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
No comments:
Post a Comment