In 2010, Candidatus Atelocyanobacterium thalassa (UCYN-A), a type of nitrogen-fixing cyanobacterium was found to have a degree of genomic streamlining, which is common in organisms involved in symbiosis. Unusually this cyanobacterium lacks Photosystem II (oxygen evolving component), RuBisCo (ability to fix CO2) and the TCA cycle all-important components of photosynthesis. It was noted however, that UCYN-A has full nitrogen fixing pathways, as well as the genes for glycolysis, the implication being that it could provide valuable fixed nitrogen to a photosynthetic partner from which it would receive all of its fixed carbon. It is possible that UCYN-A gradually lost genetic information as a result of symbiosis.
To determine the potential symbiotic partner, samples were collected, sorted by flow cytometry and screened for the nitrogenase gene, nifH using quantitative PCR. This showed that the groups associated with the nitrogen fixing cyanobacterium were members of the photosynthetic picoeukaryotes (PPE) populations. Using 18S rRNA from the PPE and 16S rRNA from the associated sample it was found that PPE were exclusively prymnesiophytes and that UCYN-A was present in the microbial community. Within the prymnesiophytes the greatest relatedness was to Braarudosphaera bigelowii and Chrysochromulina parkeae. B. bigelowii is of particular interest as it has calcareous plates at certain stages in its life cycle. It is not certain whether UCYN-A has a calcareous stage during symbiosis, as it is not known at what stage the symbiosis occurs.
This symbiosis represents globally distributed and important (i.e. carbon fixation) groups of organisms, giving environmental importance to the symbiosis. This importance stems from the organisms involvement in a large portion of the vertical nitrogen flux in the oceans. It is already known that calcified prymnesiophytes are major contributors to the POM that is distributed throughout the water column. One of the questions raised by this apparent symbiosis is why this symbiosis has not ended with an endosymbiotic relationship, specifically the formation of a new nitrogen fixing plastid. The possible answers are numerous. Some of the ideas we discussed involved nitrogenase sensitivity to oxygen. This would mean that a nitrogen fixing plastid would have to be maintained in anoxic conditions within the cell. Another potential problem is the calcification state. Perhaps calcified plates would act as a barrier and prevent phagocytosis or perhaps the symbiosis is held intact by bonding to these plates and engulfing the UCYN-A would therefore not be possible.
Halogen In Situ Hybridization and Secondary Ion Mass Spectrometry (HISH-SIMS) were used to determine the flow of 14C and15N between the two partners. This showed that 95% of the nitrogen fixed by UCYN-A was transferred to the partner and 17% of the fixed carbon was transferred to UCYN-A. This technique also showed that there was an indentation on the algal surface where attachment occurred The indentation was smaller than would be expected, and some indentation could be seen at both poles of one alga which was on the verge of mitosis, implying that the symbiosis may occur throughout the life cycle of both the algal partner and the cyanobacterium.
This paper allows us to understand that there is a symbiosis occurring between some of the most important groups of microbes in the ocean. This opens up possibilities into what we may find in the future.
Thompson, A. W., Foster, R. A., Krupke, A., Carter, B. J., Musat, N., Vaulot, D., & Zehr, J. P. (2012). Unicellular cyanobacterium symbiotic with a single-celled eukaryotic alga. Science, 337(6101), 1546-1550
George & Ethan