How to survive iron depletion

A review of Kupper, H., Šetlík, I., Seibert, S., Prášil, O., Šetlikova, E., Strittmatter, M., Levitan, O., Lohscheider, J., Adamska, I. & Berman-Frank, I., (2008), Iron limitation in the marine cyanobacterium Trichodesmium reveals new insights into regulation of photosynthesis and nitrogen fixation, New Phytologist, Vol. 179 Pg. 784-798

This is a post based on the paper reviewed in the seminar by myself and Sara Puddy.

Trichodesmium is a nonheterocystous diazotrophic cyanobacterium, which unlike most photosynthesising, nitrogen-fixing organisms uses the Mehler Cycle as a mechanism to protect the oxygen-sensitive nitrogenise. It does this by rapidly reducing oxygen back to water as soon as it is produced, thus preventing it from inactivating nitrogenise.

Trichodesmium is found in tropic and subtropic oligotrophic waters which are characterised by period pulses of iron supply and reduced iron availability between the pulse events. This means it is highly prone to iron deficiency due to its high demand of iron, both for nitrogen-fixation and photosynthesis. Therefore it is important for the Trichodesmium to have a survival strategy in order to overcome the periods of iron-depletion and the authors explore the techniques it uses for this. This was done by analysing two cultures of Trichodesmium, an iron-replete and an iron-limited culture, where the iron was slowly diluted out.

They found that the iron-replete culture grew healthily and displayed normal functions. However in the iron-limited culture, the filaments, containing cells for nitrogen-fixation, had fragmented and shortened. This is believed to be a survival strategy, as it increases the surface/volume quotient, allowing increased uptake of iron and other nutrients. This strategy is believed to be the evolutionary adaptation of programmed cell death.

They also found that photosynthesis activity was maintained throughout iron depletion and instead nitrogen-fixation had decreased significantly and after 9 days of diluting the iron, nitrogen-fixation per volume, decreased to 25% compared to the iron-replete culture and further down to 10% after another 3 days. This is because nitrogen-fixation is an energetically and iron-expensive process, so instead the energy was used for photosynthesis. Infact, pigments and proteins, specifically keto-carotenoids, associated with photosynthesis had actually increased in the iron-limited culture, some which were only a minor component in the iron-replete culture. Nitrogenase was also degraded to yield an emergency supply of iron and nitrogen for the remaining metabolism.

They also observed that although photosynthesis activity is maintained there is modification of PSI, PSII and phycobilisomes, and a reduction in carbon assimilation efficiency, in order to adapt to the lack of iron levels needed to maintain the normal process.

The study shows that Trichodesmium can survive short-term iron depletion by reducing nitrogen-fixation to compensate for the lack of iron, but that the cells can only survive in this state for approximately 3 weeks. Interestingly, they found that shortly before the cells died, there was a rapid increase in nitrogen-fixation. This was a last resort, as nitrogen starvation in cells had become too severe and so it was necessary to degrade other iron-containing proteins in order to assemble more nitrogenise. It is believed that this was done using an unknown type of restructuring of the phycobilisomes.

Another aspect of this process that is not well researched is the ability to assemble nitrogenise, as normally this requires previous nitrogen-fixation to have taken place, which is not the case for the iron-depleted culture. It is believed that an isoform of phycoerythrin (part of the phycobiliprotein family) is used, which contains more chromophores per amount of protein backbone, helping the cells synthesise nitrogenise.

This paper is a really good basis for further research. It presents us with many different processes which are yet to be investigated and also explores an interesting survival technique, one which perhaps could explain the mechanisms of other nitrogen-fixing organisms in their productivity and survival.