Role of chemotaxis in the ecology of denitrifiers

A modification of the Adler capillary assay was used to evaluate the chemotactic responses of several denitrifiers to nitrate and nitrite. Strong positive chemotaxis was observed to NO(3) and NO(2) by soil isolates of Pseudomonas aeruginosa, Pseudomonas fluorescens, and Pseudomonas stutzeri, with the peak response occurring at 10 M for both attractants. In addition, a strong chemoattraction to serine (peak response at 10 M), tryptone, and a soil extract, but not to NH(4), was observed for all denitrifiers tested. Chemotaxis was not dependent on a previous growth on NO(3), NO(2), or a soil extract, and the chemoattraction to NO(3) occurred when the bacteria were grown aerobically or anaerobically. However, the best response to NO(3) was usually observed when the cells were grown aerobically with 10 mM NO(3) in the growth medium. Capillary tubes containing 103 M NO(3) submerged into soil-water mixtures elicited a significant chemotactic response to NO(3) by the indigenous soil microflora, the majority of which were Pseudomonas spp. A chemotactic strain of P. fluorescens also was shown to survive significantly better in aerobic and anaerobic soils than was a nonmotile strain of the same species. Both strains had equal growth rates in liquid cultures. Thus, chemotaxis may be one mechanism by which denitrifiers successfully compete for available NO(3) and NO(2), and which may facilitate the survival of naturally occurring populations of some denitrifiers.