Unicellular cyanobacteria with a new mode of life: the lack
of photosynthetic oxygen evolution allows nitrogen fixation
to proceed.
The reactions of N2 - fixation catalyzed by the O2-sensitive nitrogenase and photosynthetic O2-evolution are incompatible. Most filamentous cyanobacteria that perform N- fixation during the day develop specialized cells, called heterocysts, that do not evolve O2 photosynthetically. Some of the unicellular cyanobacteria separate the two processes by performing N- fixation in darkness and photosynthetic O2-evolution and CO2- fixation during the day. However, few other unicellular cyanobacteria can fix both CO2 and N2 in light.
A group of unicellular N - fixing cyanobacteria of the oceanic picoplankton have revealed a new mode of life. These organisms, termed UCYN-A, were discovered by amplification of the nitrogenase gene (nifH) and are characterized by their 16S rRNA gene sequences. UCYN-A cells have nitrogenase gene arrangement and composition similar to those of Cyanothece sp. ATCC 51142 and of the spheroid bodies of Rhopalodia gibba.
The lack of genes coding for phycocyanin, phycoerythrin or associated linkers, the Calvin-Benson cycle and for photosystem II suggests that UCYN-A cyanobacteria cannot synthesize organic carbon by photosynthesis but are strictly dependent on extracellular sources. The cells apparently meet their energy demand by cyclic photophosphorylation. UCYN-A does contain complete metabolic pathways such as glycolysis however and possess transporters for sugars and dicarboxylic
acids. The authors are therefore tempted to assume that they assimilate dicarboxylic acids, sugars and/or amino acids from ocean waters, as other non-photosynthetic bacteria.
Another possibility is that the UCYN-A cells might be able to cleave pyruvate, but the fate of the remaining two electrons (of NADH) remains uncertain. The authors consider a symbiosis where a fermentative end product such as malate is rapidly consumed by other microorganisms, which could symbiotically supply the missing amino acids to UCYNA. But no organism accompanying UCYN-A has been detected as yet. The review then makes comparisons and takes the reader through the logic of the relatedness of the cyanobacteria to the endosymbiotic spheroid bodies of R. gibba and the acetate photoassimilating green alga Chlamydobotrys sp. of the Volvocales.
It remains to be shown whether acetate or any other chemically simple organic carbon source is sufficiently available in the oligotrophic regions of the open oceans to sufficiently allow growth of the N-fixing picoplankton. However the low population density and small size of UCYN-A requires three orders of magnitude less organic carbon than that which could theoretically be provided by Prochlorococcus. The authors concluding remarks said that the catabolism of organic carbon is not fully understood in UCYN-A. Photoassimilation of acetate, as in the alga Chlamydobotrys, is only one of the possibilities that may happen in their carbon metabolism. Nature may hide further microorganisms with entirely new modes of life.
6 comments:
Hi Alice,
I was late for the seminar and did not have a clue what was being discussed. Good thing you posted a precis of what sounds like a pretty complex paper.
It is a very interesting review which underlines how diverse metabolic processes (and niches) are in the marine environment.
Did the authors talk about or give any insight into growth rates for this cyanobacteria at all? Also you mentioned they do not have genes for accessory pigments, PSII and Calvin-Benson cycle ( I think I prefer to call it the Benson-Calvin Cycle because it was Benson's work that led to the discovery while Calvin won the Nobel Prize ). Despite this, they are still able perform cyclic photophosphorylation. Does this mean they have thylakoid membranes or did the authors not elaborate on what this cyclic photophosphorylation pathway was?
It is quite intriguing to hear about possibly novel metabolic pathways.
The paper was hard to say about in a seminar and did go off on a tangent quite alot.
The paper did not mention growth rates, instead the authors determined how much carbon the bacterium would require by size and population density as a whole.
I think the bacterium does have thylakoid membranes, almost all cyanobacteria have concentric thylakoids but the paper does not appear to actually say that it does! The paper mentioned that energy (ATP) is generated mainly by cyclic photophosphorylation around photosystem I in Chlamydobotrys (which is something that was compared to the bacterium) however again it is not clear if this is what the bacterium does. i will ask Matt and Katty if they know better.
Well a new mode of life would be interesting if the paper actually made some better conclusions. The comparisons were logical but not clear. I guess further research is required as usual!
The paper does mention growth rates but just to say they dont know what they are. They do have thylakoids but they have no idea what type or if they have been modified in anyway. They do seem to have a stab at what the pathway might be but theres no real evidence for it and they even say themselves that their equations dont add up. It does seem quite hard to study these pathways when the organism cant be cultured.
I do think it was an intresting paper, but one critism I have is that I feel that they should not have spent so long speculating the different ways in which they could produce energy. I feel an overview of the different methods would of been better. Obviously finding the pathways of energy generation will be hard while they cannot be cultured.
i agree.
There are 3 types of photosynthesis (type I, type II and type I/II) that can be cylic and non-cyclic.
It seems Type I/II cyclic photophosphorylation only using photosystem I is common in cyanobacteria with the reaction centre and electron transport components embedded in the thylakoid membranes. It is oxygenic (which is contrary to the paper reviewed) and only found in cyanobacteria and chloroplasts (which descended from cyanobacteria). It utilises plastocyanin and a cytochrome b/f complex.
The paper talks about not having genes for phycocyanin but did the authors indicate that a gene for plastocyanin was observed? If it was mentioned, then that may well be confirmation of cyclic photophosphorylation (and oxygenic photosynthesis) in PS1 by the cyanobacteria that they isolated
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