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As nitrogen availability is often the limiting factor to growth and biomass production, nitrogen fixation is one of the most important biological processes on Earth. Now, new research has revealed the reasons behind the differences in the global distribution of heterocystous and non-heterocystous nitrogen-fixing cyanobacteria.

Heterocystous cyanobacteria are believed to be better adapted for N₂ fixation than non-heterocystous cyanobacteria — because the heterocysts (differentiated cells enveloped by a glycolipid layer) are thought to protect nitrogenase from inhibition by molecular oxygen. Despite this however, the dominant N₂-fixing cyanobacteria in the tropical oceans are the non-heterocystous Trichodesmium spp. Indeed, there are significant differences in the global distribution of hetercystous and non-heterocystous cyanobacteria — heterocystous cyanobacteria are the dominant species in freshwater lakes and brackish environments (such as the Baltic Sea), but Trichodesmium spp. are dominant in the tropical oceans, yet unsuccessful in temperate or polar oceans.

Now, reporting in Nature, Staal et al. have provided answers to two important questions regarding these differences in distribution; namely, why are free-living heterocystous cyanobacteria not the dominant N₂-fixing organism in the tropical oceans, and why are Trichodesmium spp. not able to thrive in temperate and polar regions, and in freshwater and brackish environments?

Using an acetylene reduction assay, the authors compared nitrogenase activity in the heterocystous Nodularia spumigenia strain CCY 9414 and Anabaena sp. strain CCY 9901 with the nitrogenase activity in Trichodesmium sp. strain IMS101, and showed that N₂ fixation in the dark in both heterocysts and diazocytes (the specialized N₂-fixing cells of Trichodesmium which do not have a glycolipid layer) is limited by O₂ diffusion. In the dark, respiratory ATP production limits N₂ fixation due to a limited influx of O₂ to the heterocyst or diazocyte. However, in the light, ATP production by the photosynthetic electron transport chain ensures that ATP availability is not a limiting factor.

The authors also examined the effect of increased temperature on the gas flux and enzyme potential in the N₂-fixing cells. Although the concentration of dissolved O₂ decreases with increased temperatures, the gas diffusion coefficient increases — resulting in a net increase in the gas flux into the N₂-fixing cells. However, the enzyme potential increases faster than the gas flux, so with incerasing temperatures, nitrogenase activity becomes substrate limited, particularly in those cyanobacteria with an efficient diffusion barrier — heterocystous cyanobacteria. In addition, by modelling the influx of O₂ to both heterocysts and diazocytes, Staal et al. calculated that O₂ flux increases by approximately 25% when the salinity is decreased from 35 (sea water) to 0 (fresh water) — thereby, highlighting the need for an efficient diffusion barrier, such as the glycolipid envelope of the heterocyst, in freshwater conditions.

So, in freshwater conditions, the glycolipid envelope of the heterocyst provides a selective advantage over non-heterocystous cyanobacteria — explaining the absence of Trichodesmium from freshwater seas. But, in pelagic environments at high temperatures, the glycolipid envelope is a disadvantage — explaining the dominance of Trichodesmium in the tropical oceans.