photosynthesis.

Cells from samples of seawater from an oligotrophic area in the North Pacific Ocean were initially sorted by flow cytometry, based on size. Prymnesiophytes were identified by their size as well as by their photosynthetic ability. Samples were analysed with red fluorescence to measure chlorophyll-a concentration, DAPI staining and CARD-FISH (catalysed reporter deposition fluorescence in situ hybridization). This latter technique produces a stronger fluorescent signal and is more sensitive than the FISH method. Cells were formally identified using gene clone libraries of 18S rRNA and 16S rRNA that were amplified from samples of the entire prymnesiophyte population. To measure the uptake of carbon and nitrogen into the cells, HISH-SIMS (halogenated in situ hybridization nanometer-scale secondary ion mass spectrometry) and NANO-SIMs were utilised using ¹⁵N₂ and ¹³C-bicarbonate markers for quantitative isotopic analysis.

Most of the detected nitrogen-fixing UCYN-A nifH genes were detected outside of the prymnesiophyte cells, indicating that UCYN-A is a separate epiphytic cell. UCYN-A cells were also easily dislodged from the surface of the prymnesiophyte. The isotopic analysis showed that UCYN-A transferred fixed nitrogen to the photosynthesising prymnesiophyte, which in turn transferred carbon to UCYN-A. In addition, a comparison of ¹²C to ¹³C showed that ¹³C /¹²C enrichment was lower in the UCYN-A symbiotic cells than the partner cells and the paper explained this was due to UCYN-A cells being small, slow-growing heterotrophs, therefore having a lower carbon requirement. Both the UCYN-A and the prymnesiophyte cells were N¹⁵ enriched, however, ¹⁵N/¹⁴N enrichment was higher in the prymnesiophyte cells, presumably from the high level of fixed nitrogen that had been transferred from the UCYN-A cells. Fixation of nitrogen by the UCYN-A cyanobacterium, the carbon fixation and possible calcification by the prymnesiophyte partner may have important consequences for biogeochemical cycles.

There is no definitive conclusion as to whether the partnership is endosymbiotic or whether the UCYN-A lives on the exterior of the prymnesiophyte. As the UCYN-A cells can be easily dislodged from their partner cells, the association between the two organisms may be connected to the shedding of calcareous plates of the prymnesiophyte if it has these. The authors suggest a mutualistic if not obligate relationship between the two cells. Such a mutually beneficial relationship could offer an explanation for the survival of a cyanobacterium lacking a carbon fixation pathway within an oligotrophic environment. It could also offer an insight into the occurrence of primary endosymbiotic events, including the evolution of nitrogen fixation pathways, N₂ fixing plastids and mutualistic phytoplankton relationships.

Thompson et al. (2014) have published further research into the diversity of the UCYN-A symbiont relationships, which Emma will be reviewing shortly.

Written by Bekki, Emma and Anita

Reference: Thompson, A.W., Foster, R.A., Krupke, A, Carter, B.J., Musat, N., Vaulot, D., Kuypers, M.M.M. and Zehr, J.P. (2012) Unicellular cyanobacterium symbiotic with a single-celled eukaryotic alga, Science, 337, 1546-1550.

http://www.sciencemag.org/content/337/6101/1546.long