Acetylene Reduction Activity in Free-Living Cultures of Rhizobia

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Abstract
A survey of 272 free-living rhizobia strains showed that a majority of the Rhizobium japonicum and unclassified Rhizobium isolates are capable of growth and acetylene reduction on a Casamino Acids-minimal medium. Two other taxonomic categories of Rhizobium were discernible: R. meliloti and R. trifolii, which grow but do not reduce acetylene; and R. leguminosarum, which fails to grow on the minimal medium. Sixty-nine strains were acetylene reducers obtained from root nodules of the plant hosts: Albizzia, Cassia (partridge pea), Baptisia (wild indigo), Crotalaria, Ulex (thornbloom), Indigofera, Lespedeza, Desmodium (Florida beggarweed), Arachis (peanut), Phaseolus, Strophostyles, Stizolobium (velvet bean), Pueraria (kudzu), Vigna (cowpea), Erythrina and Glycine (soybean). The extent of acetylene reduction varied depending on both the Rhizobium strain and O2 tension, suggesting that reduction activity differs according to the cellular organization of individual strains.