Keeping Oxygen away from Nitrogenase

Approximately 50% of natural nitrogen fixation occurs in the oceans. Although this may not sound like a particularly large amount as our oceans cover 71% of the Earth, it occurs almost exclusively where the surface temperature is 25^oC or higher. The concentrations of nitrate are higher below the euphotic zone due to sedimentation but they may be pushed to the surface in upwelling zones and therefore be available to the cyanobacteria.

The paper discusses some of the ways that cyanobacteria may be well adapted to protect nitrogenase from oxygen inactivation so that they can perform the oxygenic process of photosynthesis as well as N₂ fixation. One group takes an ‘avoidance’ strategy, only fixing N₂ in anoxic conditions and inhibiting phoytosynthesis.

The second group is able to confineN₂ fixation to differentiated cells known as heterocysts. The heterocyst is able to generate energy using PS-I which is anoxygenic but has no PS-II so no oxygen is reduced. It is not able to fix CO₂ but receives sucrose from neighbouring non-heterocysts and also gives fixed nitrogen to neighbouring cells. The heterocysts cannot differentiate between O₂ and N₂ so must allow some O₂ in but this is quickly respired to keep the cell anoxic. It is thought that heterocyst cells may have dynamic pores so would be able to close during the night when nitrogenase activity is low to keep O₂ out and maintain an anoxic cell but even if this were the case it would only apply to some species of heterocystic cyanobacteria as some have been found to fix a lot of nitrogen at night.

The least is known about the third group which separates the process of N₂ fixation from photosynthesis by performing them at different time. There has been a lot of research into Trichodesmium and it has been found that it may also have differentiated cells, similar to heterocysts, although another study was unable to replicate these results. Another theory is that the cells are able to perform both and individually switch between the two.

Whichever group the cyanobacteria are in they must be able to respire any O₂ that enters with the N₂. If the temperature is high enough the nitrogenase activity is controlled by the O₂ flux into the cell (Transport Control) but at lower temperatures the respiration rate is not fast enough to maintain anoxic conditions so nitrogenase activity is largely reduced (Reaction Control). Different species of cyanobacteria switch from transport to reaction control at different temperature. Although these temperatures do reflect the environment each cyanobacteria comes from it must be noted that the values are absolute limits and in order for the cyanobacteria to compete they must be in higher temperatures.

This paper doesn't seem to come to a conclusion as to why the N₂ fixing cyanobacteria are mostly limited to temperatures of 25^oC or higher but after reading the paper it seems that the decreased rate in respiration with temperature may play a part in this as anoxic conditions cannot be maintained so N₂ Fixation cannot occur.

Reference: Stal, L. J. (2009), Is the distribution of nitrogen-fixing cyanobacteria in the oceans related to temperature?. Environmental Microbiology, 11: 1632–1645