Effect of Protein Additives on Acetylene Reduction (Nitrogen Fixation) by Rhizobium in the Presence and Absence of Soybean Cells

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Abstract
Acetylene reduction was promoted on the basal medium supplemented with 1.4 mg of N/ml supplied as aqueous extracts of hexane-extracted soybean (Glycine max [L.] Merr.), red kidney beans (Phaseolus vulgaris L.), or peas (Pisum sativum L.). Commercial samples of .alpha.-casein, or bovine serum albumin also promoted acetylene reduction at a concentration of 1.4 mg of N/ml of basal medium, but egg albumin supplying an equal amount of N to the basal medium completely suppressed acetylene reduction. Autoclaving the aqueous extract of hexane-extracted soybean meal had no effect on its ability to promote acetylene reduction. The presence of 40 mM succinate decreased acetylene reduction with leguminous proteins supplying 1.4 mg of N/ml but promoted acetylene reduction by Rhizobium 32H1-soybean cell associations on media containing .alpha.-casein, bovine serum albumin, or egg albumin supplying 1.4 mg of N/ml. Similar results were obtained with both cowpea Rhizobium 32H1 and Rhizobium japonicum 61A96. Pure cultures of Rhizobium 32H1 developed acetylene-reducing activity in the presence of soybean extract on basal agar medium and in vermiculite supplied with N-free mineral salts plus crude soybean meal. In certain situations, free living Rhizobium may reduce N2 under field conditions.