The effect of oxygen concentration and temperature on nitrogenase activity in the heterocystous cyanobacterium Fischerella sp.

Lucas J Stal

doi:10.1038/s41598-017-05715-0

Abstract

Heterocysts are differentiated cells formed by some filamentous, diazotrophic (dinitrogen-fixing) cyanobacteria. The heterocyst is the site of dinitrogen fixation providing the oxygen-sensitive nitrogenase with a low-oxygen environment. The diffusion of air into the heterocyst is a compromise between the maximum influx of dinitrogen gas while oxygen is kept sufficiently low to allow nitrogenase activity. This investigation tested the hypothesis that the heterocyst is capable of controlling the influx of air. Here, the thermophilic heterocystous cyanobacterium Fischerella sp. was analysed for the effects of oxygen concentration and temperature on nitrogenase activity. Dark nitrogenase activity is directly related to aerobic respiration and was therefore used as a measure of the influx of oxygen into the heterocyst. Above 30% O2, the influx of oxygen was proportional to its external concentration. Below this concentration, the influx of oxygen was higher than expected from the external concentration. A higher or lower temperature also triggered the heterocyst to increase or decrease, respectively, dark nitrogenase activity while the external concentration of oxygen was kept constant. A higher dark nitrogenase activity requires a higher rate of respiration and therefore a higher flux of oxygen. Hence, the heterocyst of Fischerella sp. is capable of controlling the influx of air.

Highlights

Nitrogenase reduces N2 to two molecules of ammonium - and simultaneously produces at least one molecule of hydrogen (H2) - at the expense of protons, electrons that are usually derived from reduced ferredoxin, and 2 ATP per electron[1]
Many species are capable of fixing dinitrogen but this seems enigmatic in organisms that thrive in aerobic environments and evolve O2 as a consequence of their photosynthetic mode of life, even though photosynthesis can satisfy the demand of energy and low-potential electrons for N2 fixation[4]
With increasing O2 concentrations, Nd increased to its maximum at 30% O2, while Nm decreased to compensate nitrogenase activity (Ntot) to its maximum

Summary

Introduction

Nitrogenase reduces N2 to two molecules of ammonium - and simultaneously produces at least one molecule of hydrogen (H2) - at the expense of protons, electrons that are usually derived from reduced ferredoxin, and 2 ATP per electron[1]. Another property that is shared by any nitrogenase is the extreme sensitivity of the enzyme for oxygen. The heterocyst develops a special glycolipid layer which serves as a gas diffusion barrier[7]. The glycolipid layer of the heterocyst, serving as a gas diffusion barrier, must be a compromise between the maximum influx of N2 and the capacity to scavenge the O2 by respiration or otherwise. The imported reducing equivalents are partly used for respiratory scavenging of O2 and partly used to reduce ferredoxin, which serves as the electron donor to nitrogenase[12]

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