Both nitrogen fixation and photosynthesis are extremely important mechanisms for marine cyanobacteria. But there has always been limitations allowing both to occur in one organism due to oxygen inhibition of the important nitrogen fixing enzyme nitrogenase. Ways of conquering this include heterocyst’s, specialised nitrogen fixing cells separate from those containing oxygen, and also fixing nitrogen at night with photosynthesis occurring in the daytime. Trichodesmium however does neither. It is non-heterocyst diazotrophic cyanobacterium and can fix nitrogen during the day and at night. The Mehler cycle, which quickly reduces oxygen to water, as well as a high dark respiration rate, protects nitrogenase from damaging oxygen and these are the keys to multitasking here. However, in oligotrophic waters, iron depletion acts as a limiting factor towards this system. Iron is part of the nitrogenase complex and is also found amongst the proteins in photosynthetic units and depletion usually causes sudden death. However, adaptations to Trichodesmiums system in these tough conditions allow it to continue living until the next influx of iron appears in their waters.
Two cultures were set up, an iron – replete and an iron limited culture of which the concentration of Fe was steadily reduced. Morphological changes were studied under transmitted light. This included looking at the filaments of the cell. After one week the filaments of the Trichodesmium cells in the Fe replete culture were healthy and normal, however in the iron limited culture, filaments had shortened and fragmented; a survival technique which increases the surface volume ratio bettering the chance of Fe uptake. No other morphological changes were noted.
Photosynthesis levels were maintained throughout the iron depletion but after 9 days of dilution nitrogen fixation was reduced to 25% of that seen in the replete culture. This continued to decrease as less iron became available but interestingly, at day 20, nitrogen fixing increased to near normal levels once again, seemingly a final survival attempt. NifH, dinitrogenase reductase, followed this pattern during Fe limitation, decreasing throughout and then increasing suddenly towards the end of the experiment. Subunits of the proteins found in PSII, which have a cofactor containing iron did not show any sign of decreasing, in fact keta carotenoids, yellow pigments and blue pigments like phycocyanin, increased to improve photosynthesis. Nitrogen fixing is sacrificed allowing for the energy making process of photosynthesis to carry on, a technique not seen before.
The experiment noted that there was an obvious down regulation of nitrogen fixation during iron limitation. It includes the breakdown of nitrogenase and the utilisation of its Fe protein for photosynthesis. However the increase of this enzyme seen towards the end of the experiment (20 days) suggests that degradation of other Fe proteins must have taken place to assemble the new nitrogenase. It is unknown as to where this extra Fe is accumulated but an isoform of phycoerythrin, an important part of the light harvesting antennae, phycobilisomes, has an increased number of chromophores compared to protein backbone and can therefore help to synthesize nitrogenase without previous nitrogen fixing. This needs further investigation.
Trichodesmium is an exciting cyanobacterium with a completely new system for overcoming the problem of photosynthesizing and nitrogen fixing at the same time. It would be nice to see more studies completed on this, with specific focus on how there is an increase in nitrogenase synthesis at the end of the experiment. However, there are numerous results given for each finding presented in this experiment which makes this an excellent first paper on the subject. It would be interesting to find more papers that show similar mechanisms being used, or is Trichodesmium the only one.
A review of: Hendrik Küpper, Ivan Šetlík, Sven Seibert1, Ondrej Prášil, Eva Šetlikova, Martina Strittmatter Orly Levitan, Jens Lohscheider, Iwona Adamskaand Ilana Berman-Frank (2008) Iron limitation in the marine cyanobacterium Trichodesmium reveals new insights into regulation of photosynthesis and nitrogen fixation
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