Nitrogen fixation and nitrogenase activities in members of the family Rhodospirillaceae

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Abstract
Strains of all 18 spp. of the family Rhodospirillaceae (nonsulfur photosynthetic bacteria) were studied for their comparative N2 fixing abilities. All, except Rhodocyclus purpureus, were capable of growth with N2 as the sole N source under photosynthetic (anaerobic) conditions. Most rapid growth on N2 was observed in strains of Rhodopseudomonas capsulata. Within the genus Rhodopseudomonas, R. capsulata, R. sphaeroides, R. viridis, R. gelatinosa and R. blastica consistently showed the highest in vivo nitrogenase rates (with the acetylene reduction technique); nitrogenase rates in other Rhodopseudomonas spp. and in most Rhodospirillum spp. were lower. Chemotrophic (dark microaerobic) N2 fixation occurred in all species with the exception of 1 strain of Rhodospirillum fulvum; O2 requirements for dark N2 fixation varied among species and within strains of the same species. The capacity to fix N2 is universal among members of the Rhodospirillaceae but the efficacy of the process varies among species.