Nitrogen Fixation in Obligate Methanotrophs

This paper reports on the distribution of the N2-fixing character of all the representative strains of obligate methanotrophs in the University of Warwick Culture Collection and investigates the effects of O2 and ammonium ions on the activity of nitrogenase in these organisms

J. C. Murrell; H. Dalton

2009

Scholarcy highlights

  • N,fixing methane oxidizing bacteria have been known to exist for many years, initial attempts to measure N,fixation by the classical acetylene reduction testwere unsuccessful(Whittenbury et al, 1970).It was subsequently observed that acetylene was a potent inhibitor of methane oxidationbut that whole-cell nitrogenase activity could be measured if a suitable electron donor such as methanol or formate was included in this system
  • In the few methanotrophs that have been shown to fix N,diazotrophic growth was sensitive to O2 due to the intrinsic sensitivity of their nitrogenase proteins.This sensitivity was manifest in the characteristic ‘bell-shaped’curvesfor nitrogenase activity versus applied PO, values or by the ‘switch-off’phenomenon observed by Drozd & Postgatewhen aerobic cultures were subjected to a sudden increase in dissolved oxygen tension
  • All methanotrophs exhibited some growth on these nitrogen-free plates, type I methanotrophs showed far less growth than the type I1 methanotrophs
  • Type I1 methanotrophs appeared to be less sensitive to O2 than MethyZococcus capsulatus under high p 0 2 values suggesting that their nitrogenase proteins were either less sensitive to O2damage than the enzyme from Methylocoecztscapsulutus(Bath) or that these organisms could augment respiratory activity to protect nitrogenase
  • The rapid ‘switch-off’of nitrogenase activity by NH,+ is an unusual phenomenon so far only observed in a few photosynthetic N,fixersand it will be interesting to learn from future studies with Methylococcus capsulatus if a similar short-term control mechanism of nitrogenase activity operates in methanotrophs

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