Aerobic denitrifiers isolated from diverse natural and managed ecosystems

Twenty-eight bacterial strains were isolated from an ecosystem adapted to f luctuating oxic-anoxic conditions. This ecosystem comprised a mixture of di fferent natural and wastewater treatment environments. Among the 28 strains isolated, 10 exhibited aerobic denitrifying activity, i.e., co-respiration of oxygen and nitrate and simultaneous production of nitrite by 4 of them and of nitrogen gas by the remaining 6. Comparisons between the 16S rDNA se quences of the 10 strains showed that 3 of them were identical to M. aerode nitrificans, whereas RAPD profiles showed that the 3 strains were identical to each other but that they were different from M. aerodenitrificans. This implies that alternating aerobic-anoxic conditions allowed the isolation o f a new strain of this aerobic denitrifier. Moreover, other denitrifying ba cteria belonging to the genera Paracoccus, Thiobacillus, Enterobacter; Coma monas, and Sphingomonas were isolated in this way. These data imply that a wide variety of bacteria are able to carry out this type of metabolism. M. aerodenitrificans was also detected in methanogenic, denitrifying, nitrifyi ng, phosphate removal, and activated sludge ecosystems by two-step PCR ampl ification. After 4 months of acclimation to oxic-anoxic phases, the strain was also defected in a canal and in a pond. This suggests that there is no specific natural ecological niche for aerobic denitrifiers but, as soon as selective pressure such as alternating aeration conditions is applied, this metabolism is amplified.