Nitrogen Fixation Associated with Development and Localization of Mixed Populations of Cellulomonas sp. and Azospirillum brasilense Grown on Cellulose or Wheat Straw

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Mixed cultures of Cellulomonas sp. and Azospirillum brasilense were grown with straw or cellulose as the carbon source under conditions favoring the fixation of atmospheric nitrogen. Rapid increases in cell numbers, up to 109 cells per g of substrate, were evident after 4 and 5 days of incubation at 30°C for cellulose and straw, respectively. Nitrogen fixation (detected by acetylene reduction measured on parallel cultures) commenced after 2 and 4 days of incubation for straw and cellulose, respectively, and continued for the duration of the experiment. Pure cultures of Cellulomonas sp. showed an increase in cell numbers, but CO2 production was low, and acetylene reduction was not detected on either cellulose or straw. Pure cultures of A. brasilense on cellulose showed an initial increase in cell numbers (107 cells per g of substrate) over 4 days, followed by a decline presumably caused by the exhaustion of available carbon substrate. On straw, A. brasilense increased to 109 cells per g of substrate over 5 days and then declined slowly; this growth was accompanied by acetylene reduction. Scanning electron micrographs of straw incubated with a mixed culture under the above conditions for 8 days showed cells of both species in close proximity to each other. Evidence was furnished that the close spatial relationship of cells from the two species facilitated the mutually beneficial association between them and thus increased the efficiency with which the products of straw breakdown were used for nitrogen fixation.

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