Denitrification by Rhizobium meliloti, 1: Studies of free-living cells and nodulated plants
1996
Rosen, A. | Lindgren, P.E. | Ljunggren, H. (SLU, Uppsala (Sweden). Inst. foer Mikrobiologi)
The denitrification activity was investigated in thirteen Rhizobium meliloti strains, of which most are proposed inoculant strains and one is deficient in nitrogen fixation (Fix-). One strain each of the species Pseudomonas aeruginosa and Bradyrhizobium japonicum were used as references. We found a great variety regarding the denitrification capacity within R. meliloti strains, most of them with low or no detectable activity. Four strains showed a denitrification rate 200 times higher or more than the strains with low activity, and the highest rate was found in R. meliloti 6 963. The average denitrification rate (microg N2O-N mg-1 protein h-1) by free-living bacteria was 100, 50, and 26 for P aeruginosa, B. japonicum, and R. meliloti 6 963, respectively. Of nine R. meliloti strains tested, three expressed nitrous oxide reductase (Nos) activity, while eight strains gave positive hybridisation signals to a gene probe derived from nosZ of Pseudomonas stutzeri. The Fix- strain, with a great denitrification capacity, was phenotypically Nos- and did not hybridise to the nosZ gene. Bacterial growths during nitrate respiration as well as consumption and production of nitrogenous oxides were investigated in a closed bioreactor with a limited access for oxygen. The growth rate of R. meliloti was more influenced by decreasing oxygen pressure than P. aeruginosa. Both bacteria accumulated detectable levels of nitrite and nitrous oxide during the nitrate respiration, but R. meliloti seems to be much more sensitive for accumulation of nitrite than the pesudomonad.
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