Effect of oxygen on continuous cultures of Azospirillum spp. and diazotrophic rods closely associated with Kallar grass

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Abstract
The effect of oxygen on NH4+-dependent growth of two Azospirillum strains and two diazotrophic rods closely associated with roots of Kallar grass (Leptochloa fusca) and one Azospirillum brasilense soil isolate as reference was studied. To enable precise comparison, bacteria were grown in dissolved oxygen controlled continuous cultures at a constant dilution rate. Similar O2 responses for all strains were found. Steady states were achieved from about 1 to 190 .mu.M dissolved o2, all of them carbon limited. The response of all steady-state cultures to increasing O2 concentrations suggested that neither oxygen limitation nor oxygen toxicity occurred within the range of O2 concentrations applied. Growth of the plant-associated diazotrophs on combined nitrogen did not explain the colonization pattern of the different root zones of Kallar grass.