Nitrogen fixation by Methanobacterium formicicum

Methanobacterium formicicum utilized molecu-lar nitrogen as the sole nitrogen source for growth as monitored by methane production and culture turbidity measurements. The rate of methane production by the bacteria was correlated to ni-trogen gas concentrations. In the absence of ni-trogen gas or any other nitrogen source, the bac-teria completely stopped growing. The presence of selenium and molybdenum in the culture medium was vital for the growth of the bacteria under nitrogen fixing conditions. Methanobacterium ivanovi [4] and Methanolobus tindarius [10] have been demonstrated to be able to fix molecular nitrogen. The above-mentioned species have been shown to possess nitrogen fixation (ni l) genes [4-6]. For Methanobacterium forrnicicum strain MS1 the presence of nil genes was proven [5], but the genes have yet to be shown to be expressed. This work was designed to demonstrate that Methanobacterium formicicum strain MS1, which is an endosymbiont of the anaerobic iliate Meto-pus striatus [7], is also capable of fixing molecular nitrogen. 2